Tektronix 2232 scope.... in need of a doctor.. - Page 1
Forum section: electronics / repair

author: Vince
author_rank: Super Contributor
timestamp: June 05, 2017, 07:49:29 pm
content: Hi every one,OK, so I like tearing old gear apart to restore them and see how they are put together... and try fixing them when required... I guess I should be "happy" today... ?!.... my Tektronix 2232 scope just gave up the ghost a minute ago ! Grrr... I love that little scope, been searching for one for some time, not easy to find one in France, in a good shape, fully working, for a reasonable price, and a seller willing to bother packaging/shipping it. So... I must fix it....Today I was fixing an issue with the big time base button on the front panel. the x10, Cal and secondary time base where behaving fine, but the main time base part of the knob was acting weirdly, it had become unusable. Expecting a complicated piece of mechanical engineering marvel at play, I wasted no time and removed the cabinet and front panel... OK so the main logic board was fully exposed, but I did not mess with it in any way, and too great care not to disturb it, not even put my greasy finger prints on anything. I left it alone. the front panel... some of the buttons are not held by a screw, they just snap onto the shafts. The first button I pulled on... broke (the little clip inside of it, that grabs the shaft). OK, so that's bad luck, plastic got brittle over time. Next button... same luck. third.... again. Fourth.... worse : the button itself disintegrated in to a million pieces ! And I swear I was not using any pliers or any tools, I was grabbing and pulling it with two fingers, I swear ! So, so much for the front panel removal... I gave up. Then later I figured that I didn't have to remove the front panel nor even take off the cabinet/cover.... all the time base knob needed was tightening a few screws and resetting it in the correct position....So, now I have a working time base knob... but 4 missing knobs for the vertical and horizontal position controls.... which can be found on ebay... in Greece, for like 10 or 20 buck A PIECE ! My goodness.... no other choice sadly :-( Well at least while I am sourcing buttons, I can still use the scope just fine without theses buttons, I can just grab the shafts directly no worries.So, I reassemble the scope, and proceed to test it to make sure everything is fine.Yes... the time base buttons now works like a charm, and using the delayed time base feture is a delight... so after a few minutes playing with that, I switch to something else and leave the scope unattended. 10 seconds later I hear "grrr.... POOF ! ", then it shuts down abruptly. Oh no, was working just fine ! :-(I proceed to check the fuse... sure enough it was blown. OK it's a 30 year old fuse, maybe it decided to die... if fuses can die on their own ?!........ though the fuses was a transparent/glass type, and it clearly blew, not to mention it was pretty warm, so no doubt some energy passed through the poor little bugger.Was a 2A slow blow, 6x32mm. only 6x32 I had was the 10 pack of 10A Fast fuses I bought for my collection of Metrix multimeters... OK I knew I was taking a risk there, but just to rule out a defective old fuse, I put it that 10A fuse in the scope anyway, thinking if the scope is fine, then 10A or whatever won't make a difference. Yes you guessed it,... the old fuse did blow for a good reason ! LOL .... As soon as I pressed the power button, a distinct/audible "GRRRR. .. gzzzz" that laster a couple seconds tops, then followed by a big " POOF " then shut down like the first time.Given that the scope was working just fine before hand, I take it the digital stuff is not at fault, and even if it somehow developed a big short on one of the rails... the power supply is supposed to cope with that, right ? So... I guess it's more to do with the power supply... old filter caps going bang, or something ?!That will be my starting point anyway....Luckily the service manual and schematics is available, have a copy of it... though I have not yet checked if the schematics for the power supply specifically, are in there... I hope it's not like my TDS 310 scope with a third party PSU where the schematics are just not available, grrrr....Anyway, I created this topic for those who might be interested in the repair, and so I can ask for help/ideas/suggestions here and there if required.Obviously, not being very experienced in repairing stuff, and not having that much time available, I expect this topic to be open for a few weeks at best, most probably a few months... but well, now it's been created, I can add to it as I go.... little by little.I will try to get motivated to start work on it real soon, but most probably it will have to wait until the summer break, starting end of July, for 3 weeks.See you in a bit for the beginning of the repair, then...

author: tautech
author_rank: Super Contributor
timestamp: June 05, 2017, 09:25:10 pm
content: These run a SMPS don't they ?For those following along it would be a good idea to post up a link to the SM so we can all play along.Tek knobs:http://www.sphere.bc.ca/test/tek-parts/tek-knobs.htmlAnd from the sticky in the Repair board:http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

author: Vince
author_rank: Super Contributor
timestamp: June 05, 2017, 09:51:19 pm
content: Hi,Thanks so much for the links, a mine gold ! They even have the square ivory push buttons for my ancient 5000 Series rack mount Tek scope ! When I went to clean these buttons, the markings on some of them came right off as soon as the sponge barely touched them !Will be able to make it look like new again, yahoo ! :-DSMPS ? Nope... I just spent a few minutes looking at the schematics. No third-party SMPS in that old scope, luckily it's all Tek stuff hence documented, and it's the linear type. Below is the relevant portion of the schematics, where one can see the transformer and all the secondary windings, rectifiers and main filter caps, which I highlighted in yellow.I can see a couple small (33 and 270uF) filter caps related to the CRT, then a big 4,700uF one across the cooling fan, then for the main power rails I can count no less than x9 1,000uF caps. So, I will check all that when I find time...I also just ordered a x10 pack of the appropriate 6x32mm 2A slow blow fuses, I anticipate I will go through a few of them during the repair...

author: tautech
author_rank: Super Contributor
timestamp: June 05, 2017, 10:06:43 pm
content: Manual:http://bama.edebris.com/manuals/tek/2232/65 MbWaiting for download on slow connection............Quote from: Vince on June 05, 2017, 09:51:19 pmGoogle dim bulb tester and make one, it will save a pile of fuses.The schematic snip does not confirm it's not SMPS.

author: Vince
author_rank: Super Contributor
timestamp: June 05, 2017, 10:26:51 pm
content: Quote from: tautech on June 05, 2017, 10:06:43 pmWhile your download is running, I attached below the full PDF pages. As you can see it's all 100% old school linear stuff. The second page "Power Distribution" holds not much more, just simple linear 3 terminal regulators.... good old 7805/7905 would you believe it (lazy Tektronix ! LOL), and just small decoupling caps and LC filtering networks, the usual stuff. Not an inch of switching in there. Given the age of the 2232 I am not surprised... I guess all the 2000 series scope, being the same era, are linear. I assume SMPS appeared a tad later, in the late 80's very early '90s, when the more modern, newer generation, fully digital 'TDS ' series was introduced. That was for Tek the opportunity to start a new generation of scopes from a blank sheet, hence the ideal time to update from linear to SMPS. Well, it's a reasonable assumption any how, I find.QuoteHmmm... no idea what this is, but sounds interesting, you got me curious !

author: tautech
author_rank: Super Contributor
timestamp: June 05, 2017, 11:02:58 pm
content: Quote from: Vince on June 05, 2017, 09:51:19 pmFew more minutes required.See P276SMPS waveforms P274

author: Vince
author_rank: Super Contributor
timestamp: June 05, 2017, 11:16:09 pm
content: P276 ? on mly end it's a board view of the alternate sweep sectionAnyway, you mean you found an DC/DC converter somewhere in this scope.. but all the stuff I showed you is linear so at least 95% of the rails in the scope are generated this way... I wonder why they would add a local DC/DC somewhere, for what benefit, other than creating noise which is not really welcome on a delicate instrument like an oscilloscope... could you give me the link to the schematics you found ? Will have a look at it, doens't seme to match the one I have. You did get the 2232 right ? You have me intrigued...BTW my service manual weights only 23MB not 60+ like yours... maybe you have more stuff than I do...Anyway, time to go to bed, give me the link to your manual and I will compare with mine... which if I remember I downloaded straight from Tektronix web site.

author: tautech
author_rank: Super Contributor
timestamp: June 05, 2017, 11:29:20 pm
content: Quote from: Vince on June 05, 2017, 11:16:09 pmhttp://bama.edebris.com/manuals/tek/2232/P276 as reported by Adobe reader.

author: Vince
author_rank: Super Contributor
timestamp: June 06, 2017, 02:01:32 am
content: OK I figured it out ! 3AM here, and must go to work... not until 13h00 luckily, but still...Could resist the temptation of having a quick look at the scope ! LOLIn the end... I look like a complete idiot, I owe you a big apology ! Thank you for your patience... I somehow, in some perverse way, like these moments where I look like a complete fool... make me more humble (and knowledgeable) when comes the next problem ! LOLSooo... took it apart, was easy, the mechanical/system engineering is excellent in this thing, thank you Tektronix.First thing, once I got the cover off, I noticed a damaged capacitor on the main/logic board. Clearly the cap didn't quite clear the cover and it rubbed against it. You can see makrs on the top of the cap, and also it's partially lifted from the PCB. Looks ugly, but it did not look like it created any damage in the area... though obviously I will replace it any way. No, the big "bang" I heard had to come from the hot side of the power supply, not some random short down the line, from which the PSU would be able to protect itself in a more silent way I assume.So I removed the cover from the PSU section and performed a visual inspection..... not big transformer... the main transformer is quite small and for all the world looks very SMPS-ish to me ! Hmmm..... how comes. Then looked at the schematics and.... I was wrong as you said... sure, this transformer with all the seconddary windings, rectifier and filter caps, looks like the typical old school linear PSU.. but.... hey wait a minute, nowhere does the schematics say that the primary side is fed by the mains voltage ! So, looked some more at the scope, hey presto, found a suspicious plastic covers hiding something screwed on the rear panel, above the PSU main PCB... hmmm.. removed the cover, hey presto ! A littel board with a couple SMPS-ish transformers, fed straight from the mains ! OK then... that must be a pre-regulator stepping down the voltage going to the main transformer. So I was halkf right/half wrong... it's not quite a full blown SMPS, but a hybrid... evolved from the ancient linear design, but improved with that pre-regulator, so they can decrease the size of the main transformer. OK....Only thing is... I could not see any integrated circuit on that regulator board, humm... there must be one somewhere though ! Then decided to removed the cooling fan to get a better view of the board in a dark corner, and hey there it was, hidden right below the cooling fan ! As soo as I read the part number on it " TL 594", I knew I had my friend ! ... Just 20 years ago at school, I remember vividly that we did experiments with that very chip, when we were taught the working principles of SMPS... well maybe not that exact part number, IIRC it was more like TL4xx something, not TL 5xx, but whatever.So.... now that the design part is figured out, let's see what's wrong with it !At this point I should now say that as I "worked" the scope to removed the cover (was quite stiff), I heard something moving around in the scope... like a screw or something ?! As soon as the cover was off, I agitated the scope until something came out of it.... and something did, a screw, and not a small one at that ! The kind that secures the back panel ! ... but none were missing, so no idea where that screw comes from... it's entirely possible it ahd been there from day one, when I bought it I mean, which not that long ago, and I have not used the scope much since then, so why not...So, at least we know now that the problem is now gone away, and we jsut need to figure out what damage it caused ! :-/The big filter caps all looked good. Here is the damage I found, cooked stuff :- burnt 3 pin connector on a big device/package that's bolted onto the heat sink... maybe the power transistor used by the pre-regulator. The package looks hardly off-the-shelf, so I guess there will be no hope sourcing it. hopefully I can come up with a modern replacement. Probably won't be the same package, but there is some space in there, so it should be able accommodate pretty much any shape and size of package.- just below this tranny, on the PCB, squeezed between the heat sink and the terminal of the main transformer, I noticed a 1W or so resistor that's cooked as well- Then under the cooling fan, the TL 594 chip is cooked as well, and so is a little passive next to it, as well as little T092 tranny, and another one not too far, although not visibly cooked, is destroyed ! It's package is broken/chipped, hmmm...The passives will be easy to replace/source I imagine, so will be the TL 594 chip, but replacing the power transistor (or whatever it is), will be less straightforward I guess.... :-/ Will look into it more closely when I have some more time.... tomorrow...

author: Vince
author_rank: Super Contributor
timestamp: June 06, 2017, 02:35:27 am
content: OK, progressing... found the schematic for the pre-régulator... page 283 in my PDF ! Go figure.Attached below. Sure helps ! :-)I can see the power tranny... a MOSFET by the looks of it, Q9070 is its name. Scrolled back 50 pages to search for it in the part list... thank god it gives all the required details ! :-)It says it's a " MTP 6N60E ", an N-Channel MOSFET, 600V, 6A, 1.2ohms, must refer to Rdson I assume.Found the datasheet, attached below. Says it's obsolete... how surprising. Well at least it was an off-the-shelf component, so we have the full specs to help find a replacement...Oh, Googled it, led me to this nice document from ST Microelectronics no less, listing replacement for many old trannies :http://noel.feld.cvut.cz/hw/st/3650.pdfSays a replacement could be an " STB 6NA600 ".... sadly can't find it on Farnell's website, the only big supplier we have available here in France (as far as I know anyway)... the other big names require importing from the US, prohibitive...Well at least we have the detailed specs for this tranny, so will do a parametric search tomorrow... really must go to bed now, 4h30 here, yikes !

author: Vince
author_rank: Super Contributor
timestamp: June 06, 2017, 02:48:51 am
content: Woops goofed it when looking up the table.. replacement is STP 6NA60 not STB 6NA600. Well, doesn't make much difference as it's also unavailable it appears..

author: tautech
author_rank: Super Contributor
timestamp: June 06, 2017, 03:39:15 am
content: You've got some studying to do, don't hurry this repair.QuoteThis one ?Series wound mains input filters AKA common mode chokes.Spend some real time reading up on the PSU section and principle of operation.The Tek Troubleshooting doc has a section on SMPS where they call it the high efficiency supply.

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2017, 03:46:13 am
content: The shorted power transistor means doing a full rebuild of the power supply including replacement of the controller, drive circuit, aluminum electrolytic capacitors, and switching diode.The MOSFET is not all that special. Look for a 600 volt part with roughly the same current rating and resistance as the MTP6N60E. Tektronix used a lot of different MOSFETs in this design over the years.

author: tautech
author_rank: Super Contributor
timestamp: June 06, 2017, 03:53:10 am
content: David, would you seriously consider a full recap on this scope too ?

author: oldway
author_rank: Super Contributor
timestamp: June 06, 2017, 12:44:29 pm
content: Everyone has his own method of repair...2236 = 2235 + multimeter/counter board2232 = 2235 + digital board.(more or less)First, I don't worry about the preregulator and feed the 42V rail with my HP6296A power supply (current limited at 2A)...(without the multimeter/counter board or digital board)I check all the power supplies for voltage and ripple and all the basic control of the scope.If all this is ok, I check the correct working of the scope with the multimeter/counter or digital board in place.Check the crowbar protection by increasing voltage up to 45V....When everything is allright, then I repair the preregulator...Replace all the four diodes 1N4007, the TL594, the Mosfet and check all the other components.Use an isolating transformer with a bulb (100W) in serie...Check if TL594 is oscilating or not and gate signal of Mosfet.In my opinion, you must have a good reason to replace components....So recap is a wrong decision if capacitors are of good quality and they did not failed without overvoltage.

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2017, 02:12:37 pm
content: Quote from: tautech on June 06, 2017, 03:53:10 amI would absolutely replace all of the aluminum electrolytic capacitors. Once access is gained to the power supply, it would be a waste of effort not to.It also is not entirely clear what causes this failure and I suspect it is the aluminum electrolytic capacitors.Quote from: oldway on June 06, 2017, 12:44:29 pmThat is basically it. There are minor differences in the main board but this series of oscilloscopes are all based on the 2235. The 2213/2215 and 2225 are different series.QuoteBesides testing the inverter separately with a 42 volt DC power supply, I would also test the preregulator separately with a 42 volt dummy load in place of the inverter.QuoteI would change the aluminum electrolytic capacitors once access is gained for two reasons; they are known to wear out with age and they may have been the cause of the preregulator failure.

author: oldway
author_rank: Super Contributor
timestamp: June 06, 2017, 02:37:17 pm
content: QuoteI don't agree...If you look at the pictures, you will see that whatkilled probably the Mosfet is a bad contact of his connector and overheating of his terminals...The Mosfet goes then in short circuit and damaged the capacitor by overvoltage.

author: Vince
author_rank: Super Contributor
timestamp: June 06, 2017, 10:02:21 pm
content: HI everyone, and thank you all for your input, much appreciated ! Looks like we have some people well versed in theses old Tek scopes, I should be covered then !Yes last night was a bit of a rush... was trying to get as much done as possible befonre going to bed. Not gonna repeat that tonight... need some sleep eventually ! LOL...So, sitting back a little tonight, digesting the information...FET :--------I removed what I thought was a funny looking/custom Tek thing... and in the end that funny package was just a big plastic piece wrapping the FET to press it against the heat sink. The FET is indeed, phew, just a regular TO220 package. So that's a relief, there are no custom/impossible to find component involved in this repair, I should be able to fix it for cheap, and enjoy my scope for a long time to come.Part number ? Indeed as one of you said, Tek doesn't stick to the BOM religiously at all : BOM said MTP 6N60E... my scope comes with 6N55.I Googled a bit and found a replacement for the 6N60E. An IRF BC40A. Exact same specs (600V, 6A, 1.2 ohms), still available today from Farnell and only 1,5 Euros or so ! Happy I am.The plastic connector of the FET is cooked, but the FET itself looks absolutely sparkling new ! No sign of over heating anywhere, be it on the terminal or the plastic body. However... I tested it with an ohm meter and I get either 2 ohms, or a dead short, no matter what pins I check and no matter the polarity ! OK I expect a short when the drain-Source diode is forward biased, but when in reverse I would expect an open circuit since it's an enhancement type FET...As for the Gate-Drain or Gate-Source, of course I would not expect a short there either...So, the FET is definitely cooked.Thanks for the debugging tips, will proceed. The schematics are great and supply vottage and waveforms n various places, how nice from Tek...Looks like the preregulator runs at 60kHz or so, I guess any FET should be able to cope with that...the inverter is even more lazy, 20+ kHz.... well I guess the transformer downstream couldn't cope with more than that...Schematics indicate 145V once the mains is rectified and fed to the FET... so that must be for the 110V mains in the USA. On my side of the pond it's 220/240, so I expect 300V+ or so.I assume the preregulator is designed for European market so as to still give the 40+Volts anyway, not 80...OK so now that hardest part is to come : get access to th board, so I check all the components, and e able to remove and replace all the cooked components.The heatsink is right in the way : most of the components involved in the accident, are right at its feet so to speak. It being very tall, makes it a nightmare. So I have not much choice but remove it :-/ The small three terminal regulator hooked onto it will have to hang in the air... should alright just for the time of the repair... Fan mountng hardware, EMI board and mains socket all get in the way too, need to move them out of the way.Not sure if the analog board, can come off the chassis without taking the whole scope to bits... crossing fingers. Will check the service manual for info on that...So, first get a good access to the part of the board that's hosting the PSU, then I can start working on it...Oh, as for isolation, I do happen to have an isolation transformer, ancient thing I have never used.. that will be the opportunity, great. Rated at 100VA max, hope it will be alright...Ah, need to check that bulb in series thing, not familiar with this trick...Thanks again for the input, will update the thread as I go...

author: tautech
author_rank: Super Contributor
timestamp: June 06, 2017, 10:36:05 pm
content: Be sure to take oldway's and David's advice, they know these scopes better than I.For the FET 1.2Rds ON is nothing flash these days and a better device could be selected for much lower dissipation. You could also pick a bit higher V device for a bit more headroom over your mains voltage.Just be sure to match Gate characteristics.

author: Vince
author_rank: Super Contributor
timestamp: June 07, 2017, 12:09:19 am
content: Yes I thought of doing that but refrained precisely because og what you said... matching the gate specs... not quite fluent enough in FET designs to take the risk to goof it up !So until I know better, I think it's best to stick to a direct equivalent... unless of course one of you has experience with a particular part model I could have confidence in, and just buy blindly... knowing it would work fine.Anyway, I have made some progress again. Not going to spend the night at the workbench like yesterday, but just having a little look at the service manual.I looked at the board view to locate and identify the components that got shot during the incident, so I can locate them on the schematics, hoping it would help me make sense of what happened. That was successful... it all seems to match the theory Oldway exposed earlier on. Here goes.The blown components are Q908, R907 and of course TL594.The FET is blown, it's a fact, it's short how ever you look at it. Burnt connector indicates that it's probably the cause of the incident rather than a consequence, since the "browning" is clearly the result of years of overheating, nothing to do with, not a consequence, of the blast that happened during the incident. So, the FET goes bad, becomes a short... it dumps the rectified mains voltage to the 42 V rail...- R902, it's 0.2 ohm shunt/current sensing resistor that lies between the FET and the middle tap of the transformer's primary winding. So, R902 blows, acting like a fuse which hopefully was fast enough to protect the transformer's primary... will have to check the windings to make sure of that though !- Q908, the little TO92 transistor that drive the gate of the FET. It's collector is connected to the 42V rail, so here again that would explain why a big chunk of it's package has vanished into thin air....- TL594 : connected to the poor R904 resistor obvioulsy, as it uses it to sense the current the FET is outputting. Sense pin of the TL594 is Pin #16... oh, surprise surprise, that precisely this area of the chip that got cooked... and looking more closely at the chip, I can see that pin 16 is... missing ! just blown away ! LOLSo, this confirms that....Two other components, Q928 and R913, next to the TL594, look cooked too. However, looking at the schematics, they don't seem related to each other nor the sens pin of the chip. So most likely, that didn't get blast, but only got covered by the black smoke emanating from the TL594, simply because they happened to be there.. bad luck. So chances are they are perfectly fine... just need a good cleaning ! LOL Will check anyway of course.... don't want to take the risk.It seems to hold water to me... everything checks out...Still, I am not too sure exactly how that FET's connector would cause the FET to eventually go bang... please explain, I want to know...What kills me though, is that I only need a few bucks worth of components, and believe it or not, say maybe just an hour before the scope went bang, I placed an order on Farnell, for a 10 Euros LCD display, then scratching my head to figure out how to spend the remaining 20 Euros to get free shipping, typical !

author: oldway
author_rank: Super Contributor
timestamp: June 07, 2017, 08:10:21 am
content: QuoteThat's right.Normally, rms current is under 2A and there are not much reasons to overheating the connector.But I have found already 2 times a far much higher rms current in 2235 oscilloscope:Why can this happen ?Pre-regulator works as a buck converter.T906, even if it has several coils, is in fact a dc inductor with gap.If inductance of this inductor is lower than required, then working of the buck converter become fully wrong, with high peak current and high rms valueIn one case, inductance was very low because ferrite core of T906 was broken, in the other case, there was an internal short between layers or turns of one of the coils.With such a problem, the oscilloscope works, but the Mosfet overheated and sometimes, you can hear a high pitch sound varying with brightness control.The best way to find this problem is to check the current waveform on shunt resistor R907Be careful, use an isolation transformer to feed the pre-regulator.For Mosfet, don't worry, almost every single mosfet 6A 600V works well.I like to use a TO220 isolated fullpack IRFIBC40G which is only 3.5A but never failed after replacement.I also advise to install a fan in the 2235 because power supply run to hot without fan.

author: Vince
author_rank: Super Contributor
timestamp: June 07, 2017, 08:10:43 pm
content: Hi there,Oldway, thanks for all the good info on that inductor/transformer !Looks like it's important that I take the time to check its good health !Did you make note of the resistance (or better yer, actual inductance if you hae an RLC meter ?) of its various coil, so I can check on mine ?Failing that, I can at least check that the core is not broken/cracked, and that the windings aren't open-circuit or shorted, then check then current doesn't exceed 2A as you mentioned.So, that's what I am planing to do in the next few days :Getting full/proper access to the board looks like a nightmare, might take time to get to it. However, in the meantime, since we now have pretty much established that the damage occurred up stream in the preregulator, I could at least bypass it and feed the inverter with my bench power supply 2x30V 3A, putting them in series should do it. This way we will see if the scope can power up like this, and the bench PSU will tell me how much current it draws, so I canThat should be easy to test, because the shunt resistor is apparently the only link/path between the FET/preregulator and the inverter/downstream. So, since the resistor is shot, I have to remove it anyway, then I can just solder a wire on the correct pad, and connect the bench PSU there... and see what happens ! ...Yeah, will try and do that this week, maybe not tonight, don't want to rush this...

author: tautech
author_rank: Super Contributor
timestamp: June 07, 2017, 08:16:42 pm
content: Quote from: Vince on June 07, 2017, 08:10:43 pmWise to do this in case something has been damaged from the PSU failure.......it was suggested earlier.

author: Vince
author_rank: Super Contributor
timestamp: June 07, 2017, 09:36:42 pm
content: Well yes, it was implied that I was doing this based on the previous advice from you guys, never claimed it was my original idea !

author: Vince
author_rank: Super Contributor
timestamp: June 08, 2017, 01:09:21 am
content: Quote from: oldway on June 06, 2017, 12:44:29 pmHi Oldway,Sorry for taking so long to react to your remark... but I didn't know what the expression "crow bar" meant... though I assumed you were talking of some clamping circuit. Looked it up on Google... I learned something nexw again, great. Not clamping then, more like shorting. I assume you are talking about the thyristor Q935, but the zener diode that accompanies it and drives the gate, is rated at 51V not 45.. am I missing something ? Does the SCR really start conducting as soon as 45V, the 51V zener is only there to clamp the gate voltage to a safe level ? Bottom line is, I guess... should I be worried if the SCR turns on at 51V instead of 45V... would the few extra volts cause damage downstream ? I guess not...I tried checking the datasheet of the SCR to have a loot at the gate characteristics, but the parts list specifies a "SCR2217" and boy, Google just can not find any datasheet for this thing ! Nor much of a web page of any kind for that matter, this thing is like a ghost ! LOL Hmmm....Anyway, the good news is that if this crow bar did its job, then the entire invert circuit, and the entire scope down stream... were protected during the blast :-)So, indeed everything suggests that only the pre-regulator suffered.I guess I would still better check that the SCR itself survived the short... might want to make sure it isn't shorted, before I apply power.QuoteSorry for being a little slow here... first time repairing this stuff .... I see 5 diodes on the schematics, not 4, and none of them are 1N4007. I assume you are talking about the 2 diodes around the FET (CR908 and CR907) and the two across on the secondary winding of the inverter output transformer ?Obviously these are "fast" diodes, as stated in the parts list. For the inverter, which runs at 20kHz, thet specified MB2501 (200ns recovery time), and for the two in the pre-regulator, which runs 3 times faster at 60kHz, they specified even faster diodes, a 50ns one (MUR 460RL) and an even faster one (4ns), a FDH 9427 mated to the little bipolar tranny that drives the gate of the FET.But the 1N4007 is a simple general purpose rectifier, it's datasheet is very short, not much specs given, and they don't even state what the recovery time is, probably because it's horrendous.The fifth diode I see is related to the crow bar, I guess when you said "4" diodes, you were implicitly not considering that one, as it doesn't play a role in the operation of the inverter nor the preregulator...So... I assume I must have misunderstood you ? I highligted the 5 diodes on the schematics I have.QuoteLooked that up too... these dim bulb tester seem like quite useful tool indeed ! Simple, cheap, yet effective.... what's not to love.Will build a nice one when I get some time, but to get me going on this repair I will just make a quick hack, no proper box nor switch nor anything, just wire a bulb and that will do it. Finding a 100W bulb will probe tough... these things have been made illegal recently, you can't buy them anymore. 100W old fashioned bulbs are now replaced by 60W halogen bulbs, to save energy. I guess I can a 60W one and a 40W one in parallel...

author: tautech
author_rank: Super Contributor
timestamp: June 08, 2017, 01:18:51 am
content: oldway may have a valid point that the mains bridge rectifier may have been damaged when the SMPS blew, maybe that's what he meant.

author: Vince
author_rank: Super Contributor
timestamp: June 08, 2017, 01:46:01 am
content: Oh OK, yes that would make sense.... 4 big 1N4007 would indeed make sense to replace the bridge rectifier ! :-)Will make sure to check the bridge....

author: oldway
author_rank: Super Contributor
timestamp: June 09, 2017, 10:16:09 am
content: QuoteNo, because it is a very rare issue.As pre-regulator works even with bad T906, it is easier to look at the current waveform than to desolder T906 and to measure its inductance.This failure can't be spotted by measuring resistance, only by measuring inductance.QuoteThis current of 2A is the current of the 42V dc external power supply, nothing to do with pre-regulator, nor with T906.QuoteThe goal is to check if the crowbar protection is not actuating at to low voltage because shorting the 42V rail (as crowbar protection doe it !) may damage the pre-regulator.The goal is not to check if the crowbar protection does actuate or not...This is not necessary, the circuit is very simple, checking the components for failure is enough.QuoteI don't understand what's your doubts about it ? The four diodes are CR901, CR902, CR903 and CR904....I am looking at the repair manual of a 2236, but I guess the layout of the main board of the 2232 is the same.QuoteCan not be sold in shops no more in Europe, but you find these 100W bulbs still in used and junk itens market.

author: Vince
author_rank: Super Contributor
timestamp: June 09, 2017, 06:26:57 pm
content: Quote from: oldwayThanks, I get your point nowQuote from: oldwayOK, so indeed you were talking about the mains rectifier. Soory if I didn't understand you the first time, but the 2232 schematics are different than the 2236 apparently : on my 2232 they use an integrated/all-in -one bridge rectifier ("CR901"), it does not use discrete diodes, so there are no CR902, no CR903 and no CR904, and not a single 1N4007. That was why I was a little confused ;-)So I can use discrete diodes, won't fit in the pad holes I guess. I will check the bridge and if faulty will hopefully find a replacement with a compatible foot print so it can fit in the existing holes in the PCB...Or do you advise on replacing the bridge anyway, just to be on the safe side ?QuoteCan not be sold in shops no more in Europe, but you find these 100W bulbs still in used and junk itens market. Yeah it should be possible to find some... actually I may well have some at home... when I moved to that old house 3 years ago, I removed the old light bulbs to put fluorescent bulbs... I might have kept the old bulbs as a backup, buried somewhere...Anyway, week-end time ! At last I will have some time to work on that PSU... stay tuned

author: oldway
author_rank: Super Contributor
timestamp: June 09, 2017, 07:51:40 pm
content: I did not know that there is another layout of the main board for the 2232.I have the original printed service manual of 2236, that's the reason why I looked at this manual, not my 2232 pdf copy.Anyway, I had a lot of trouble with these little bridge rectifiers in Hameg oscilloscopes.The most common failure is an open diode....I think Dave encountered the same problem when repairing an HP1740A oscilloscope.

author: David Hess
author_rank: Super Contributor
timestamp: June 09, 2017, 10:16:21 pm
content: Quote from: oldway on June 09, 2017, 07:51:40 pmThe 2235 and 2236 were replaced with the 2235A and 2236A starting in 1990 which is when the 2232 came out. There are minor differences between the non-A and A designs and the 2232 may be closer to the A designs. There also may have been a revision of the 2232 without a model number change which I suspect happened in 1992. You can find two different 2232 service manuals online but I have not compared them in detail yet. Also, the 2221A (single timebase 2232) replaced the 2221 in 1991.I get the feeling that Tektronix consolidated the 22xx designs in 1990 but I have incomplete information on the A designs and late 2232 if it existed so it is not real clear to me.QuoteDiodes, rectifiers, and bridge rectifiers are one of those things that I preemptively replace if they were possibly damaged. They do not cost very much.

author: Vince
author_rank: Super Contributor
timestamp: June 10, 2017, 06:50:30 pm
content: Hi chaps,Back again, spent some time at the workbench... so go grab a cup of tea and some biscuits to go with it... as you have figured by now, I always struggle to keep things short, no matter how hard I try.Point noted for the bridge rectifier, rectifiers at large... and bridges in old Hamegs too, because I happen to have one...which got to get some use today BTW, to assist in the repair of the Tek.In short : as planned, I bypassed th regulator and powered the inverter from my bench power supply. The scope is back to life, tested it without the digital board and as a 100% analog scope, it works fine... or almost, see further down.But before I order the parts to fix the pre-regulator and reassemble the scope, there are still 3 points that concern me, I could definitely do with your input on the subject.For clarity, let's unfold things in the order :First, before I took the risk of powering the inverter and the entire scope with it, I :- made some space to get access to the shunt resistor and SCR : removed the tall heat sink as well as the cooling fan mounting piece.- cleaned the mess/carbon deposits from the blast, to better/more accurately assess the actual extent of the damage. Some IPA with a ESD safe brush, helped. As I thought, the little T092 tranny, and resistor that looked carbonized, next to the TL594 chip that blew, were fine : they were simply covered with smoke because they so happened to live there, but cleaned up just fine and look sparkling new again. Obviously, I will check those two components electrically anyway, just in case...- removed the plastic plate on the bottom side of the PCB, to get access to the solder side. Found some obvious signs of rework in there, old flux that did not get cleaned. But I guess that's to be expected in any old gear...- removed the shunt resistor that blew, so I can check check for damage on the PCB underneath the resistor, because it looked like it took quite a hit !Not sure it shows well in the pictures, but the PCB survived. The fat track of the 42V rail probably got scared to death, but it is intact, and the PCB got burnt enough for the fiberglass weaving to be exposed, but other than looking scary, there is no actual show stopper.- Electrolytic caps : I checked the big 1000uF one that filters the 42V rail, because obviously if a cap had a good excuse to blow, this was the one. No visual sign of damage, so just measured it : no short that's a good start, and it's value is still well within spec, no worries there. So at the least I could assume there was a good chance that it did survive the blast, ie the crow bar did its job, along with the shunt resistor acting as a fuse and protecting it too.- SCR/Thyristor: removed it from the board so I can check it properly/out of the circuit, and also so I could manage to read its part number (impossible other wise, as it's stuffed right into a corner of the chassis, facing towards the metal work.... ) because the parts list says it's an SCR2117 or something, but Google can't find any data sheet for this. Actual markings on the beast are " MTR 72 ". At least this one is known, but obsolete "obviously".. so I will have to figure out a replacement for that as well. Shouldn't be too difficult though, I guess...Anyway, the forced and unfortunate absence of the SCR on board, would not keep me from powering up the scope, since it is powered by the bench power supply therefore safe/current and voltage limited.- mains bridge rectifier : checked all of its internal diodes, all good, and a consistent 0.45V or so forward drop which is fine by me.Second, I set the bench power supply exactly to 42.8V as indicated in the annotated schematics, and set the current limit to 2A as you said.Then I crossed fingers that nothing would blow again.... victory !The scope powered up, it was not shorting the power supply. current draw was 750mA then after warming up settled at 800mA. Scope was powered like this for many hours while I was testing it, and no incident to report.So, followed you advice : tested the thing to make sure all was well, before rebuilding the pre-regulator. This is were I found 3 problems which could do with your input... Other the these, the scope, in pur analog form for now, works just fine.1) Odd : the probe compensation signal goes NEGATIVE ?! Frequency, amplitude and signat shape are spot on, but instead of going from 0 to +0.5V, it goes from 0 to MINUS 0.5V ... go figure.2) The time base is off, and by a long shot ! Error is measured at about 4.8 to 1 . At first I though well that's all right, it's just the big know that got loose again and missed a couple steps .. but not, not that simple : knob is still firmly attached/screwed, turned it all the way from lock to lock, the dial goes where it's supposed to go... no mismatch. Plus, had the knob been out of adjustment, it would imply an error fitting the usual 1-2-5 sequence.. but here I witness 4.8, too far off "5" anyway.2 bis) The little "CAL" knob of the time base, woks only partly : The "s10" magnification works. however if I turn the knob CCW to go out of cal on purpose, it has absolutely zero effect on the trace on the screen. Yes, yes... when I say the time base off, it's obviously when the little CAL know is set fully CW, in the calibrated position...The CAL feature has a range of 10 : 1 isn't it. So it not working, and the fact that the error I get fall inside that range... it's reasonable to assume that the time base subsystem is fine and that only the var cal feature is misbehaving, it is somehow stuck solid on 4.8 : 1 ...Had a quick look at it. That knob has its own dedicated switch assembly, which is also mounted on a small dedicated PCB. So that's a starting point... There are only 2 cables going to this little board : one which goes to the digital board, which obviously is disconnected at the moment, and another smaller, 3 way connector which goes to the main/analog board, but this is one firmly secured in place. So I fear it's not as simple as a disconnected connector... and probing in that area doesn't look like fun, access looks shitty at best...3) Power rails : they all look fine except for the 5V rails which has me very concerned ! Exhaustive list of my measurements follows. Measurements taken with a fixed x10 probe sorry (not ideal to measure tiny ripple...), as my slower x1/x10 probe is out of order... need to order new ones... So, x10, and measured on the Hameg scope whose lowest setting is 5mV/DIV not 2mV like the Tek. So that gives us 50mV per division. Still good enough to get a ballpark figure though....+ 30V (CRT) : 30.3 V. Ripple : 40mVpp, 50 tops. No random garbage, but rather a perfectly clean wave form fro the inverter. The trace as jumping around a little bit but I the period was about 45us, ie 22.2kHz... and the waveforms the service manual provides for the inverter, indicate around 21.8kHz, so not bad.+100V (CRT) : 102.8 V. Ripple : same as above (clean, inverter residue, same amplitude, and frequency as well obviously), except that somehow the waveform/pattern is different. See below for pics of each of these two waveforms.+8.6V : +8.75V. Ripple: even at 5mV/DIV, it's as flat as it gets, no discernible ripple whatsoever.- 8.6V : -8.7V. Ripple : none visible.-5V : -5.8VRipple: none visible+5V: +5.46VRipple: none visible.Not 100% sure why the 30V and 100V are the only ones to display some ripple, whereas the other rails are really really flat. Probably because the 30/100 rails have fairly small filter caps, only 33uF and 270uF, whereas the other rails have much large 1000uF caps, as well as LC filtering.But obviously my big problem is with this +5V rail ! +5.46V, what the ?!At first I though nothing of it, thinking well who cares, there will be regulators down stream... then realized that 0.46V isn't much headroom for a regulator, even an LDO type which probably wasn't really widespread back then anyway. So, I started to get scared.... could the digital chips be getting 5.46 instead of 5.0V ?!So, checked the "Power Distribution" schematic page, and that got me real scared indeed : I do spot a couple 78/7905 regulators, but this is only local to the "Timing" board. What the schematics shows is that indeed, the is a bunch of TTL logic chips which powered straight from that 5.46 rail ! NOOOO !OK TTL is specified at 5.0 +/- 5%, but that only gets us to 5.25V, we are way above what is already the maximum tolerated ! :-/I thought... OK maybe the schematics says 74 "LS", but maybe it's just a generic way of saying things... maybe the actual chips on the board, are some other family which can cope with a wider range of supply voltages. But no... checked on the board, the chips are indeed bog standard 74 "LS", so 5,25V max it is !Then I thought... OK this makes no sense, you must chasing a red herring again.... so just to be sure, I measured the voltage directly directly on the two power pins of each and every one of these TTL chips... and sure as hell they all get 5.46V :-(I didn't give up : could still be chasing a red herring, maybe the multimeter is somehow off ? Unlikely especially since I checked it when I bought it, and it reads fine on the other rails.... but tried a different meter anyway just in case... no joy, still 5.46V.I don't know how the scope can still work "fine" (from a user point of view at least, bar my time base issue...), with all the TTL chips being pushed/stressed that much... but the point is, regardless of the fact that they happen to somehow function at that voltage, it can't possibly be DESIGNED to be 5.46... So now the question is... what is wrong... :-/From the top of my head :- partially shorted 5V secondary windings (internally/wire insulation problem) : I guess that would make for a voltage lower than expected, not higher... so unlikelyThen we could go upstream (primary winding, then maybe a problem with the inverter).. but that would affect all the rails... and they are all perfectly fine, only the 5V ones are way out.Oh, just an idea : maybe the blast DID do some damage to some track after all, and maybe the voltage from one of the other rails, of higher voltage, is "leaking" its way to the 5V rails a bit ? No, stupid... the damage happened way upstream in the regulator, when the various rails don't even exist... so really, I am clueless !So.... I welcome your input on the +5.46 V rail, and the preceding two problems as well !Now picture time.

author: Vince
author_rank: Super Contributor
timestamp: June 10, 2017, 06:52:34 pm
content: ...

author: oldway
author_rank: Super Contributor
timestamp: June 10, 2017, 07:50:28 pm
content: Nothing to became so nervous....Only reduce the voltage of your external power supply to adjust -8.6V to his right value.When the preregulator will be working again, you should read the adjustment procedure....You will see that there is an adjust of a trimpot to match the -8.6V rail.Without digital board connected, a lot of functions are not working correctly....you don't have to worry about this.....The goal was to test the inverter and HV generator, they are working, that's allright.After ajusting -8.6V, next step is to connect the digital board and test all the analogic and digital functions of the scope.

author: Vince
author_rank: Super Contributor
timestamp: June 10, 2017, 10:14:19 pm
content: Hmmm.... thanks for that, feeling much better already !I will proceed with the fixing of the regulator then, the time to figure out replacement for the SCR and a couple small T092 trannies as well.If someone has already replaced the SCR and can suggest a suitable replacement, I am all ears...

author: tautech
author_rank: Super Contributor
timestamp: June 10, 2017, 10:18:43 pm
content: Quote from: Vince on June 10, 2017, 10:14:19 pmHidden at the bottom of this page istheTek Xref file.http://www.sphere.bc.ca/test/tekequiv.html(27 Mb)As Sphere asks, do not link it.

author: oldway
author_rank: Super Contributor
timestamp: June 11, 2017, 05:41:31 am
content: Why do you want to replace the SCR ? Did it failed shorted ? If not, you don't need to replace it.If you need to replace it, something like TIC126 will do the job.

author: Vince
author_rank: Super Contributor
timestamp: June 11, 2017, 10:32:52 am
content: Quote from: oldwayOh no... you mean that in my mile long message, I somehow did not find the space to mention this point ? Indeed I forgot ! Sure meant to, I would have swore I wrote about it... I am getting old or what ?!Anyway... I must replace it because although it checked out just fine from an electrical standpoint, it did not from a ... mechanical perspective ! The little bugger took the concept of metal fatigue, at tad too seriously ! Even though I babied it during removal, the middle pin broke, then another pin broken when I tried to put it back on the board. These copper terminals just aren't that strong...Technically I guess I could away by soldering bit of wires to connect the broken legs to the pads on the PCB, it would probably work just fine...in case it ever needs to, that is, hopefully never again ! LOLBut I was trying to do a decent repair, hence my wish to replace it.Quote from: oldwayThanks.@Tautech : thanks for the file, I downloaded it and saved it, a 343 pages mine gold for sure. However it's not very helpful in the case at hand, since it does not give replacements/substitutes, which is what I am looking for. Plus, as luck would have it, out of the 3 or 4 active components I need to figure out, none of them are even listed in that file, somehow ?! I am cursed...Anyway, this file does not give more information than the parts list in the service manual. I assume it would be useful to someone with a discrete Tek part in his hand, to help him figure out what it is. But if the person actually has the actual test gear that this part comes from, he can just look it up in the appropriate service manual.. unless he is unlucky and the manual for his particular gear, isn't available.....

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2017, 10:41:50 am
content: Unfortunately the available Tektronix parts catalog is less useful for the 2232 because many of the newer parts including the SCR are not listed.Look for an 8 amp or greater sensitive gate (200 microamp) SCR. That limits you to modern parts like the MCR72 (8 amps), MCR310 (10 amps), and MCR8 (8 amps).http://www.mouser.com/Semiconductors/Discrete-Semiconductors/Thyristors/SCRs/_/N-ax244Zscv7?P=1yx67ruZ1yx67qrZ1z0z63x&Rl=ax244Zgjdhl8Z1yx67sbZ1yx67nvSGT&Ns=Pricing%7c0This excludes the TIC126 because it has a much higher gate trigger current although I suspect it would still work but perhaps not as reliably.

author: Vince
author_rank: Super Contributor
timestamp: June 11, 2017, 11:42:46 am
content: Quote from: David HessThanks for that. The service manual does specify a "sensitive" gate, but not knowing about SCR, I couldn't translate that in actual values that I could look for...Quote from: David HessMCR72 ?! Oh no, stupid me ! What did I say the old part was, MTR72 ? Just looked at it again, it appears I somehow misread that (that's what happens when you work on the thing at 4AM... ), it does read MCR72 in fact, so it's still available, yahoo !!!But that's suspicious... a modern part in that old scope ? So I guess this means someone already replaced it in the past ! As I said yesterday, I did notice signs of rework in the PSU section of the board, but didn't notice anything on the SCR itself.Whatever, I can buy it then, problem solvedAt the very least,The shunt resistor is nothing special, just metal film 1W 0.2 ohms. Then I need a couple of the T0-92 trannies that accompany the TL594. The one driving the gate of the FET literally exploded ( a chunk of its package flew off...), and another looks fine from the outside, but the collector and emitter appear to be shorted... unless it is the surrounding circuitry that is fooling me. Might remove it from the board to make sure...QuoteI won't take chances then.. I will just put a new MCR72 since it is available, does have a sensitive gate, and because well... we now know for a fact that it can do the job, not only did it protect my scope, but it also survived. So, I will just whack a new one of those in thereGreat, I am getting there....About the 5V rail climbing mount everest at 5.46V, I think oldway was right (of course ;-). The adjustment procedure won't solve the "problem", since it just fine tunes the drive of the primary of the transformer, therefore moving all the rails in unison. Since they are already pretty much spot on (only need shave 40mV off of the reference -8,6V rail, that's half a percent), that will only shave 20mV or so from the excess 460mV, hardly enough. However as Oldway said... once you put the digital board back in the game, it will bring things back to normal. After all, there are so many digital chips crammed in that board that it's a reasonable assumption ! It certainly would make sense, and I fail to think of another theory that would make sense of all this. So... here I am, relieved !I will search for substitutes for the smaller trannies, hopefully order all the parts today and, well... hopefully I will get the parts sometime next week and I will able to repair the scope next week-end, can't wait !

author: oldway
author_rank: Super Contributor
timestamp: June 11, 2017, 08:25:06 pm
content: Reliability of the crowbar circuit is higher using a SCR like TIC126 than using a high sensitivity SCR.Why ?- Actuating is the same: when 42V rail reach 51V + 0.8V (gate/cathode voltage), VR935 became conducting and there is no resistance to limit the current flowing in the gate of SCR, so there is no difference of actuation between a sensitive gate thyristor and a non sensitive one.- spurious actuations....what can't occur is a spurious actuation because erratic shorting the 42V rail may damage the pre-regulator....I remember you that the current limitation of a TL594 is slow and not able to eficiently protect the Mosfet....at this point of vew, regulator's IC controlling current pulse by pulse are far more secure. (like UC384xA family)So a non sensitive SCR is far more reliable than a sensitive one because it is less prone to erratic actuations.If you repair old stuffs, you must be clever and understand what you do because these old stuff are full of errors and bad projects.For exemple, I am repairing an amplifier Sansui AU-999 and I had to make some modifications because the original schematic has some bullshit errors.

author: Vince
author_rank: Super Contributor
timestamp: June 11, 2017, 09:58:41 pm
content: Hi Oldway,Thanks for sharing your thoughts.Honestly,as everyone has gathered by now... I have next to zero experience at fixing stuff. I started collecting old stuff only 6 months ago, so I can build myself a decent lab, after 20 years of dreaming that one day I would have a decent lab at last. So far my 2232 is the only gear I have had that required to dig fairly deeply into it ! I did repair my ancient 5000 series rack mount Tek scope (below), but it did not require any serious troubleshooting/thinking... all that was wrong with it were the dozen trimmers on the CRT boards, that literally were falling apart ! Replaced all of these, and now work like a charm. Finding the parts was not as straight forward as I thought so : these trimmers looked nothing unusal, but in fact they were, because of their pin pattern mostly. Had no choice but order them from the US from Mouser, and pay an arm and and a leg (and kidney too !) for shipping + duty taxes etc.... but I wanted to repair it with the proper parts ;-)Anyway, just mean to say that I am clearly not qualified to argue or debate with you or David on this subject, I clearly know shit on the subject, and have next to zero experience !So when I repair stuff, I am forced to make the assumption that the engineers that designed whatever gear I am working on, knew much better than I do, hence it's safer/reasonable to just replicate what they did, in this case, use a sensitive gate. But obviously I also know that no matter how expensive/professional a given piece of gear, is.... the engineers behind them are just mere mortals and human beings like us... and humans are never perfect are they...I do suck every bit of information/thoughts you guys deliver to me, though, it does not go to waste. Whatever the final decision on this part will be, the various arguments/thoughts I read on here, is precious material to me.What I propose is... given that as always the price of the parts is ridiculously cheap compared to the 30 Euros minimum order at Farnell, I will just get 2 or 3 of both the MCR72 and the TIC126.... then by the time I get the parts and actually get some time to work on the thing, that gives us 'til next week-end for you and Dave to discuss the gate sensitivity subject, because well, I don't enough to argue with either of you !Oh well hold on, Mouser marks the TIC126 as obsolete, and Farnell, which I must use, does even list it at all.So if if it's better to switch to a less sensitive gate, it will have to be some other part.Using the parametric search (quite nice to use on Mouser BTW, like it...) : T0220 package, >= 8A, >=600V , we have these puppies in the "less-sensitive" gate department :5mA : TYN612MRG, TN1205H-6T15mA: S6008x20mA: MCR12

author: Vince
author_rank: Super Contributor
timestamp: June 12, 2017, 01:27:34 am
content: Hmmm.... just looked at Farnell's offering for the shunt resistor : 0.2ohm >= 1Whttp://fr.farnell.com/w/c/composants-passifs/resistances-valeur-fixe/resistances-traversantes?resistance=0.2ohm&type-de-boitier-de-resistance=axial|axial-leaded&range=inc-in-stock|exc-direct-ship&st=0.2+ohm+resistance&sort=P_PRICEThe first 4 in the list are wire-wound so my first reaction was no-no... not a good idea to upset the regulator circuitry with an unwanted inductance... let's just take a film resistor, one of those blue ones from "Ohmite", they look like like your typical metal film resistor, so no inductive behavior, that's what we want.... then looked at the datasheet to check physical dimensions to make sure it would fit on the board, and if the beefier 3W might fit and.... uh ? It says even those resistors are wire-wound too ?!So... could the shunt resistor Tek used originally actually be wire-wound ?! I tried to examine the dead body of the one that blew, but it's so completely carbonized that it's hard to tell anything from the remains...So, now I am trying to find arguments to convince myself that maybe it's actually OK to use wire wound despite it being used in an SMPS :- 0.2 ohm is tiny and maybe it's not easy to produce it using metallic film technique, so wire-wound is the only possibility ?- 0.2 ohm is not much, so it probably needs only a very few turns of wire, hence producing very little inductive behavior, not enough to upset the regulator circuitry, especially given that it runs at 60kHz which is not much.- The shunt is downstream with regards to the regulator, it's outside the feedback loop, so the regulator actually doesn't give a shit ?No hold on, OK it's after the FET but still in the feedback loop, my bad.. makes sense I guess, even with a tiny 0.2ohm value, it would still drop a non-negligible 0.4V given the 2A figure it might encounter in the machine, if I understood well. So, you can't afford to leave it outside the loop.- the service manual DOES precise "FILM" when they want to use film resistors...and they did NOT specify this when listing the shunt resistor... meaning it's irrelevant probably.Unless I missed something and did not search Farnell properly, it seems that I don't have much choice anyway...Any thoughts before I do a mistake ordering this ?!Bugger, datasheet says that non-inductive versions ARE available... but the specific part numbers Farnel offers, are unfortunately not this type...

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2017, 03:16:00 am
content: Quote from: Vince on June 11, 2017, 11:42:46 amThyristors have not changed in decades. The smaller SCRs used in the earlier 22xx oscilloscopes are also still manufactured.http://www.mouser.com/Semiconductors/Discrete-Semiconductors/Thyristors/_/N-ax23v?Keyword=C106&FS=True

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2017, 03:30:34 am
content: Quote from: oldway on June 11, 2017, 08:25:06 pmI suspect Tektronix did not use sensitive gate SCRs without reason.There is another SCR failure mode which may apply. At high currents which will occur when shorting out the preregulator's output capacitor, low gate current can result in hot spots in the gate area because turn on time is increased. Large gate currents help prevent this.http://www.electronicshub.org/scr-turn-on-methods/

author: oldway
author_rank: Super Contributor
timestamp: June 12, 2017, 08:59:10 am
content: When pre-regulator Mosfet goes in short circuit, what happened with SCR is of little concern....What's really matter is to protect the inverter and all the other power suplies.When things are going so horribly wrong, you cann't be sure the scr will survive or not.To protect a scr, you must have selectivity between the scr and the fuse he must blow to clear the fault.i²dt of the fuse must be at least 30% lower than the i²dt of the scr.In such a circuit where you not only have to blow the mains fuse but also have to dissipate all the energy , not only of the electrolytic capacitor of the 42V rail, but also of the 400V input capacitors, you cann't predict what will happen with SCR.Happily, they generaly fail going to short circuit, what is safe in this application.I think it is stupid to loose your money and time to look after an original component, when you can buy a cheap TIC 126 or TIC 116 in every electronic shop and that will change absolutely nothing in the working of the oscilloscope.

author: Vince
author_rank: Super Contributor
timestamp: June 12, 2017, 11:07:22 pm
content: Hi chaps,Thanks for your thoughts on the SCR, I do appreciate. I see there is non consensus on the matter, so since I hardly mean to ignite a diplomatic incident, I will leave it at that. Will order both types and decide at the very last minute what to whack under the hood.... might flip a coin, just kidding !Anyway, whatever happens, I will take the full responsability of course.. if the scope blows for good this time, my fault, I won't blame anybody but me.I asked for your views since I knew shit on the subject, now I have the information I asked for.. the rest is on me now...anyway, any thought on the shunt resistor ? I posted about this right after my message on the SCR... but I knew there was a risk that it might go unnoticed because the SCR subject was getting a bit hot, pun intended !Checked the TL594, that ancient beast is still available, even in DIP package, what do you know !Just need to look at replacements for the small signal trannies around the regulator, and I will be ready to order all the parts...

author: David Hess
author_rank: Super Contributor
timestamp: June 13, 2017, 12:37:26 am
content: It looks like Tektronix never changed the resistor. It is indeed an inductively wound wire wound resistor but I doubt the inductance amounts to much with such a low value. Here is the modern datasheet for the old Dale RS1A series:https://www.vishay.com/doc?30204http://www.mouser.com/ds/2/427/rsns-226497.pdfAnd the modern version of the original part is still made if you want it:http://www.mouser.com/ProductDetail/Vishay-Dale/RS01AR2000FE12/?qs=sGAEpiMZZMtTURnxoZnJAHlhF1kLzuAqSP3bVTNPViY%3dhttp://www.mouser.com/ProductDetail/Vishay-Dale/RS01AR2000FE70/?qs=sGAEpiMZZMtTURnxoZnJAHlhF1kLzuAqxV5PAsnHGbU%3dThe alternatives would be a different wire wound resistor or maybe a metal oxide resistor. A 3 watt version of the same resistor is available however since the failure was do to catastrophic overload, I doubt it is worth using. For a metal oxide resistor, I would use the largest which will easily fit which is 2 watts.

author: Vince
author_rank: Super Contributor
timestamp: June 13, 2017, 08:21:39 pm
content: Thanks for that.Would love to have Mouser here in France / Europe but sadly no... so will have to go with what Farnell offers. Mouser always have more choice than Farnell but well... Farnell is the only one that cares about Europe and has operations here so... they get my money almost every time, if they have something suitable.I think I will get the Ohmite branded one. The only one that actually does 1W and has the appropriate physical dimensions. As a bonus it's a tighter tolerance (5 times the price of the 5% Vishay though...) even though it does not really matter of course. A 3W one I give up... thinking about it, it's probably a bad idea ! The little 1W one blew quickly (faster than the main fuse which is the slow type), and is the first in line, right after the FET. So I like to have this "fuse" there, being able to react quickly and "locally". A higher power rating would only mean thicker wire inside, which could only make for a slower "fuse", not what we want he....OK. So tonight I will figure out replacements for the small signal trannies around the regulator, and try and order all the parts tonight... hoping to fix the scope this week-end ! We shall see...

author: Vince
author_rank: Super Contributor
timestamp: June 14, 2017, 08:34:34 pm
content: Hi there,Here I am again, for the next episode of this breathtaking saga...Last night spent nearly 4 hours checking each and every component of the regulator, systematically., lifting terminal when needed, or even removing parts altogether sometimes, to make I was not measuring garbage and wasting time chasing red herrings.I am glad I took the time because well... a lot of components were having issues ! So no way this scope is getting fixed this week-end, unfortunately.I attached a schematic of the regulator, highlighting what components checked OK and which ones didn't...- FET, shunt resistor and TL594, and the little tranny driving the gate of the FET, were already known.Now some more :- CR908 and R909, that go with the FET's gate driver. The 39 ohm gate resistor is open circuit, and the diode across the Base-emitter junction of the tranny, is shorted.- Q930 : which supplies power to the TL594, is shorted.- Less obvious : C917 is dodgy : I removed it completely from the board so I could grab it firmly with alligator clips, but still, it gives erratic readings, dancing all over the shop. Always near the expected value (0.5nF up to 2nF, for a 1nF face value), but completely unstable. His other friends of similar constructions, don't do this, they give perfectly stable readings. I noticed some slight browning on its package to add to that. So no reason to take chances, I am replacing.- R912 : no visible sign of damage, but reads to low, 20% too low. It's the bottom resistor of a precision (1%) voltage divider that gives the reference/threshold for the current limiting feature. 20% off for a 1% resistor, can't be justified, and I am not willing to mess with the settings of the current protection either, so putting that back to specs. In the case at hand though, I would have been on the safe side, the current limiting would have happened 20% earlier, not later. Still, I am fixing this.- and now for the real bad news.... T906, the main coil... is out of the game I am afraid ! See pics. The two halves of its casing are split, and there is a big crack too, as a result. The main coil/ "primary" winding is.. open circuitWell even it weren't, the condition of the case itself calls for replacement, obviously...All 3 secondary windings are probably good though : they are not open circuit nor shorted nor abnormally high values. They all measure at a couple ohms, which sounds plausible at least.I will carry out an autopsy of the coil just out of curiosity, and so I can learn something from that catastrophic failure... but now I need to find a new one !Nothing I can find on ebay. Just check that "sphere" site you showed me recently, they have one..... well, HAD one, it's marked as "sorry, sold" !http://www.sphere.bc.ca/test/tek-transformers.htmlOK, I will post on this forum in case someone may have one, but I am not holding my breath so.... look like my poor 2232 which I loved as my general purpose scope... won't be working for a long, long, lonnnng time unfortunately, the time to come across a coil !Well, I might still order all the other components, so I can start putting the thing together and not have a mega pile screws that I won't even remember how to put together again.. 2 years from now, once I manage to get hold of a coil...The absence of the coil should still allow me to power up the TL594, and check that it all works fine, oscillates as it should etc. Then once I get the coil I can just throw it in there, fit the new shunt resistor, and hopefully call it done...

author: wasyoungonce
author_rank: Frequent Contributor
timestamp: June 17, 2017, 12:21:55 am
content: wasn't there a guy over atYahoo Tek scopesre-winding these?

author: Vince
author_rank: Super Contributor
timestamp: June 17, 2017, 08:29:11 am
content: Is there really ? Taht would great !Not sure how active this old Yahoo group might be today, given the popularity of the EEVBLOG forum ? Will have to join to find out (can't even read past messages un til I subscribe). Unfortunately for me, I keep clicking on the "Join" button, but nothing happens at all, the web browser does not even seem to be trying to reach any server, it just does nothing at all... not goodAnyway, been thinking of rewinding the coil myself actually (nothing to lose after all), if just for the sake of seeing if I could even manage to do it. Sadty I fear there is not much point in doing so, given that the case is split and cracked... unless this outer/visible part is only there for show, doesn't not really matter, and the actual ferrite core is inside, and might still be intact...But first I must get when of these "Dremel" miniature tools, if I want to have a remote chance of opening it up without destroying everything... all I have is an angle grinder and hacksaw, not really the most precise tools for that job !Other than this, last night I ordered all the (other) parts I need to fix the regulator. Figuring out replacement for the MPS2907A trannies and FDH9427 diode was easier than I feared. Trannies were just regular 2907A which I should have guessed from part number... the PNP companion of the famous 2222 NPN. Now they make a PN2907 version with just the same specs.FDH9427 diode, marked as "super fast" in the service manual, believe it or not is still super fast by modern standards. I did a parametric search on Farnell's site and 4ns is about as fast it gets even today. The 1N4151 has spot on the same specs : 4ns, 2pF, 150mA continuous, and a tad more head room with regards to reverse voltage, 50V instead of the original spec of 40V. Can't be a bad thing.The few resistors I needed will get an upgrade.... just out parts availability, the 5% will get to be 1%, and the 1% jobbies of the precision voltage divider setting the current limit threshold, will be 0.1% for the lower resistor, and 0.5% for the upper one, from memory. Can't be a bad thing.Even the mundane 39ohm resistor driving the gate of the FET will get an upgrade from 5% to 1% again because no choice and not much cost penalty anyway. Won't make any difference whatsoever to that part of the circuit though, obviously...

author: wasyoungonce
author_rank: Frequent Contributor
timestamp: June 17, 2017, 09:33:40 am
content: Vince the Tek group is very active. You will even find guys and articles by members who worked for Tek in that era. Also a Google search for "rewinding Tektronix transformer" brings quite a few hits as well as "rewinding Tektronix pre-regulator transformer".Most are hits for HV transformers but you get many for the pre-reg txfmr. Of course this PSU is similar on all 22XX scopes. I think its the 120-1439-00 or 120-1469-01 transformer. Would be nice to find a blank bobbin and make a new one...except these are probably all gone as well.Edit:Ummm Sphere have qty 2 used 120-1439-00 transformers avail. I think these are pretty much the same???. Mine is a Tek 2236 and this part is on my parts list...120-1439-00. Even if it isn't compatible...and I'll eat mu hat if it isn't...then you can use its bobbin for a re-wind!http://www.sphere.bc.ca/test/tek-transformers.html

author: Vince
author_rank: Super Contributor
timestamp: June 17, 2017, 11:20:02 am
content: OK then, will try and get my computer to cooperate and subscribe me to this Tek Yahhoo group...I am wary of getting a different part-numbered coil from another 22xx series scope because... there must be a reason why they don't carry the same number, and unless I know what it is exactly, I can't make an informed decision... Sure it looks the same from the outside (I guess so anyway), but are the inductances exactly the same, same number or turns for all 4 windings ? Are all 4 wires the same gauge ? A bit risky/hasardous at the least.Since I don't have reference measurements for the resistance and inductance and wire gauge of all 4 windings of the 2232 coil, I can't compare to other 22xx coils in any meaning/useful way...But I will definitely join this Yahoo group then, since it's still active and there are knowledgeable people there, and since this coil issue is apaprently very common. People there might have data on the various coils and know if I can actually use a 22XYZ coil on my 2232, and maybe some people already have done it and can report...Problem as always is that buying from the US is prohibitive for me being in Europe. Probalby economical to actually wait to find a donor 2232 locally and salvage the parts I need.As you said though, if I can find a coil for a reasonable amount of money once shipped, then yes even it's not the correct part number I could still use it as a blank, then I can do the autopsy of my broken so I can replicate the exact same windings.Anyway, great to see that it's actually technically possible to rewind them (I guess the two halves of the body are just glued together then, easy to put back together ?). Will practice on my broken unit and join that Tek Yahoo group to benefit from their expertise on the subject...In the meantime I will replace all the other damaged components, and then after that, fixing or replacing the coil might take several months maybe, or more who knows, depends how lucky I get. But I thought what the hell... I can still get the scope going in the interim, by putting together a 40V power supply and strapping it to the rear panel of the scope...

author: Vince
author_rank: Super Contributor
timestamp: June 17, 2017, 12:03:38 pm
content: OK done, subscription to the Yahoo Tek group is being processed by the moderator, waiting for the green flag...

author: David Hess
author_rank: Super Contributor
timestamp: June 17, 2017, 01:18:21 pm
content: Rewinding transformers from tube era oscilloscopes is common because fewer spares are available. There are lots of 22xx parts donors available and as far as I know, the -00 and -01 transformers are interchangeable. Incidentally, the 4th winding between chassis and earth ground was a clever last minute addition for EMI suppression.QService is a good place for Tektronix parts if you are located in Europe but they do not have any of these transformers at the moment. They have a 2213A main board available but given the cost, I would look for a cheap parts donor.TekScopes@yahoogroups.com is very active and is completely accessible via email which may be preferable to the web interface. TekScopes2@groups.io made the jump away from Yahoo not long ago and is worth checking as well.The small signal transistor types are not critical. 2N4401s and 2N4403s are suitable replacements.I know of at least one person who just replaced the preregulator with a 48 volt off-line power supply module which could be adjusted to 42 volts.

author: Vince
author_rank: Super Contributor
timestamp: June 17, 2017, 06:23:05 pm
content: Thanks for the pointers Dave, will check that Qservice site regularly. They say you can e-mail them your request because not everything is listed on their site... will do !Rewinding has two advantages for me : not being in the US with plenty of Tek parts available for cheap (no duty taxes no VAT no outrageous shipping fees), rewinding is a way to get my scope fixed in a reasonable time frame, rather than waiting a couple years hoping the right par comes along in my neck of the woods. Second advantage being... I have never rewound a coil/transformer and find it cool... resurrecting a transformer, I like the idea. My broken Tek just gives me a good "excuse" to get some hands on experience on the subject, and satisfy my curiosity...Most probably what will happen is that I will eventually a coil at a decent price, within a few months, which I will fit, but whilst waiting for this to happen I will "play" with my broken coil

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: June 17, 2017, 06:38:00 pm
content: Just noticed that no one pointed you to THIS document:

author: Vince
author_rank: Super Contributor
timestamp: June 17, 2017, 06:52:43 pm
content: Thanks for that. It helped me a bit with regards to the way the PWM choice is powered. Looking at schematics, I could not make heads or tails of it. I was assuming there would be some kind of crude voltage divider of some sort to lower the voltage for the sole purpose of powering the PWM chip, but with this assumption then I can male no sense of the schematics. The ground pin of the chip is being connected to the 40V rail... and a diode feeding that back upstream humm... was starting to think of some kind of boot-strapping mechanism but hardly familiar with this kind of stuff... At least this document confirms that indeed I was on the right track. Still need to figure out/analyze exactly how it all works in practice...

author: Vince
author_rank: Super Contributor
timestamp: June 17, 2017, 07:04:57 pm
content: Dave, I am checking Qservice's site and unless there are several similarly named companies bearing the same name... then it looks like they are not operating in Europe sadly for me. Their "About us" page tells their story but doesn't even say where they are located ! However their "Contact us" page mentions a street address which is clearly in the US... Los Angeles California...Still, with Sphere and Qservice, I now have two major parts suppliers that I can check regularly, that's good... gives me hope, in the long term at least (go figure when they will have the magic part available again...).

author: David Hess
author_rank: Super Contributor
timestamp: June 17, 2017, 07:41:02 pm
content: When I ordered parts from QService, they shipped from Greece. The padded envelope they used for packaging had a very strong smell of tobacco which reminded me of my grandfather. TheirEU web pageshows a Greek address.Skevou Zervou 10Rhodes Island 85132 Greece

author: Vince
author_rank: Super Contributor
timestamp: June 17, 2017, 07:52:30 pm
content: Oh, thanks for that !Glad they operated in EU too !The site I found a minute ago was this one, probably their headquarters :http://www.qservice.tv/vpasp/shopcustcontact.aspI have already contact them, so I guess they will redirect me to their US site.Greece what a good idea... that country is collapsing completely, so probably the cheapest/most profitable place in Europe to locate their EU operation !Greece hum... make sense now : a few days ago when I searched Ebay to replace the little plastic knobs that fell apart on the 2232, the only place selling some in EU was... from Greece ! And the pictures and everything were clearly very professional, it was clear someone was doing that for a living.So that means they sell their parts on ebay, which I check for Tek stuff every single day, so hopefully when they eventually get the coil I need in stock, they will advertise it on Ebay as well, increasing my chances to catch it...Obviously it would be daunting to create Ebay ads for every single part they have for sale, but since it seems this coil is a common failure, hence many people looking for one, it's worth taking the time to advertise it as it would sell quickly I guess.

author: tautech
author_rank: Super Contributor
timestamp: June 17, 2017, 08:22:14 pm
content: @ VinceIf you do go down the transformer resurrection path this site might help with tips and techniques:http://ludens.cl/Electron/trafos/trafos.html

author: Vince
author_rank: Super Contributor
timestamp: June 17, 2017, 10:31:54 pm
content: Thanks, will have a read ! :-)Will also see on Youtube if I can find something specific to the Tek coil.. everybody uploads to youtube these days, maybe some Tek aficionado recorded his coil rewinding, who knows...I just posted on the Tek yahoo forum, the mandatory presentation. They required one, understandable of course, we are civilized being aren't we. So, a presentation they got ! Vince-style.... 9 pages long.They will probably hate me and not be willing to help me as a result ! LOL But well, a presentation they wanted, a presentation they received !

author: oldway
author_rank: Super Contributor
timestamp: June 18, 2017, 06:10:58 am
content: You will need a new pot core.It is a gapped 30x19 pot core.I don't know how much is the gap but it should be easily be calculated from the data of primary inductance and primary number of turns.

author: Vince
author_rank: Super Contributor
timestamp: June 18, 2017, 02:34:05 pm
content: Thanks for that ! So these things are actually standard parts one can use to do their own coils... how great ! Just checked Mouser, indeed these pot cores can be bought, and for cheap ! Brings a smile to my face...Gap needs to be known but alos maybe also the specs of the material used for the core ? Looking at data sheets on Mouser, it seems to be an important information ?!I couldn't help, last night I tore the coil apart ! Sorry I am so impatient. No need for a Dremel in the end.... what looked like a plastic body was obviously as you know, the pot core itself, argh. So, obviously as it's brittle, no other choice but break it into bits in order to get to the windings. Was cracked in the first place any way ! It looked like it was held in place with some transparent orange glue of some sort... which too was very hard and brittle. Tried to soften it with my hot air station, hoping the core halves would come off nicely... but no joy what so ever.Underneath the Tek sticker, bearing the part number(s), I noticed markings printed on the core itself, so most likely OEM information about the core... which would be useful to figure out it's specs I would hope ! But sadly didn't think of writing it down before "disassembly"/destruction... I just tried to reconstruct the core from the bits laying around on the bench... could only do partial job, but luckily looks like it's enough to read the markings, see below.It reads:GF 4301916 00F 170As for the gap, I can't measure inductance as I don't have an RLC meter and the core was broken to start with, but I guess that's something that someone with a 2232 could fairly easily measre on this own scope, and report. Will have to ask nicely on this forum and the Tek Group...As for number of turns, that would require destruction so can't ask anybody for that... never mind since I did it on my own coil as I said. Data follows further down.So, looks like there is hope... maybe I can actually get the parts to make my own coil, for a reasonable amount of money. Will keep digging the subject while searching in parallel for a used coil on Sphere Qservice and Ebay. I might get lucky you never know. Would still find it fun to do my own coil though.I would also need a new coil "former" (looks like that's how it's called, if Mouser is anything to go by ?), because mine as you see, is... not in good shape ! It's "tilted" like the old Pisa tower, probably due to overheating ?!... overheating which might also explain why the inner winding managed to sculpt grooves into the plastic !So, I need to find a new coil former with the appropriate dimensions...Then of course I need enameled wires (of 3 different gauges), but I guess that should not be too difficult to find... might be expensive though, copepr not being cheap, but I only need 4 meters wires total, after all... I don't need a kilogram of the stuff...So, here are summarized my findings about the windings, and some pics of course, I love pics... reworked to be light weight of course...Winding (pins) | Wire | Résistance | Number of turnsOuter listed first | Diameter | | and orientation/phase, viewed from the top----------------------------------------------------------------------------------9 - 10 | 0.20mm | 0.26 ohm | 5, CCW from 10 to 96 - 7 | 0.20mm | 2.18 ohms | 50 (x1 layer), CCW from 7 to 64 - 5 | 0.45mm | 1,65 ohm | 159 (x6 layers), CCW from 5 to 4, green colored1 - 2 (primary) | 0.30mm | Open/cut | 151 (x5 layers), CCW from 1 to 2In the end it was nothing extra-ordinary, I don't think it would be a problem to rewind it, even though I have never done it before. Obviously that's assuming I can find the appropriate pot core and former, but as we have seen, there is hope...The primary was indeed open circuit/cut. I found that the cut/burn in the wire was located 15 turns in.Don't why, but one of the 4 windings is colored green, go figure. Maybe because it's bigger than the other wires, some sort of wire gauge color coding ? Or maybe it's a standard way of saying "this winding serves this particular purpose". No idea... any suggestion ?Anyway, what I found interesting (because I know nothing about SMPS and how all these secondary windings are used exactly...), is that unlike what intuition would suggest, the primary is actually NOT the winding using the thicker wire, as you can see from my table above. The green secondary winding is by far the thicker, at 0.45mm diameter, and the primary winding is "only" 0.30mm.Anyway, just thought the curious out there might be interested in this mini tear down

author: Vince
author_rank: Super Contributor
timestamp: June 18, 2017, 03:18:55 pm
content: Google just served me this interesting catalog from pot core manufacturer :http://www.mmgca.com/catalogue/MMG-Ferrite-PT2.pdfIndeed all these pot cores have standard sizes, how great. 30x19 is in there no problem.One interesting thing is that the cores they sell, can be adjusted with a screw, to adjust the inductance. I assume they mean that it adjusts the gap.. adjustable gap, how practical eh ? Well that's assuming that once adjusted, you Loctite it firmly into place so that the gap stays put ! ....So that means if someone can just measure the inductance of their primary coil for me, I don't would not have to worry about the gap.. I would just adjust it until I the primary reads the same inductance. No ?That being said, make me feel a bit nervous.. if the screw were to move at all, might make the coil overheat and the scope blow again ?! Noooooooo... please no.That said... thinking about it... although the Tek coil had no screw in it, it did have the inside of the core (where a screw would go) filled with some white glue/gunk... maybe Tek too adjusted the gap "by hand', then locked the two halves of the core into place with that white gunk... so the idea might not be as silly as it sounds ?Any way, food for thought at the least !

author: Vince
author_rank: Super Contributor
timestamp: June 18, 2017, 03:27:37 pm
content: Hmmm, another interesting link, "Magnetics" web site, a core manufacturer :https://www.mag-inc.com/Products/Ferrite-Cores/Ferrite-Pot-CoresLooks like the " AL" value , the "permeability", actually defines what material the ferrite is made of. So, if there was way to figure that permeability ? That would solve the problem and I could order the pot core... or,since they appear to be fairly cheap, I guess I could order one of each material/AL value, and try them all until the coil measures at the expected inductance ?I mean, most of the work is wiring all the windings on the coil "former". But once this is done, you don't have to touch it ever again, you can fit whatever pot core on top of that, try 20 different core models if need be, not a problem I think ?!

author: Vince
author_rank: Super Contributor
timestamp: June 18, 2017, 03:55:55 pm
content: Hmmm... guess work :See below the page from the PDF catalog from 'MMG' which I linked to earlier.Their table for the 30x19 core, lists a model with an AL value of 1600... could it have something to do, with luck, with the "16 00" marking on the Tek pot core humm.... If it does then problem solved for the gap : table specifies it, it's 0.08.

author: Vince
author_rank: Super Contributor
timestamp: June 18, 2017, 06:33:30 pm
content: Making some progress on the gap side... what do you know, the coil former just snapped ! No I swear... I did not brutalize it !Was hardly my intention, but I guess it was destiny : that allowed me to see for myself the gap between the two halves of the ferrite core.See picture below. They is a partial bead of orange glue between the two parts. That bead was actually squeezed, therefore we know for a fact that the thickness of that bead is an exact image of the gap that used to be in there. Unfortunately the bead is split, but there is one spot where 100% of the bead is on one core only. So, I proceeded to measure the bead at this spot, as accurately as humanly possible using calipers. Did all I could to get a decent accuracy, though obviously you can't expect much in these conditions. But still. Even without measuring anything, the first thing that catches the eyes is how large the gap is, many tenth of a millimeter, at least 0.5mm and under 1mm. MY measurement falls in the range so I would say it's plausible.. though I still find it very high...Here goes : I estimate it at 0.80mm tops, with a likely-hood of being a tad under that. 0.75 or maybe 0.70, but not much less...I could maybe make a more accurate and reliable measurement at work, using a dial gauge, will see what I can do. But 0.75mm is the ball park figure any how.Hey hold on a minute... looking at catalog page I uploaded just above, 0.70mm actually does exist ! Material code " F5A " and AL = 250.What do you know...Does any of what I said in the 5 previous messages make any sense to any of you ?? Or am I just talking BS again... God I need to learn the basics of coils... we didn't learn about this at school, and it's far away... I need to get hands on experience on real/practical stuff... like I am doing right now... learning along the way, thanks for your patience...

author: Vince
author_rank: Super Contributor
timestamp: June 18, 2017, 09:12:38 pm
content: Just got a reply from Qservice about the coil. I didn't get lucky, their physical stock does match their on-line stock : no 2232 coil available, all sold. Apparently the things sell like hot cakes...That makes me want to really go ahead and rewind my coil. Once we have figured all the practical details, will be nice to have a turn-key solution for the numerous people who will need that coil in the future. A "howto" guide with all the technical information needed (like my table above, a good start), what core to buy exactly, wires etc... so they can feel more comfortable doing it themselves.

author: oldway
author_rank: Super Contributor
timestamp: June 19, 2017, 10:27:02 am
content: It is very interesting to see how you spent your time to know in detail how this inductor was made.I have always repaired this kind of failure by recovering the inductance on other non-repairable oscilloscopes.There are two versions of this inductance, the 00 and the 01 .... I do not know what the differences are, but I think they are interchangeable.These are the same inductors on the 2235, 2236, 2232, ....Your findings confirm what I have always written in different forums: this inductor works too hot, that's why it often ends up losing the insulation between turns or layers.I always advise to install a fan in the 2235, as I did in mine.From a ferrite quality point of view, I would choose a ferrite with the least possible losses at about 40Khz due to the overheating problem of this inductor.It is also necessary to check whether this ferrite has a high induction saturation level.But in general, ferrites saturate at about 2000 Gauss (0.2T) and this does not vary much.You should also care about insulations between coils....Are there any ?

author: Vince
author_rank: Super Contributor
timestamp: June 19, 2017, 08:30:29 pm
content: Quote from: oldway on June 19, 2017, 10:27:02 amYeah, I check the local ads for Tek gear every single day.Sorted by price, here are the Tek 22XX I can find right now :https://www.leboncoin.fr/annonces/offres/pays_de_la_loire/occasions/?q=tektronix%2022*&sp=1The cheapest one at 60 Euros, looking very sorry but sellers says it's working:https://www.leboncoin.fr/informatique/1133453935.htm?ca=18_sNext cheapest one is at 100 Euros, starting to get pricey for a donor scope...https://www.leboncoin.fr/equipements_industriels/1148674764.htm?ca=18_sBut these are 221X and 222X... looking at your list above, looks like only the 223X are compatible between one another... so the 221X, 222X and 223X are technically distinct families with different designs ? Too bad... a 223X surely is never going to be as cheap as this 221X...OHHHH !!!!! What do you know !!!!I just ran a similar search on Ebay and looooook at that !Just the part number I need !I am contacting the guy RIGHT NOW !!!!http://www.ebay.fr/itm/Tektronix-120-1439-00-01-SwitchmodeTransformer-for-22xx-Oscilloscope-/282530665982?hash=item41c823d5fe:g:VFwAAOSw-0xYf5hjIn the US but item is small and light so he states he can ship it abraod for only 13+ bucks ! Taht will be under 30 bucks delivered, I'm in !Might buy two of them just in case I get unlucky and one of them is "noisy", as he states.Says it's compatible with other 22XX scopes as well, not just the 223X series.. that answers my question.There are two versions of this inductance, the 00 and the 01 .... I do not know what the differences are, but I think they are interchangeable.That chap on Ebay seems to know what he is talking about, and agrees !I always advise to install a fan in the 2235, as I did in mine.Luckily the 2232 had one, phew !From a ferrite quality point of view, I would choose a ferrite with the least possible losses at about 40Khz due to the overheating problem of this inductor.You mean 60kHz ? That's the frequency that regulator operates at, if I am to believe the waveforms in the service manual. Or were you talking about something else entirely ?!It is also necessary to check whether this ferrite has a high induction saturation level.Wow, I don't master this aspect at all.. .and don't know how we could figure it out.I guess I can only do so much in trying to come up with an adequate core. Then I would test it in situ to make sure it operates properly, and if it does then I guess that means I did an OK job ?! First, before even whacking the coil back in to the scope, I can check that teh inductance of the primary is correct, compared to another working scope. The I will solder it and check waveforms to make sure it oscillates properly at 609kHz, with adequate waveforms. Then check for any funny noises, then check for the 40V rails accuracy, the put the cover back onto the PSCU section, fan spinning, and let it warm-um, then check temperature of the coil and check all the rails again, especially ripple. If all is fine then.. I think we can call it doneQuoteYep sure, didn't mention it but yes of course the windings are insulated from one another. A couple turns of transparent tape, not more. The outer tape is orange but I guess that's just for show...

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: June 19, 2017, 08:53:46 pm
content: Maybe you should look at thishttp://www.hakanh.com/dl/surplus.htmor talk to that guy. That's where I got the document from.

author: Vince
author_rank: Super Contributor
timestamp: June 19, 2017, 09:02:56 pm
content: Thanks for that.guy on Ebay already replied to me, how quick. Said he would (at my request) send two coils with the white gunk in the center hole as usually more "quiet". I had that type in my scope and it was quiet. Said he would mail them tomorrow !

author: tautech
author_rank: Super Contributor
timestamp: June 19, 2017, 09:13:06 pm
content: As you complete this repair I would have the inductance of each winding measured and note it in this thread.As time goes by these SMPS transformers will get harder to source but if each windings info is known and can be matched with n# turns, wire gauge and core parameters it shouldn't be hard to replicate DIY.Who knows, you might even be able to start a little sideline making these and supplying the market.

author: Vince
author_rank: Super Contributor
timestamp: June 19, 2017, 09:24:40 pm
content: Quote from: tautech on June 19, 2017, 09:13:06 pmMy intention/thoughts exactly !I have ordered two coils. Will repair my scope with one of them, so I can fix it quickly, and I will use the second coil to take inductance measurements .... later.... because I don't have an RLC meter just yet ! ... and no money to buy one, have much higher priorities on my plate currently...Well, until I can afford a decent/good RLC meter, maybe I can get a cheap/crap one for a few tens of bucks, just to get an idea...QuoteLOL Well I would have to sell an awful lot of them, and for a lot of money in order to make it worth the time spent winding... plus, since all the specs would be freely available on this forum, anybody could do the same as me, and profits would plummet ! LOLBut I will try and make myself a new coil, sounds like fun, I want to do that... at least once in my life. I find it educational... and being abel to buld your own stuff to repair your old gear, rather than buying old parts that eventually will run out anyway.... I find it satisfying.

author: oldway
author_rank: Super Contributor
timestamp: June 19, 2017, 10:17:49 pm
content: QuoteMost of them are noisy because they are faulty. When inductance is too low (internal short circuit between turns or layers) , there are high peaks currents who are stressing the ferrite core to saturation and the noise is coming from sub harmonics of magnetostriction of the core.QuoteHow to measure an inductance without RLC meter when you have a function generator with counter, a capacitor of known value and a working oscilloscope ?This is very easy, I let you guess the answer......

author: Vince
author_rank: Super Contributor
timestamp: June 19, 2017, 10:46:31 pm
content: Quote from: oldway on June 19, 2017, 10:17:49 pmhmmm, thanks for thatSo I can just measure the resistance (now I have the data from my old coil) of the windings to see if they are shorted or not...QuoteA resonant circuit yeah why not... I am rusty on the theory/math though, I must admit... I will dig out my old text books... 1/2*PI*sqrt(LC), something like that... shame on me. I mean I studied this stuff 20 years ago, and zero practice since then... yes of course I got rusty !No worries, I will get my old text books and refresh my memoryI don't have a counter BTWMy TDS 544A can measure frequency though, obviously...I have a 1Mhz analog generator, and a 2MHz "DDS" Fluke arb gen but it's in pieces... I am in the process of replacing the original LCD, with a back lit unit, because the original LCD has no backlight and it's just a real pain to read...Anyway, I could also just wire up on a bread board, a simple first order RL filter, look for the 45° phase shift / 1/sqrt(2) amplitude reduction (the TDS544A could help with its automatic measurements) and figure out what value L is...Well OK, in practice it would not be as accurate as measuring the frequency of a resonant circuit, I admit...OK OK....I will do a resonant circuit. Google refresh my memory please !https://www.electrical4u.com/resonance-in-series-rlc-circuit/Yeah, phew, I was correct earlier, 20 years of inactivity have not erased everything just yet, I am not as hopeless as I thought...

author: oldway
author_rank: Super Contributor
timestamp: June 20, 2017, 10:55:04 am
content: QuoteA short circuit between turns does not change the coil resistance enough to be measured and detected. But the inductance is reduced and even more the damping factor of the inductor associated with a capacitor in a RLC parallel ressonante circuit.The short circuited turns works as a short circuited secondary of a transformer.The definitive diagnostic is made "in circuit working" by measuring the current waveform on the shunt resistor and to compare it with waveform of service manual.

author: tautech
author_rank: Super Contributor
timestamp: June 21, 2017, 05:51:00 am
content: VinceFurther study for you when you get to rolling your own tranny:https://www.eevblog.com/forum/other-blog-specific/power-transformers-the-lost-art-of-custom-winding-bsfeechannel-5/

author: Vince
author_rank: Super Contributor
timestamp: July 09, 2017, 10:03:09 pm
content: Hi there, back in the game ! :-)Thanks to who it was that posted this link :http://ludens.cl/Electron/trafos/trafos.htmlI read his other pages on his site, related to coils.. this guy really seems to enjoy coils at large, and looks like he really knows what he is talking about !He designed some cool little tools like this one, with which you can determine the core properties... cheap and easy to build so will definitely make one and play with it, to characterize the Tek coil, so I know for sure what core to buy in ordr to wind a new coil by myself.http://ludens.cl/Electron/lmeter/lmeter.htmlI received a bunch of parts from Farnell, so I can rebuild the regulator.. 90% of it ! :-/I also received the two coils I ordered from a chap on Ebay in the US, as was affordable enough, 15 bucks the coil and only as much for shipping to France.. so I bought two of them, one to fix the scope quickly, and the other to play with, so I can determine it's exact specs and build/wire my own. Unfortunately, buying two of them also caused the death of one of them ! They must have banged against one another inside the package (package itself was fine, on the outside), so one of them lost a tooth so to speak ! LOL A chunk of the ferrite core, broke ! :-( Rendering it pretty much useless then.. not gonna risk putting it in the scope ! Can't use it to characterize the coil either I guess... So, I contacted the seller about it, and still can't believe it but he agreed to send me a new one and even pay for shipping ! That's one honest guy, a rare sight these days ! So I think it's worth pointing people to him, if you need a coil or something else he might have :https://www.ebay.fr/sch/shockcore/m.html?item=282530665982&ssPageName=STRK%3AMEBIDX%3AIT&rt=nc&_trksid=p2047675.l2562Said he would send it promptly.In the mean time, today I finally started work on the 2232. I replaced all the components that I found faulty 3 weeks ago...I put the TL594 on a socket because accessibility is a nightmare and getting rid of the old one was a real pain, had to break it in pieces and removed the bits one by one, not fun. So should the chip blow again, I would rather be able to remove it easily and quickly, and "gracefully" ! The thing hardly runs at high frequencies so I don't think having it raised a little from the board will upset it at all... though I will see that for myself when I probe around, come time to actually power it up !So... I am quite close to powering it again and start checking that this regulator now works as it should. But I am not ready just yet :- Today I removed the 51V Zener diode from the crow bar, because although it tested fine in forward bias (0.7V as regular diiode would), it didn't quite test as I expected in REVERSE : should have tested "open circuit" because obviously the multimeter only puts out 2Volts or so, hardly enough to make the Zener conduct. But somehow, it DID read something ! 1+ Volt or so ! Eh ?? At that point, the SCR was still absent from the board, so I don't see how the surrounding circuitry could have affected the test. So, not wanting to take any chances with this safety related device, I decided to remove it so I can put it on a bread board, to better test its reverse bias spec. My lab power supply can do 2x30V, so 60V in series, enough to test that 51V Zener then. But... somehow... once I had removed the Zener from the board, and laid it on the work bench... I grabbed the bread board, then went to pick the Zener on the bench.... but nowhere to be found, I lost it !!!! For the life of me, I lost sight of it for only a few seconds, and I just can't find it any more !So I am screwed, must get a new one ! Hopefully I won't have to spend yet again 30+ Euros at Farnell (minimum order to get free shipping) JUST for a bloody Zener diode ! I mean I hope local electronics shops have these in stock !.... :-(Anyway, making progress !Oh, and I also need to DIY a light bulb tester before I take the risk to power it up ! I did buy a x10 pack of the appropriate fuses, but if I can save blowing all of them, the better ! And I want to build this bulb tester anyway, will be quite handy whenever I will work on a mains powered gear.

author: Vince
author_rank: Super Contributor
timestamp: July 09, 2017, 11:17:56 pm
content: Found 51V Zener diodes from a "local" shop.. though 45 miles from me ! Shipping by mail is only 5 bucks, so I went for that. Should have them this week I would think, in time for this week-end any way. In the mean time I will get round to building a DIY bulb tester...

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 09, 2017, 11:22:44 pm
content: Good work Vince! I'm going to comment so I see how it comes out.Somewhere is a rift in spacetime where all the surface mount components and tiny screws I have lost are all collected, don't feel bad.

author: Vince
author_rank: Super Contributor
timestamp: July 15, 2017, 03:19:52 pm
content: Back again !I may have lost that bloody Zener, however I found something else that compensates.... that lonely big long screw that came out of the scope when I wiggled it, that I had no clue where it was supposed to go.... I eventually figured it out I think ! When I put the main heat-sink back in to place, I screwed the FET onto it, then looked at the pair of power transistors at the bottom of it (the two trannies from the output of the inverter, that drive the primary of the main transformer). There is a plastic holder, similar to that of te FET, that grabs both transistors at the same time. The holder is screwed onto the heatsink of course... the mystery screw is a perfect fit in there ! And it's also a pig to screw because accessibility is a pain, no room to manoeuvre the wrench, was a real pain to screw back in, a 1/16th of a turn at a time... I am 100% sure I did not have to remove that screw when I pulled the heat sink out of the case.. because I remember vividly saying to myself, when I pulledit out the other day "hey that was much easier than I thought i would be ! " ... no way I would thought of it in these terms, had I had to painfully unscrew that thing !!!!Anyway, I received the replacement Zener diodes from the local shop. Local shop means I obviously could not find the exact replacement for that Zener.OEM Zener was specified as 1N978BRL 0.4W 5%The one I could get my hands one is a BZX55C51It come in a DO35 package which looks a tad smaller than the OEM Zener, but somehow the power rating is a tad higher, not lower, at 0.5W vs 0.4W.Accuracy-wise both are 5%, hardly critical. I measured the replacement Zener before hand just in case, and although it's a few tenths of a Volt above 51V, it's still waaay within 5% so not worried here.And over the course of the week, I also worked on building my first light bulb tester ! Had to buy the bulb holder, quite expensive at 8 buck, and another 2 bucks for the halogen bulb 77W not 100 W. Could not find a 100W one as was suggested earlier by someone. I did find some 100W old fashioned bulb, but they were the bayonet style fitting, not the screw type, and the holder/socket was only available in the screw type. The rest of the hardware I had in already in stock (switch & mains socket combo, power cord, scrap wood).Tried to make something decent looking and practical, so took a bit of time (two days alone just to let the 2 coats of wax/oil dry)So, I powered up the scope at long last.... standing back and anxiously maneuvering the switch, crossing fingers..... and.... and.......To be continued next week !No just kidding !The good news : NO catastrophic failure (like when then original failure happened a month ago), no big "BANG" (pun intended), no smoke, no nothing. So at least that's good isn't it ?!The bad news : it does not work either.The light bulb does not stay on or off either... it keeps cycling between the two states at a very stable/clock-like pace. At the same time I can hear a hissing sound, presumably from one of the transformer. Of course I made sure to check every component before putting them back onto the board, and triple-checked the orientation of diodes and transistors, but I am just a human being so I guess I could still have managed to get one of them back to front...Obviously I checked the transformer too. Core intact of course, and all windings check OK (compared to old transformer).Made a short clip of it so you can better understand. If you crank up the volume you can hear the transformer.https://www.youtube.com/watch?v=9jd9idAlXDY&feature=youtu.beSo here I am, some more work to do, but getting there I guess...I am real happy I made this light bulb tester, thanks for the advice ! First, it looks cute. Second, it indeed saved me a lot of fuses already. Third, blowing a box full of fuses would not have given me as much useful information as the light build tester did : fuse would have just blown, leaving no clue as to what happened, whereas the bulb cycling tells me that it's not a catastrophic failure, because the scope keeps trying to recover over and over again, which means that some over current protection (PWM chip of crow bar) is actually being activated.Now I guess it could one of two things : either of the current limiting protections either react to an actual overload event, or there is no overload condition but the protection triggers anyway. Not relevant in the case of the PWM chip, as it would cut off power rather than short the SMPS. However the crow bar does short things out , so maybe the crowbar is not working as it should, and shorting the 40V rail when it should not. I replaced both the SCR and the Zener diode, so maybe I screwed things up there.so I guess the fist thing to do is, again, to remove the 0.2 ohm shunt resistor so I can isolate the inverter from the regulator... and hook it up to my lab power supply and carefully ramp up the voltage to see how the crow bar behaves.... if it's not shorting at 42V or so, then it's good and the short comes from the regulator side. At this point I can unhook the bench power supply, still leaving hte inverter disconnected from the regulator, and power the scope from the mains... and see what the regulator is doing...If I said anything above that makes your skin crawl "no don't that, you will blow the thing again ! ", then time to speak up !

author: Vince
author_rank: Super Contributor
timestamp: July 15, 2017, 07:32:45 pm
content: Bummer... had to remove the heatsink again so I could get access to the shunt resistor/R907 in order to isolate the inverter... grrr..;Inverter & scope still work like a charm ! I am relieved ! So now we know it's not the crow bar that's shorting the regulator, nor the inverter nor anything down stream. So, something is going in the regulator... I tested (again) the FET : removed it's connector.. all the pins test as they should... though I did scare myself for a moment because the drain -source was NOT open-circuit when the internal diode was biased in reverse ! That's too bad for a brand new part... too bad too, that the other same part (I ordered two of them just in case...)... I assumed/hoped I was just doing something wrong and chasing red herrings, but at the same time I didn't want to risk mains voltage to destroy again the scope again ! So I thought, maybe the gate, though being a power FET therefore beefy, can get charged enough just by being left floating, to make the FET partially conduct (it measured at around 50Kohms). So I put it on a bread board, shorted the gate and Source, then measured the drain-source again.... open-circuit, relieved I was !So... since the crow bar, inverter and and scope aren't shorting the SMPS, since the FET is not shorted/defective... where the hell the short could be coming ?!Was starting to think well maybe the regulator is fine and the short is upstream, in the common mode filter daughter board or main filter... or mains rectifier or filter cap. But I had already checked these, and the common mode filter well, it somehow it got shorted, I doubt it would "heal" itself in a second, then fail again, and heal again, never mind at very regular rate that the light bulb test is indicating.So I was starting to wonder if maybe... the scope is perfectly fine, and it's just oscillating due to the presence of the bulb. Haven't thought of this in detail, but the thermal inertia of the bulb filament, coupled with the filter cap getting charged and discharged... maybe there is some back and forth game between that filter cap and the bulb, which would explain this oscillation, and it's perfectly stable frequency ?!Bulbs and electronics.. all of sudden it reminds me of an experiment we did at school, where a miniature filament bulb was used as part of the feedback loop in some op-amp circuit, for automatic gain control IIRC.The fact that an SMPS presents a big filter cap to the mains, meaning the voltage is lagging, make it prone to oscillation, the light bulb tester thing was probably "invented" before SMPSes were the norm ? With a linear PSU, hence a big transformer, the light bulb tester sees a inductive load, not capacitive, so the current is lagging, rather than the voltage.So I guess the light bulb tester is indeed an excellent tool when restoring older tube Tek scopes that use a big transformer.. but for more modern Tek scopes with an SMPS/ capacitive " front-end " so to speak...Anyway, that was just a crappy guess of mine, a shot in the dark to keep me going in my trouble-shooting.So as a result, I took the risk of powering the scope withOUT the light bulb tester, just to see what would happen... thinking what the hell, there is still the mains fuse that can blow, if need be.So... did that and... nothing happened ! But... the fuse did NOT blow either ! So maybe my reasoning above, was not 100% BS ? Maybe only 99.9% BS !I checked a few voltages : there IS voltage across the mains/rectified filter cap : 300V+ DC, as you would expect ! Bear in mind this is running 220+ V not 110V.That voltage was perfectly stable.So looks like things are getting better now :- Rectified DC present- FET OK / not dumping the mains- inverter & scope OK- Crow bar not shorting the regulatorSo at this point I am starting to assume that maybe there is no short, and I might take the risk of soldering the shunt resistor back into place, to re-establish the link between the regulator and the inverter.But before I take that risk, I wanted to try and probe around the regulator... I probed the power supply of the PWM chip... supposed to be 14+ Volts, determined by a Zener diode. Good news is, I do get around this... bad news is that it is very unstable and keeps jumping around, going up and down, say + and - 5V around the nominal 14V. So my guess is that it's something to do with the start-up/boot-strapping mechanism used here ? I am really out of my comfort zone here as I have no experience of these things... but I guess that since I have removed the shunt resistor, the output of the FET is not connected to the 40V rail filter cap anymore (C940), hence the output voltage has no way of being smoothed out and stabilizing, so it's a chicken and egg situation and the PWM chip has no way of producing a stable supply voltage for itself. Am I having this about right ? So basically, I have no way of testing/probing the regulator, until I have re-attached it to the inverter.. bummer, risky. At least the above means that the PWM chip is probably oscillating and trying to produce a voltage... so that's rather good news, I say !OK.... I am feeling lucky today... with every I said, it looks like the scope won't blow up if I reconnect the inverter to the regulator, and that i might even work... or at least it will be in a state where I can actually probe/troubleshoot it properly/in detail.Taking a deep breath..... see you in a moment for the result ! Wish me luck !

author: Vince
author_rank: Super Contributor
timestamp: July 15, 2017, 07:43:17 pm
content: Oh ! just watched the video clip I uploaded above... and I noticed that on the foiurth and last "on" moment of the bulb, the scope was actually just starting to draw a trace on the screen !!!nothing happened in the first 3 strokes, but I guess that was just because the CRT was not warm enough.So, indeed looks promising !Too bad my light bulb tester made me chase red herrings !Would still like to get that light bulb tester working here though ! I was advised 100W... my modern halogen bulb is only 77 Watts, the more powerful I could find.How does one calculate what power rating is required in order for the gear not to oscillate ?? Does that the fact that it's a halogen bulb changes things a little bit compared to an old fashioned bulb, and that would need to be taken into account ?For now, I guess the answer is : if it oscillates, just put a more powerful bulb in there, until the oscillation stops ?!I guess I will need to make a large version of my tester, so I can fit two bulb sockets and combine various bulbs to get a combination that works for a given gear.Any way, back to the work bench, see you in a bit !

author: tautech
author_rank: Super Contributor
timestamp: July 15, 2017, 07:52:34 pm
content: When I use a dim bulb tester I first look at the DUT manufacturers rated wattage consumption and select a bulb greater than what's spec'ced on the label. Rarely I've had to use a 150W bulb.For various gear you might need to use the bulb tester on it's a good idea to have a selection from ~40W and up.

author: Vince
author_rank: Super Contributor
timestamp: July 16, 2017, 12:22:15 am
content: Thanks Tautech, will go and source a more powerful bulb... even if that means I need to order it online, since local shop won't do higher than the 77W I found.I am back from the bench... VICTORY !!!The light bulb tester made me chase red herrings and worry needlessly : my repair of the regulator, somehow, despite the numerous components that I replaced... worked first time ! Thanks to all who helped me through this !My first decently involved repair... sure was more involved than me replacing a tactile switch in my 10 bucks alarm clock !Was my first time working on a real world SMPS, so I learned a lot along the way, thank you every one.I am so glad my little 2232 is back to life, it's my main scope and I love it to bits.I adjusted the trimmer in the inverter section, following the calibration procedure in the service manual. They take the -8,6V rail as their reference, so I set that spot on to the required 8,60V, and all the other rails were spot on too... except for the +/- 5V rails which were way off, but I didn't care since I assumed they would comply once I would plug the digital board back in. Sure it helped... but then all the other rails moved in unison ! They dropped enough to be out of spec ! So, I readjusted the -8.6V again to be spot on 8,60V. and all was good again. all the rails are within spec now... though the 5V rails are on the high side : +5,17 and -5.25V (max allowed). The 8.6V rail is also on the verge : 8,64V for an allowed maximum of... 8,64V...So I am half-temped to drop the -8,6Volt reference just a tad (-8,56V minimum allowed) so as to bring the +8,6 and 5V rails a little bit on the safer side of things... what do you think ??So, the scope it working again, nice... but after a quick test, I see there as still some minor issues I would like to fix, at least try to.... if it's not too involved.1) Probe compensation signal is inverted : shows up as 0 to minus 0.5V instead of +0.5V. Had this problem when I tested the scope without the digital board, while powering the inverted directly from the bench power supply. Well now that the digital board and everything is put back together... the problem is still there sadly !I tried on both channels, and they behave the same. So I hooked the probe to the compensation signal from another scope, my old Tek 5111 (100Hz 400mV signal), and the 2232 displayed it just fine ! What a relief ! So the scope itself is fine... it's just the probe compensation signal itself that's upside down... only a minor annoyance you might say, as this doesn't keep one from compensation his probes.. but still, it's not normal so if I can fix it... why not.If it were my 5111 it would be easy. If it were my TDS 310 or TDS544A it would be easy too... the signal is generated locally on the front panel assembly, which easily pops out to let you work on it... but the accessibility of the front panel on th e2232 looks a nightmare... will check the service manual and schematics.2) When in digital/store mode, I witness too issues : the trace is not centered anymore : it is shifter down a little bit, by roughly a small division. Also, when I use the vertical position knob to readjust it, the traces moves rather jerkily. See video clip...One would say that the pot just needs cleaning, expect that this very same pot works like a charm in analog mode...https://youtu.be/ug77tJohVSA3) Fan is noisy... after 30 years I guess it's to be expected eh. Makes both a low pitched rumble as well as a high pitched noise that drive my ears a little mad to be honest. the latter noise does not come from the SMPS... my new regulator transformer is absolutely silent, the other transformers too. Any advice on a good replacement (brand and model ) ? The 'PC' gamer nerds, IIRC, are well versed in this domain... but IIRC desktop computers use only 80mm and 120mm fans right ? So this crowd won't be much help for the Tek, as I measured the old fan and it's a smaller, 60mm unit ! Bummer. Still, they could advise on brands, I presume.Specs are as follows :Brand : " Sunon "Model " MD 1206 PTS1 "12V DC, 1.5 Watt60x60x25,4mm.Sticker also states "Brushless", but aren't they all ? So not a criterion...4) I am a bit worried about the orientation of two little, identical, 4 way connectors, on the edge of the digital board (CRT side). Picture below. I am worried because although I am 100% sure that I put them back in the same orientation they were when I took the scope apart (took plenty of pics at that stage, precisely to help me with re-assembly), I noticed that the Pin #1 on the cable, is NOT matching the Pin #1 labeled on the board, on the silk screen !Looking at the board view in the service manual, these are connectors J2111 and J2112. Looks like they are directly related to some analog circuitry shaping the two channels before they are fed to the ceramic proprietary modules, one per channel. So, these connectors are bound to be important... so I would rather like to be 100% sure of their correct orientation...Can anyone tell me how they are oriented on their scope, for those with a 2232 or similar ?I mean, maybe it's "normal".. maybe it's a mistake from Tek, who got the Pin #1 marking wrong ? Happens even to the best I guess...Things is, the scope has been opened before, by some previous owner, that's for sure as I found old flux on some reworked solder joints in the SMPS section of the main board. So maybe the guy who did that, put these two connectors the wrong way around, I can't know.In the mean time, I will keep studying the service manual closely to try to figure it out...

author: Vince
author_rank: Super Contributor
timestamp: July 16, 2017, 01:17:19 am
content: OK, I am getting there with Problem #4 above.These two connectors are as could be figured just by looking at the board, the two analog channels which are undergoing some final analog processing before they are fed to the ceramic modules which are obviously the ADCs, which then go to that big ceramic/gold chip which of course is the big acquisition chip.Anyway, PDF below. First the acquisition board, where it tells us where to find where these connectors go : sheet #2.So, although I still don't know how Tek mean to orientate these connectors, at the least it seems that regardless, no damage can occur, as the pin-out is kinda symmetrical : the two outer pins are both ground, and the two inner ones are a differential pair carrying the signal from the corresponding channel. It's not referenced to chassis ground any more at this point, apparently.I am still surprised that the most vital and fragile signals in the scope... the input signals, travel from board to board across 8 inches or so, on a simple flat cable instead of a coax ? I guess that is is carried by a differential pair, surrounded by ground on both sides as well, keeps it from picking up too much noise along the way.

author: Vince
author_rank: Super Contributor
timestamp: July 16, 2017, 03:03:22 am
content: Now having a look at problem #1 above, inverted probe compensation signal.Found it in the schematics, see below... it's generated locally, directly within the front panel. A simple text-book op-amp astable, at least I could figure this outHowever all the stuff after it, I can't make heads or tails of it... The waveform they give for the output of the op-amp spans 15V (makes sense, +/- 8.6V rails, minus the limited swing of the 741output which can't fully the rail, 15V or so sounds about right) so that means all the stuff after the op-amp is supposed to reduce this to 0/500mV.So... that analog stuff must probably take that symmetrical +/- 7.5 Volts output from the op-amp, and maybe rectify it to keep only the positive part of it, that would solve half of the problem. Then it must somehow reduce that +7.5V square, down to 0.5V. Since it's a square wave and we are not too fussy here, it can do it either by scaling it with a voltage divider, or simply by capping it ! Now with this in mind, let's try to make sense of the schematic...Just had a look at the front panel... the back of it is partially accessible/exposed, one can probe the top half of it, and for once I got lucky : all the circuitry for the probe compensation signal is accessible !Will probe that...... later, 5AM here, time to go to bed, done enough for today I think !

author: BradC
author_rank: Super Contributor
timestamp: July 16, 2017, 07:46:34 am
content: Quote from: Vince on July 16, 2017, 12:22:15 am10 seconds of google :https://www.digikey.ca/product-detail/en/sunon-fans/KDE1206PTV1.MS.A.GN/259-1358-ND/1021207On fans. Be very wary of anything recommended by the "'PC gamer nerds". PC's don't have to last anywhere near as long as quality test equipment, and the fans are built down to a price. Airflow ratings and static pressures are generally rated in PMPO Chinese watts. Additionally, PCs don't have irreplaceable ASICs and other expensive parts that fry when your 'buck fiddy' "ultra-mega-whisper whopper-flow hoosit waffle-cone bearing super flower blade' PC fan bearing carks it and it grinds to a silent halt.

author: Vince
author_rank: Super Contributor
timestamp: July 16, 2017, 12:45:56 pm
content: OK, good point (s)... noted.So let's take this fan replacement more seriously then... Indded Digikey has lots of fans for sale, lots to choose from... well that's if I lived in the US. Choice will be much more reduced here in Frog land, obviously.But good news is that " Sunon " the OEM, still is in busines, so I can always contact them to firgure ou a suitable replacement, and potential improvement, over their 30 year old product. Well assuming they are nice enough to reply to me, hum hum...At the least, looks there are 3 parameters to take take into consideration :- Fan speed at nominal voltage, 12 DC here.- Static pressure, no idea what that means in practice... the pressure that the fan can produce before the air flow starts dropping ? If so, then I guess the higher the better.. as long as it does not compromise other factors like noise.- Air flow .. well that one at least sounds more self explanatory..So, armed with the OEM part number, let's try to find what these parameters are... so I can make an informed decision on what replacement to get.The one you pointed to has a very close part number to the original unit, so at least that's a starting point.As for the probe compensation after-opamp shaping circuitry, I am thinking maybe1) Not all of it is meant to shape the op-amp output, come to think of it, maybe some of it could be there to protect the op-amp/scope from people accidentally applying voltage to the probe test point on the front panel, rather than hooking their probe tip on it.... Not strictly necessary, but it being a Tek / quality stuff, maybe they wanted to make the product as robust/real world-proof as possible.2) If they drop the voltage using a voltage divider rather than just capping, then R988 and R989 might serve this purpose: they have weird values so they must be accurate... indeed parts list lists them as 1%. That would make sense for a voltage divider. If they were part of a protection circuit then most likely they would not need to be that accurate...On the other hand, the 200ohm resistor at the end of the chain, near the 5V Zener, is also 1% ! Hummm... This resistor and the Zener are physically mounted next to the probe test point, away from the op-amp/rest of the circuitry. So I guess that means the Zener and 200ohms resistor work together, and are there for input protection...3) Diode CR989, given how its mounted... I don't see how any positive voltage could ever come out of that probe test point !So.... could it be that I am losing it ? Could it be that this scope has always showed me a negative compensation signal and it's only NOW that I actually happen to pay attention to it ??I still find it weird... all my other scope give a positive signal.I think I am losing it.... please call the doctors !

author: tautech
author_rank: Super Contributor
timestamp: July 16, 2017, 12:55:34 pm
content: So is TP 56 waveform correct ?If it is there's only a few components to check but I'd suspect any of the 3 diodes, including the zener.The 2 silicon diodes are probably just signal diodes, 1N4148 or similar and the zener, well 5.1V is pretty common so sub something in there just for a check.

author: Vince
author_rank: Super Contributor
timestamp: July 16, 2017, 01:02:37 pm
content: OEM Fan specs : Thank you Tek... I didn't think I would get THIS lucky, but the part list gives all the specs about the fan !Does not talk about static pressure though, but all the other info is there, even tells you what type ofb earing is used !12V, 1.5W, this we already knew, was written on the fan itselfBearing : sleeve bearingAir flow: 19cfmnominal speed : 4600rpmThere I go !Now the question is : can I improve on this : maybe a different type of bearing would be more silent, for example ? Also, intuitively, an increased air flow might sound like a good idea, but this static pressure thing intrigues me as I don't fully understand it and it's relationship with the all important air flow. I mean, I guess the airflow is measured "unloaded"/unrestricted, but once inside a closed cabinet, by how much will the air flow drop.A fan with a higher "nominal" air flow is no good if that flow drops by so much that it ends up lower than maybe a fan with a lower airflow but that can actually maintain that flow better than the other fan, once in the enclosure.I guess I need a little training on cooling fans eh ?!........But until I can figure this out, at least I have the specs of the old fan as a safe starting point...

author: Vince
author_rank: Super Contributor
timestamp: July 16, 2017, 01:07:53 pm
content: Quote from: tautech on July 16, 2017, 12:55:34 pmI still have not been in the lab today... but going there in a minute !QuoteYes, just checked them on the parts list. It's the same general purpose small signal diode that they seem to throw everywhere in the scope, same that I replaced in the regulator in the SMPS.

author: Vince
author_rank: Super Contributor
timestamp: July 16, 2017, 02:54:48 pm
content: OK, so after some probing... see the screen captures below. I kept the same vertical gain on all 3 pictures to make it easier to compare them at a glance : 2V/DIVSorry the square waves don't look really square... the old Hameg is only 60Mhz,30pF input capacitance, it's too much for the compensation range of the 500Mhz probes I got for the TDS544A. I do intend to buy more general purpose, slower/ switchable x1/x10 100MHz probes, to go with my slower scopes, but there are so many items on my "to buy for the lab" list, and so little money... well I will buy some, just don't know when...I did notice while having a quick glance on Ebay though, that there are some dirt cheap Chinese 100MHz probes, like way under 10 bucks each, I wonder what problem to expect from them, or if they are actually any good. For slow stuff I guess it does not take rocket science to make a decent product ? At any rate they are so cheap that I don't have much to lose... if I like them, I keep them... and if I don't well I can just get something else. I guess the problem is that I don't have a function generator that can go as high as 100+ Mhz, so I would not be able to plot their attenuation vs freq response, to see if they are decently "flat" or not...I might post on this forum later to get opinions/reviews on this subject...... and here I just added yet another item to my list : an RF generator.... more money again !No worries... I will just have to get a broken/cheap unit, and that will make for another repair topic !Anyway, waveform #56 is fine, matches what's in the service manual. The op-amp oscillates as expected between the +/- 8,6V rails, though as expected it can't really reach the rails. Around 7,5 positive and 6.4V negative. 2.2V away from the rail, but who cares, shouldn't make any difference here. Plus, it's still well within specs, according to the datasheet I pulled at random (Texas instrument from 1998). It even says that the minimum supply voltage ought to be 10V, not 8,6 ! So we can't really complain really... Plus, even if it were operating in the recommended voltage range, 2.2Volts from the rails would still be plenty within specs... yes, they are very, very wide....So, voltage levels don't really matter at this point... the little bugger oscillates between the rails and that's all we are asking from it...Then I probed downstream, step by step.First step is test point 'A' (at the anode of CR988) : here the positive half of the signal is capped. Signal now goes from -7.5V to +1.6VSecond step, test point 'B' (at the cathode of the same diode) : now the lower half of the wave is trimmed as well. Signal now spans -1.2V to +1.0VThird and last stop, the output signal : as already mentioned, it goes from -0.5V to 0V, correct/expected amplitude, but purely negative signal, rather than the positive one one would expect...Both signal diodes tested fine. The Zener diode however I could not test in situ : gives 200mV both ways... because R990, which is 200ohms only, is right across the Zener. The multimeter injects a calibrated 1mA current into the circuit, so obviously I get bang on 200mV across the Zener/resistor...But at least we know it's not a dead short...But again I just don't get how one can possibly dream of having a positive signal out of this circuit, given which way around, CR989 is mounted...I am really lost.I will try to see how difficult/risky it is to pull the front panel assembly out of the scope so I can get sufficient access to play with individual components... but I guess/fear it involves removing all the knobs from the front panel, and they are so fragile that I have already broken 4 of them... so am a bit reluctant to take any chances with the remaining onesHell... I looked at the user manual.. nowhere does it say that the probe compensation signal is supposed to be positive... nor negative, not symmetrical... they never, ever reference it to ground... they just say "500mV square wave"...Admittedly that's indeed all we care about when compensating a probe, but still... feels strange to me to have it negative ? I am starting to feel like I am losing it... where are my pills... could people with a 2232 or at least some 22XX scope, tell me what polarity is their signal ??

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 16, 2017, 04:33:20 pm
content: While I don't, in general, disagree, there ARE some high quality fans targeted to the PC enthusiast market. I would point in particular to the offerings from Noctua (particularly theirindustrialseries), andNidec Servo(the OEM of the well regarded "gentle typhoon")Quote from: BradC on July 16, 2017, 07:46:34 am

author: David Hess
author_rank: Super Contributor
timestamp: July 16, 2017, 09:14:38 pm
content: Quote from: Vince on July 15, 2017, 07:32:45 pmThe problem with dim bulb testing a switching power supply is that the input to the switching power supply has a negative resistance input characteristic; at low voltages the switching power supply draws more current and at high voltage it draws less current. This can result in the switching power supply latching off or continuously restarting.Quote from: Vince on July 15, 2017, 07:43:17 pmThe bulb resistance needs to be low enough that the input voltage to the switching power supply does not drop below the level where the under voltage lockout trips. The 2232 switching power supply does not have an under voltage lockout however at low voltages, the duty cycle and current of the switching power transistor goes up and eventually either trips the current limit causing shutdown as the output voltage collapses or the switching power transistor self destructs from excessive power dissipation do to high current and high duty cycle.Quote from: Vince on July 16, 2017, 01:17:19 amExactly, this signal is differential so twisted pair or in this case a twin lead transmission line made from some ribbon cable works fine; the grounds control the impedance and reduce external coupling. Schematic 2 shows where these signals originate; channel switches direct the differential signal from the vertical preamplifiers to either the non-storage or storage display circuits. Note that the signal going to the storage circuits is a current and only terminated on the receiving end which you can see on schematic 14.If you reverse the connector, then the storage display for that channel will be inverted compared to the non-storage display and trigger of that channel.Quote from: Vince on July 16, 2017, 12:45:56 pmThe probe compensation output has always been negative. It shifts between ground and -0.5 volts.When the output of the operational amplifier goes positive, CR989 disconnects it from the probe output and the output settles to ground via R990. When the output of the operational amplifier goes negative, CR988 disconnects it from R989 and the output voltage is determined by R990, CR9889, and R989. Current through R990, CR989, and R989 becomes -2.5 milliamps. -2.5 milliamps across R900 produces -0.5 volts at the output. VR9900 does not conduct much in this case and can be ignored. VR9900 is there for some type of protection but I am not sure what.The virtue of this design is that the output voltage only depends on the ratio between R990 and R989. The high value of R989 makes the current through CR989 relatively constant so the forward voltage drop of CR989 has little effect.

author: Vince
author_rank: Super Contributor
timestamp: July 16, 2017, 10:15:38 pm
content: Hi Dave, thanks for the comments especially the operation of the probe signal circuitry, that was interesting.I just asked on the Tek yahoo group and someone confirmed that indeed it's negative. So I feel relieved, I can stop chasing red herrings, and stop trying to understand how this circuit can deliver a positive signal, because it obviously can't !So that's one less problem in the list.I removed a few more problems too, on the digital board :- identification of the two ribbon cables : Tek did not label them, so no easy way at reassembly time, to know which is CH1 or CH2. Easy to figure out though : just swap the cables in store mode if they don't show up where they should...- orientation of the above cables : again just enter store mode and flip the cable(s), live, 'til you get the correct polarity on the screen.. result : yes, Tek indeed did NOT match pin #1 of the cable connector, with pin #1 of the socket on the board ! How "nice" from them.... grrr...- trace not centered in store mode : while looking at the ribbon cables, was obvious that there was some analog circuitry with trimmer, before the signal get to the ADC modules. So some of that circuitry maybe allowed to adjust the offset, which then would be of course unique to the digital board and would explain why the trace was perfectly centered in analog mode and only off-center in store mode. Looked at the service manual, indeed this analog circuitry is there to shapoe the signals prior to sampling, the usual/expected stuff : offset and gain, as well as "HF peak" which I don't know anything about. But whatever, I was only interested in the offset. Checked the service manuakl again to see if the adjustment procedure might be more complex than meets the eye, but it was not.So I just tweaked the trimmers on both channels and now I am pleased to report that store mode now shows perfectly centered traces on both channels, the trace does not "jump" when I switch back and forth between store and non-store mode, it's now seamless... lovely.So..... I am getting there !I think I have solved all my problems now !The power supply is fixed, store mode is now feeling better than ever, and the probe signal was a red herring.I just need to replace the fan because I don't feel like opening the scope again later just for that, and IU will done !It's great to have my little 2232 back in business, it's a great all rounder for general work I find.For the fan, I obviously can't order from Digikey, so as always I will have to do with what Farnell has to offer. 2,000 fans vs 12,000 on Digikey... not in the same league. They don't carry the same brands though, and somehow appear to have a good selection of appropriate fans. 24 items, if I include all types of bearings.There are two Sunon models in the lot. The "KD' series, at only 10 bucks, cheap because apparently it is being phased out. The current model "Maglev" series) is more expensive at 23 bucks, but price drops down like a rock : only 8.8 bucks/Euros for just 5 units ?! It's almost worth buying 5 of them !So just need to get a fan, then I can put the scope back together and move on to the next repair.... my recently acquired TDS 544A is misbehaving...Oh, and I am disappointed... earlier I said that the transformers where not making any hissing noise... and that the fan was. No... I stopped the fan by hand and the hissing is still there... noooo... I wonder which of the transformer is doing it. I guess it's the regulator coil, because when I bypassed it to feed the inverter with the bench power supply, it was silent. Anyway, hopefully once the cover is put back in, I won't be able to hear it that much. Plus, I already have another coil coming in the mail, should be there soon. and of course I don't give up on my (educational..) pan of wiring up my own coil/transformer, so I would have two more to try ! Maybe one of them will be silent...

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 16, 2017, 10:39:00 pm
content: good job Vince!

author: Vince
author_rank: Super Contributor
timestamp: July 16, 2017, 11:56:44 pm
content: Thanks !But of course the real thank you goes to all the knowledgeable people who helped me through this ! Thanks a million chaps !Fan is sorted : just ordered the two Sunnon models I talked about.... turns out they were the ones that made most sense anyway.I compared all the fans from the various manufacturers that Farnell would list, looked at a few datasheets.. and in the end they really are pretty much all the same... the laws of physics apply to all of them, so there is no reason why one manufacturer would achieve performances significantly better than others.The physical dimensions of the fan kinda dictates what can be achieved... basically, there is no miracle : the "silent" fans are silent simply because they run slower, and therefore give lower airflow. There are 3 speed ratings: "slow" (2000+ rpm), "medium" (3000+ rpm) and "Fast" (4000+ rpm), each speed gives a flow and noise combo figure.The old fan was putting out 19cfm at 4600 rpm, fast type then, hence noisy.Modern units must have improved aerodynamics of the blades a bit, because now the 19cfm class fans fall in the medium speed range. The high speed ones therefore achieve better airflow, 23cfm or so.I ordered the older "KD" series, being obsoleted, hence cheap, which is the fast kind hence with increased airflow. I also ordered the modern replacement "Maglev" series, which produces the original 19cfm but runs slower so should be less noisy. I will try both and see for myself. The fan is given for 60,000 hours life expectancy, so I will die long before the fan does.Farnell has less choice than Digikey, so I could not get the sleeve type bearing that Tek specified. Sunon does offer it, on both of these models, but it's not the ones Farnell chose to pick. The old KD series will come with a ball bearing, and the modern Maglev will come with the " Vapo" type bearing, whatever that is. Seems like the latest hype in the fan bearing scene, I assume.I will compare them and pick the one that pleases me best, the other one will still be there handy, as a back up if need be.As for my question about the relationship between air flow and static pressure, the manufacturers give graphs that plot the latter vs the former.Hopefully I will receive these fans this week and be able to reassemble the scope for good in the coming days... and then move on to the next "project" in the pile...

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 17, 2017, 12:19:55 am
content: There has been a great deal of effort spent on optimizing impeller design in recent years, yes. It is actually pretty impressive how much noise to flow/static pressure performance they have been able to squeeze out of these constrained form factors IMO. One would have thought that this would all have been sorted years ago, but no.

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2017, 01:21:21 am
content: Quote from: Vince on July 16, 2017, 10:15:38 pmThat diode disconnect method is common in Tektronix designs and particularly suitable for a operational amplifier based one because it removes amplitude variation do to operational amplifier output saturation.QuoteI had to do the same thing on my 2230 on which I spent weeks tracking down a problem in the storage channel switch. Swap the cables to get channel 1 and 2 right and then flip the cables to get the polarity correct. See the photograph below.Quote from: Vince on July 16, 2017, 11:56:44 pmWatch out for the voltage specification; a 12 volt fan is specified but it is actually running on 8.6 volts. It is not real clear to me if the 2232 fan specification of 12 volts, 1.5 watts, 4600 RPM, and 19 CFM is suppose to apply at 12 volts or 8.6 volts where they used it. Sunon makes a fan with asuspiciously close part numberwhich is more powerful than the Tektronix specification so I suspect this may be the case.Quote from: WastelandTek on July 17, 2017, 12:19:55 amThe fan on the older 2230 has to be seen to be believed; it looks like a model of something NASA would use for a wind tunnel. But the fans on both my 2230s and my 2232s are practically silent so I am dubious of modern improvements in aerodynamics or at least that they are commonly used.

author: tautech
author_rank: Super Contributor
timestamp: July 17, 2017, 07:47:15 am
content: Nice work Vince and well done for taking the time to write it all up.......thinkings out loud included.

author: wasyoungonce
author_rank: Frequent Contributor
timestamp: July 17, 2017, 07:58:10 am
content: Well done Vince....you have that puppy nearly tamed!

author: Vince
author_rank: Super Contributor
timestamp: July 17, 2017, 11:36:09 pm
content: Thanks chaps. Yes I try to think loud and document... so that other newbies like me can learn as much as possible when following this repair.David for the fans well... when I get them, the two nes ones and the old OEM one, I think I will power them all at the same time on 12V, and get a feel for what kind of airflow they produce so I can compare them.While I am waiting for the fans to come in, I thought I would do some measurements on the regulator and inverter, for educational purposes (and for peace of mind as well).Below are PDF copies of the SMPS schematics and waveforms.The waveforms are remarkably similar to those in the service manual, shape, amplitude and frequency, but was still interesting, see below. In order of appearance :- WF #63 : ramp up voltage for the PWM oscillator.- WF #64 : PWM output : duty cycle is less than advertised, but I guess that's to be expected since Tek based his measurements obviously on US mains, 115V. It's 230 on my side of the pond, so obviously the duty cycle has to be lower in order to compensate for that.- WF #65 : collector voltage on one of the two paired transistors driving the primary of the main transformer.- WF #66 : If I read the schematics about right, this waveforms is an image of the current flowing through the primary winding. The waveform in my case is a little weird as you can see : kinda "doubled"... but the trace is actually perfectly stable on the screen. Frequency is double that of the collector voltage, note. So my explanation would be : one of the two transistors is drawing a bit more current than his brother. Hence the waveform is the same shape, but slightly taller. My analog scope obviously has no means to tell one cycle from the other, so both get super-imposed, making the trace look doubled. But I guess if looked at the same signal with my digital scope, and did a single sweep, then we would see what is actually going on : a tall cycle, followed by a shorter one, then a taller one again, etc. Yea, I might look at it with my TDS544A to confirm that. Can't make the 2232 probe itself like I would "normally" do, because obviously the SMPS ground is not referenced to chassis ground. I don't want to blow my scope again...Now the question is : should I be worried about this current difference ? Is it caused by the primary winding(s) not having exactly the same resistance/number of turns.. that would seem strange. Is it cased by one of the transistors not being fully saturated ? That would seem unlikely too, as I expect them to be waayyy over-driven to make sure they don't overheat/cook/die by dissipating abnormal amounts of power. I guess that's easy enough to figure out, I could just measure the voltage drop across both transistor, and also power off the scope and measure the resistance of the two halves of the primary winding. But the scope is working just fine so I am not overly worried ! ;-)Then some more interesting stuff I did not expect ! Might be of interest to newbies, sure caught my attention ! Before I started to check waveforms, I checked DC levels where indicated on the schematic.Main rail is fine, 42+ V as expected.The " fun " part came when I tried to probe across R927, it says it should be 6.8V. As soon as I put the multimeter probes on these terminals... the scope would immediately shut down !!! And would start again just as soon ! And this was perfectly repeatable, like a Swiss watch !So, detective cap on again !That resistor is part of the voltage divider that drives the base of Q928 which itself drives Q930 which provides the power supply for the PWM chip... hence has the magical power of turning the whole scope on and off at will... so what I was witnessing was not completely unreasonable.What seemed unreasonable though, is why Q928 would "trip" when I measure the voltage across R927 !1) I thought maybe there is a cold solder joint on that tranny, and putting the probe on that nearby resistor just so happens to flex the PCB a little, and upset the dodgy solder joint/transistor. that said, I was hardly pushing on the PCB with my multimeter probes, I was just "caressing" it, barely touching it. So that seemed strange. But just to be sure, I grabbed a piece of plastic and applied pressure on the PCB right on the transistor.... nothing happened. So, not a cold solder joint... joint looked good anyway, and that tranny was actually the only one that did NOT get replaced when I refurbished the regulator, so it could not have been me doing a bad soldering job...2) Then I thought OK, if it's not mechanical, then has to be an electrical problem... but what could be happening. If it were a FET, hence enormous gate impedance, I could understand that it could be possibly trip "just like that", by picking up ambient noise,... assuming the gate was floating... but 1) it's not a FET it's a BJT and 2) its base is hardly floating... and its solder joint is good so there is a good connection to the rest of the circuit.3) The biasing voltage divider has quite high value resistors, so I thought maybe the 10M resistance of the multimeter, across that 100K resistor, might upset/load the biasing just a tad... just enough to trip the tranny. But... 10M is a couple orders of magnitude higher than this resistor, so no way, stop kidding me please...4) maybe the multimeter impedance is NOT 10M... maybe somehow it's faulty/lower than this. So I used another multimeter to measure the impedance of the first meter... 2.5M instead of 10 ! Eh, what's going on ?? So cross checked.. and the other multimeter also presents a 2.5M impedance not 10 ! Suspicious... likely it's some problem with the way their ohmmeter works. No definitive answer on this subject, but I didn't want to waste time chasing red herrings, because 1) the fact that they both read 2.5 instead of 10 likely means that they are both good and that there is a valid explanation for this unexpected reading and 2) even 2.5M should still be plenty high enough not to upset the biasing of that transistor anyway !!!So I started random experiments to try to get some clues.6) I tried touching the resistor with only ONE probe/test lead leaving the other probe floating, resting ton the bench... this way the meter would not load the biasing circuitry anymore. I first put that unique/lonely probe on the bottom terminal of the resistor, the one that is NOT connected to the base of the transistor... oh oh, it does NOT power off anymore, some change !7) did the same thing but this time probing the upper terminal of the resistor, hence probing the base of the transistor : shuts down immediately !So at this point I knew that the transistor "tripping" has nothing to do with the impedance of the meter (whatever it actually is...) loading the biasing circuit, and I also knew that it powered off only if I made direct contact with the base of the transistor.So, again the symptoms were similar to a FET with its gate floating, picking up noise, presumably from the long test leads.But again, it's not a FET and it's not floating ! Still, that was my best shot. So I did more experiments in that vein :I contacted/touched the base of the transistor with a plastic piece (the handle of an anti-static/ESD safe brush), thinking it's picking up noise, it should not pick anything via a plastic element ! Bingo : that did NOT power off the scope !9) So, metal is has to be. Then assuming it's picking up noise because the test leasds are both metallic and very long... maybe using a metallic piece but that's very short, would not trip the tranny. I grabbed a tiny/precison screw driver... touched the base, nothing ! Put some more pressure... nothing ! it took quite a few attemps to get it to trip the tranny ! So it was clearly much, muuuch less susceptible with a short piece of metal.10) I though OK, maybe it's nothing to do with the length of the metal piece, and more to do with the mass/weight of metal at play, quantity of charges at play. So, I grabbed a large screw driver... whose mass would be of the same order than that of the copper in the test lead, yet still be much shorter. Result ? It did NOt power the scope off !Bingo.... so looks like that base is indeed tripped by the test lead picking-up noise. So now the problem becomes, again : it's a BJT so much lower impedance than the insulated gate of a FET ! So how could the base we THIS susceptible to noise ! GRRRR !!!So I now was looking at the schematics, trying to figure out how to make sense of all that..... So I turned memory lane channel on, rolled back 20+ years earlier, school days, rusty lectures... what did that electronics teacher say about BJT biasing, with regard to impedance matters.... hmmm....1) The circuit must be designed so that the impedance of the base, as seen by the biasing circuitry, is at least 10 times/an order of magnitude, higher than the impedance, OK resistance here, talking DC, of the biasing circuitry. usual > x10 rule of thumb to limit loading of cascaded stages, fine.So, the biasing circuitry is just a simple voltage divider, but the resistors are quite high value, 100K and 120K, so aournd 50K equivalent impedance. So... Tek must have made it so, that the base presents an impedance of at least 500K . now that's not quite a FET gate, but that's on the high side none the less... so at least the noise pickup hypothesis is not as ridiculous as it first seemed to me at the very beginning of this investigation !2) lecture two, god only knows how I remember this 20 years later without practicing at all during all that time but : the impedance presented by the base equals the resistance at the emitter of the transistor, multiplied by the gain / Hfe of the transistor...from memory ? The resistor at the emitter is actually not a resistor but a Zener biased in reverse, which I am not mistaken, behaves like a very high impedance ?So, we have an unknow but high value impedance, multiplied by the Hfe of the transistor which is also unknown but also high since it's a smalll transistor, so let's say 100 minimum. So 100 times the impedances of the Zener.... now yes, that gives us a pretty high impedance at the base, doesn't itSo maybe yes, it is possible that it therefore susceptible to noise pick up ?If none of this lousy theory makes any technical sense, please forgive me.... but it was my best attempt...So, assuming the above is right, then I guess the transistor is just fine and should not worry about it, and I can put the scope back together with total peace of mind ?Anyway, just when I thought the repair was finished, something as insignificant as probing the voltage across a resistor finally ended up in quite some head scratching ! I like that... when things go wrong when you least expect it, and you have to pull you hair out... that's where you really learn the art, I find ! ;-)

author: Vince
author_rank: Super Contributor
timestamp: July 18, 2017, 12:33:26 am
content: Back again...I don't know if I made sense with my base impedance crap, however I think I was right about my explanation for the "doubled" trace on waveform #66, the current flowing through the primary winding(s) of the main transformer : I could not help but did what I suggested, fired up the TDS 544A so I can have a new look at the signal, and bingo it confirmed my supposition. The trace is not doubled anymore, and we can see the succession of tall and short traces, as the transistors cycle between the two primary windings.Sorry about my ugly mug reflecting on the screen. I did try and make hard copies on a floppy disk, since the scope can do that... but then realized how useless that was, considering that I don't have a floppy drive on my desktop computer to retrieve the pics ! So, sorry, old fashioned camera pictures will have to do...I think I will invest in one of those SD memory card readers that emulate the old floppy drive so that it is transparent to the scope. Someone showed me info about this some time ago, there are some "slim" units that can fit in these scopes IIRC.

author: Vince
author_rank: Super Contributor
timestamp: July 18, 2017, 10:13:51 pm
content: Quote from: oldway on June 07, 2017, 08:10:21 amQuoting an early message from Oldway here. Figured I would probe one last thing before I reassemble the scope and loose accessibility to the board.Remembered Oldway's message a month ago, so for peace of mind, not wanting to have a disaster waiting to happen again... I decided to monitor the current flowing through the FET and its coil. After all I guess that's the most vital signal to in an SMPS... See below. The 2232 is of course powered via an isolation transformer, so I could probe the voltage right across the terminals of the shunt resistor.Waveform seems perfect to me ? There are big spikes of course, every time the FET switches the current in the coil on and off, but I guess you have to live with that in SMPS designs...and the LC filtering networks on every secondary winding (but the two CRT ones), are able to very effectively get rid of those spikes, I could see that for myself when I checked all the rails for ripple : they are dead flat/"clean".So, if we forget about the large spikes, then I would say it's working just fine... You can see the current ramping up nicely/linearly as expected, during the short time that corresponds to the narrow duty cycle that the PWM chip is driving the FET at.Then again linearly ramping down.Amplitude wise, we are way below 2A so Oldway, that means I can sleep on both ears, the coil is fine and not agonizing, waiting to blow the scope again ?!Voltage amplitude averages at 275mV, spanning 250mV up to 300mV.Shunt resistor is 0.2 ohms, so 5A per Volt. So the current in the coil averages at 1.38A and lives inside a 1.25A / 1.5A. window.Today I received the 2 new Sunon fans. I compared them with the old / original Sunon fan. The noise difference is not that obvious at first, but it becomes much more tangible once the new fan get actually installed in the scope rather running in free air on the bench. I am happy with the result.Now the dominant noise is the hissing noise noise from the regulator coil. I received the replacement coil from the US (to replace the one that got damaged during transport the other day). Might try to install it in the scope to see if it's less noisy than one currently installed.One thing I did not think of/anticipate, is that the new fans did not come with a connector ! So had to cut the old connector and solder it on the new fan. Put electrical tape to insulate the solder joints, but the life expectancy of this tape, especially inside a hot PSU... is about 2 hours 3 minutes and 45 seconds, I would say ! Then the wires will short and bang... trouble again. But that was just temporary, to check how the new fan would perform in its actual environment. Now that I am happy with this new fan, I need to get some heat shrink stuff to properly insulate the solder joints... so that will delay the re-assembly of the scope a little bit.I would also like to replace the connector of the FET, the one that overheated. Its plastic did not get hot enough for it to melt or get deformed, and the contacts in it are still fine (no oxidization, not burn marks, metal looks healthy and shiny), and they are not lose... but they aren't very firm either. So if I can find a new connector, why not...Anyway, the scope is basically fixed and it's now only a matter of days before I put it back together for good. Can't wait.

author: David Hess
author_rank: Super Contributor
timestamp: July 19, 2017, 02:39:08 pm
content: Quote from: Vince on July 17, 2017, 11:36:09 pmSure it does; either set the holdoff control so triggering happens on alternate cycles or use a separate trigger channel.QuoteDifferential pair Q938 and Q939 form an error amplifier which controls the bias of the output transistors through driver transistor Q944 so the output transistors are operating in their linear mode to control the output voltage; transformer T944 switches the bias between output transistors. Primary side feedback through CR947 and CR946 controls the secondary side output voltage.I suspect hfe mismatch in the output transistors contributes more to the imbalance than the transformer and as I recall, at some point Tektronix was grading or matching the output transistors. You did not mark scale factors on your diagrams so I am not going to bother evaluating this. I consider the readout provided by the 22xx series DSOs in analog mode to be a feature; it makes documentation easier.QuoteThis point is high impedance, about 33 kilohms ignoring the base impedance, and your multimeter coupled enough signal into it to shut off Q928. See below.This circuit really should have some decoupling at pin 12 but in practice the circuit works reliably.QuoteThe multimeter's differential capacitance is 10s to 100s of picofarads and its common mode capacitance is 100s of picofarads. Special voltmeters are used when lower input capacitance is required.QuoteCharge pumping from the multimeter's ADC and input circuits can cause this problem which is why I have a couple of older "inferior" multimeters which have no charge pumping at all to use as a sanity check. This is an example of how modern technology makes cheaper test instruments in the sense of quality.QuoteThe multimeter's common terminal is connected internally to the multimeter's shielding which capacitively couples to everything around it yielding 100s of picofarads to all kinds of things. The multimeter's input terminal is capacitively coupled to the multimeter's common by 10s to 100s of picofarads. The leads themselves add more coupling capacitance.QuoteThat only applies when the transistor is operating in its linear region. Q928 is saturated with a collector current of 7.2 volt / 6800 ohms = 1 milliamp and a base current of 66.5 microamps for a forced beta (hfe) of 15. Once about 50 microamps is drawn out of the base circuit and Q928 starts to shut off which in turn starts to shut off Q930, the voltage at pin 12 drops further lowering the current into the base of Q928 and positive feedback latches the circuit off until the voltage across C925 rises above 23 volts turning Q928 back on.Because of the positive feedback and fast response time of the circuit, Q928 only has to be momentarily shut off to cause shut down and the input capacitance of the multimeter is obviously sufficient to do this. Assuming that the multimeter's input resistance is 10 megohms, put a 10 megohm resistor in series with the tip of the multimeter lead to isolate the input capacitance and then double the measured value.And now you know why I hate cheap multimeters which have an input resistance that changes with different ranges. Having an infinite input resistance on low voltage inputs is sometimes acceptable and makes the above technique easier if the voltage is in range but I wish they would indicate or otherwise mark it and have some way to disable this "feature". Oddly enough my old and obsolete DM502 has a jumper to select this feature which is almost ideal. One of Fluke's old handheld meters had the ability to select infinite input resistance mode using the controls. I do not know of any modern multimeters which allow this but that is the price of progress.

author: David Hess
author_rank: Super Contributor
timestamp: July 19, 2017, 03:03:46 pm
content: Quote from: Vince on July 18, 2017, 10:13:51 pmThere is a good chance that the spikes or a good part of them are a measurement artifact. It is difficult to probe circuits like this and get accurate results. For low impedance circuits, I often solder a coaxial pigtail to the point to be measured and then attach the high impedance oscilloscope probe via a coaxial BNC to probe tip adapter. If using a differential probe, then a short length of twisted pair transmission line can be used.QuoteKapton tape works great for this but I usually use heat shrink tubing.QuoteI would remove the connector and solder the wires directly to the transistor pins. I thought Tektronix released a change notice about doing this but could not find it.

author: Vince
author_rank: Super Contributor
timestamp: July 19, 2017, 09:04:52 pm
content: Hi Dave,Thanks again for these explanations ! Will read them again and again...I went to the lab to do some more experiments following your comments, to bring some more food to the table so to speak but.... first thing I did was try my Fluke 11 voltmeter in its low impedance mode (a few kohms from memory, can't remember), don't ask me why, even I would be hard pressed to give a plausible explanation... seems so stupid now. Obviously it shorted the base of the transistor and the scope turned off, no surprise here but.... problem is that it now will NOT power up again !! The scope keeps blowing fuse after fuse ! So I plugged my dim bulb tester, and now unlike the first time where it was "blinking"/oscillating with the SMPS... now it' s a solidly lit bulb, a dead short ! Oh no...... sorry, can't experiment further with the scope, must fix it first !A dead short... I guess it should be easy and quick enough to trouble shoot... crossing fingers. Hopefully I will have it figured tonight... I was really hoping to finish this scope by this week-end, the start of a 3 week vacation during which I was really hoping to start work on a more "serious" problem... the TDS544A now has POST error messages, acquisition problem mostly...Anyway, never a dull moment !

author: Vince
author_rank: Super Contributor
timestamp: July 20, 2017, 02:00:59 am
content: OK, 3:30AM, fixed it !Turned out the FET blew ! WHAT ?! "playing" with that little tranny to turn the scope on and off... how could that have killed the FET ?And how comes that this time, the failure was completely silent ? No magic smoke escaped, no smell, no noise no big bang, no nothing... FET blew silently ?!Regardless... this time I was able to fix the thing in record time, getting good at it !Removed fan assembly and pulled the painful heatsink again, to get access to the components on the board. lifted a pin of the shunt resistor, fed the inverter with the bench power supply... scope was fine, wonderful.So checked the regulator. Coil survived, shunt resistor as well. The FET is shorted but the gate somewhat survived : 20ohms or so. So I thought maybe the PWM chip survived too. So checked the circuitry that's between the PWM output and the gate of the FET (Q908, R909, R908, CR908), and all these components were fine. So that meant that the mains didn't pass through this section, hence the PWM chip behind it was most likely fine.So, I replaced the FET (I had ordered two of them "just in case".. glad I did ! ), taking that opportunity to get rid of its overheated plastic connector, that turned out to be so brittle that it shattered like it were a piece of glass ! So no regrets... I soldered the wires straight onto the pins of the FET. added some heat shrink tubing I bought this morning, was cheaper than I feared, so ended up buying 9 meters of the stuff, in all, in various sizes and colours.Anyway, once the FET was replaced everything was fine, phew !!!Added some heatshrink tubes to the fan solder joints too then, colour coded to boost, what a luxury... my 2232 will be the prettiest in town !Then I could resume my experiments :- variable hold-off : sorry I looked dumb ! Last year I spent some time reading the 2232 manual about the dual time-base so I could master this magical thing and be really comfortable using it. But the variable hold-off feature I had not yet investigated ... I am glad this SMPS gave me a real world example of the usefulness of this feature ! Indeed my old Hameg could display a perfectly clear trace once I had played quickly with the variable hod-off knob, lovely !As for scale in my Hameg screenshot, sorry I didn't not mention it, thought I did. The screen capture on the TDS show all the relevant information, though.200mV per division. Shunt is 0.51ohm so about 2A per volt. Waveform shows (on the TDS) 4.2 DIVs for the shorter trace/smaller winding current, and 4.8 DIVs max for the taller trace, 4.7 average, 0.5 DIV more than the shorter trace. So that gives us about 1.70 A and 200mA more for the other winding.But since you said the output transistor driving the windings actually used in their linear region hence the whole is designed to cope with the trannies dissipating power, then I guess there is no need to care any more about one switching more current than the other... it's not gonna cause any harm...Thanks again for your the circuit analysis of the inverter. Will read it 50 times...Anyway, I think I have fixed this scope properly and probed/learned quite a lot in the process... so might be time to put the cover back on,, before I blow it again (again) !Now I just need to buy 4 new little plastic knobs for the front panel, to replace those that disintegrated the other day, and I think this scope will be sorted, and I can start working on the TDS 544A now...

author: Vince
author_rank: Super Contributor
timestamp: July 22, 2017, 10:55:06 pm
content: David (Hess, of course), may I bother you one last time with circuit analysis, then I put this scope back together and I leave you alone for good I promise...I am glad my scope is fixed of course, but this all turned out to be an SMPS introduction to me, so the second point of this repair thread is for me (and other silent newbies reading this maybe), to make sure I understand the basics of SMPS operation... because there are still some basic things I don't really understand, I am not happy with that ! ;-) I would like to make sure I get the basics right. I swear I did try to understand, I am not asking to be spoon fed... but since I failed at figuring it out by myself, I have the choice between not bothering you, but remaining ignorant and not having learned as much as I could from this repair.... or asking you, and becoming smarter... so I chose option #2 !So here are my remaining questions on this circuit, ie, hight-voltage/mains connected SMPS in general, I guess. Some of the questions are somewhat inter-related.INVERTER____________1) to finish with the inverter side of things : you said the output transistors work in their linear region, and come to think of it, the waveform #65 (collector of the upper transistor) kinda confirms this, given how slowly the voltage ramps up, hence dissipating a fair amount of power. Why can't we drive the transistors more abruptly so to speak ? Is this meant to purposefully reduce the rise time, for what purpose ? Is it to generate less harmonics, so as to induce less noise/crap on the secondary windings, to make filtering/"cleaning" easier downstream ? Or could it be that the transformer is more efficient when driven by a signal that's as close as possible to pure sine wave ? clueless about transformer, so just a shot in the dark always, sorry...REGULATOR_____________There are still some basic things that I don't get about the design of this thing, I am hoping maybe you can help me understand.1) The PWM chip appears NOT to be referenced to ground. Its ground pin (pin #7) is connected to the ouput/ 40V rail... its sitting on its head, make my head spin. Why isn't it possible to simply reference is to ground ? a common ground for the input, ouput, and regulator circuitry ? Sure, the PWM doesn't care/know where its referenced... all it sees is a suitable potential difference between its power pins, its all it needs to operate I guess. But still... why not make it simple, with everything referenced to ground ?2) Generating the power supply for the PWM chip : basically it looks simple to me : we start from a mains/rectified/filtered, a nice DC voltage... quite a high-voltage sure, 300V in my neck of the woods, but still a nice DC votlage. Why can't we just derive a lower voltage from that, to supply the chip... in a simple way ? There is already this fancy high-voltage DALE 150K resistor that feeds the regulator circuitry, and we have already a Zener as well in there there. Why can't we just (silly me I guess) just connect the Zener to that 150K resistor and supply the chip with that ? Why do we need the extra complexity of Q930 switching power to the chip, itself switched by Q928 and 5 resistors around it ? Something to do with impedance ? A simple resistor + Zener would not provide a suitably low impedance to supply the chip and make it operate properly ? It can't be drawing that much current can it ? Can't find much info about it in its datasheet...3) This all "start-up" thing, where you kinda have to prime the pump to get it going, so to speak... why is it needed ? I mean, a low voltage/"local" DC-DC regulator doesn't appear to need this : you just give it it's DC input voltage, and as soo as you power it up, it gets going. Why can't the same be done with a high-voltage/mains SMPS ? The input voltage is much higher for sure, but the principle of operation is the same no ?4) Granted, we have tis start-up sequence, so let's try to see how it operates exactly... what happens exactly, at power up, step by step ? I can see on the schematic what components/paths could be involved, but saying I fully understand what happens at power up, would be a big lie....I can see the upper secondary winding of the transformer (pins 6 and 7), which provides a direct path that can feed the output voltage back to the power pin of the chip, via CR920 which presumably keeps the rectified mains from being dumped to the output in the process...The best I can come up with is this : at power up, the filter caps of the 40V rail, and C925, are discharged, so the rail is sitting at zero Volt. These caps are progressively chargin up, hence voltage increasig, as the rectified mains trickles through this speicla purpose DALE 150K resistor, R926. The ouput voltage rises, rises... and is fed back to the power circuitry of the PWM chip, via the upper winding of the transformer and the diode CR920. Once the voltage is high enough, Q928 turns on, the Zener conducts, and the voltage is then stabilized, and green flag is given Q930 "supply is good and stable, you can wake up/power the chip now ! ". At this point, the voltage would still increase until the the 40V rail actually reaches the rectified mains voltage, which would be catastrophic (but crow bar is there to protect the inverter of course)... if it were not for the PWM chip now starting up, and it regulating the voltage at 40V, keeping it from increasing to dangerous levels.Please tell me that I got at least some that kinda right (and correct the rest !), other wise I would be kinda depressed...5) On the role of the secondary windings. That is one of the main things that always made real/practical SMPS seem like black magic to me, compared to the very simple theory we learn at school, where only a simple coil is used. We have no less thant 3 windings. I guess after working on this SMPS I am a lot less inthe dark, btu still.- Upper winding : OK, so as seen above, I guess it's used as part of the start-up circuit.- Middle winding (pins 9 and 10). It's connected to nothing ?! I seem to remember someone, on this thread not sure, that is was some clever trick to help with EMI emissions/compliance ? That would explain why it's connected to no other components in the circuit. I suppose it picks up (some) of the noise from the coil, and "dissipates" it to the earth connection, and ground plane (according to the symbols I can see) . Capacitor C908 I guess is used to capacitively couple that noise, because it there was a DC path, it would "load" the coil/primary too much ? I am really out of my comfort zone again, so I am doing the best I can but probably not good enough I know...- Lower winding (pins 4 and 5). This one I can not make heads or tail about it at all. On side of the winding is connected to ground, Then CR907 keeps current from flowing from flowing from the output rail back upstream.. but why not simply make no connection at all in this case ?! So that means you would want current to flow TO the rail... but where would that current come from ? It can not come from the secondary winding, as it's connected to ground so not much happening there. So it would have to come from the main coil/primary/mains ? But there is C907 in the way ? And well, why would you want the voltage from the coil, to leak to the output, hence interfering with FET that's trying hard to regulate the voltage ? That does not look like team playing...So I could really do with some explanation about the role and operating of this winding and C907 / CR907 that seem to go with it.MISCELLANEOUS________________1) Where the hell does the hissing noise from the regulator coil come from ? I mean OK the FET is switching current abruptly in it, so generates "noise"... but the switching frequency, 60+ KHz, is well above the audible range... so if that caused/induced a corresponding mechanical vibration/sound via the pot core, then we would not be able to hear it either, right ? OK, maybe the pot core resonates with some harmonics... but these would be at even higher frequencies by definition (?) so even less audible, so again not a problem. So... is there some physical phenomenon by which the 60Khz of the current flowing the coil, could somehow induce a mechanical vibration at a LOWER frequency ?! Looks stupid maybe, but I would like to know !The humming/low frequency noise of a transformer connected to the mains, makes sense I guess, since it's running at 50 or 60Hz, so in the audible range... though not by much. But a coil switching at 60 KHz ?!I guess that goes for any coil... they also often make noise in old CRT computer monitors for example...2) In more modern SMPS, I notice they almost always use an optocoupler, so as, I hear "provide feedback to the primary side while maintaining the galvanic isolation the main transformer provides". OK but... feedback for what/where to ? so that the inverter or/regulator can adjust their voltage in a closed-loop ? The Tek from what I can see is open loop, the inverter is just tweaked with a trimmer and the tongue at the right angle, then you just hope that it stays there... and it seems to work just fine. So feedback would allow to stop having to pray, and bring peace of mind that the inverter really is guaranteed never to drift ? Isn't it a bit overkill, given that in practice, open loop operation proved to work just fine ?I can see maybe a use case where it could be useful, though ? In the case of an adjustable lab power supply, where you could have a pre-regulator that would track the (adjustable/variable at will) output voltage so that the ballast transistor always drops the minimum amount of voltage hence minimum heat dissipation. Just enough voltage to be able to regulate without risking to become unstable, and not too much voltage so as not to waste uselessly power. this way you can increase the life expectancy of the power transistor or I guess, as well as make for a more compact power supply/smaller heat sink requirement.So in this case, yes I can see that feedback would be very useful indeed. But for a PSU producing fixed/known voltages levels ??If you (or who ever volunteers ! ;-) can shed some light on any of the above, I will have learned something...Oh, and I just found one last little job to do on the scope before I can put it back together ! I almost forgot... need to rewind the thread back to the beginning : There is a filter cap (-8.6V rail from memory) on the digital board, that got caught/rubbed on to the cabinet when I slid it off the chassis. Cap stil measures fine somehow, but it got bent quite a lot, and the bodty of the cap slid off on of it's terminal... I though that would kill it, shorting all the layers inside.. but no, still works fine somehow ! So I need to replace that one, have loads of caps salvaged from....25 years ago. don't want to put a bad cap in the scope, so I first need to get an ESR meter to find a good one... but I guess they will allbe bad after such a long time, since I understand they go bad just by sitting on a shelf. However I alos understand that they can be healed/brought back to life, so an ESR would still be useful, to ensure that the healing process performed as required... or I can just play it safe and buy a brand new cap... but placing an order just for a little cap, argh.... :-/ Will check my local store to see if they have something suitable, but not holding my breath.Anyway, the reassembly is again delayed a little bit, but not by much luckily

author: tautech
author_rank: Super Contributor
timestamp: July 22, 2017, 11:15:46 pm
content: I've got a few thoughts but to start with I too would be worried about audible noise and wonder if the switching bipolars unequal current waveforms could be the cause. Time to get a matched pair ?The cap on the -8.6 rail is an electrolytic, they don't self heal. Replace it.More later..........

author: Vince
author_rank: Super Contributor
timestamp: July 23, 2017, 01:43:34 am
content: Quote from: tautech on July 22, 2017, 11:15:46 pmSeems that most SMPS make hissing noise so can't be that dangerous... but is sure is driving my ears mad....The transistor pair is not at fault I think, because they drive the main transformer and this one is quiet, and is the transformer in the inverter. When I fed the scope/inverter with the bench power supply, there was absolutely no noise whatsoever, absolutely zero.Nope, the hissing noise only comes from the coil/transformer of the regulator, driven by the FET.Maybe because the FET is switching it so hard/fast rise times, that it generates lots of energy at frequencies that happen not to suite our ears. Still, how can a coil driven at 60KHz, end up producing mechanical waves in the audible range. I am truly interested in learning a well grounded technical explanation. I want to understaaaand what's at play here !I also noticed something "interesting" : from COLD, the hissing noise is very high pitched and very loud, but over the course of a 2 or 3 minutes, the noise progressively decrease in amplitude to a much, much more bearable level. The sound frequency also get a little less high-pitched.So I take it that the SMPS warming up has some effect on this coil, direclty (the coil itself warming up internally), or indirectly (warm up affecting the surounding circuitry which in turn would affect the coil's operating conditions) though the exact mechanism is still to be determined...QuoteI might be mistaken in some way then, but I gathered that fans of ancient Tektronix tube scopes from the '60's routinely "revive" their 50 year old electrolytics.I also recently came across this video I found very interesting :It's up to date, ( a year old), an engineer from from "Kemet", a major capacitor manufacturer, giving a talk about the most common types of capacitors, to an audience of H/W design engineers. It's 50 minutes long but I found every second of it very interesting. Worth taking the time. However you can get straight to the part where he talks about electrolytics, if you seek directly to 16minutes and 35 seconds or thereabout. Lasts for 5 or 6 minutes.The guy clearly explains the mechanism by which elecgtrolytics self destruct "by design", teh second they leave the factory, hence why the have a shelve life. He also explains how/why they also do self heal. Later in the talk he also talk about self healing in the case of tantalum capacitors.So from what I understand, the wear out mechanism that invariably destroys all electrolytics (the commonly used/wide spread ""wet" electrolyte type at least), is as follow : the dielectric is a thin oxide layer grown on the aluminum anode "plate". Physics make it so that the electrolyte actually "eats" this oxide layer, the moment the cap is manufactured, even when in storage. When in use, this effects is also present and even amplified, and the higher the applied voltage, the faster it east the oxide layer/dielectric. So, that's why "derating" is common place when using electrolytics : it slows down the process, increasing the life expectancy of the caps. It's a way to buy some time, short of being able to actually avoid the problem completely. Derating is commonly 50%, meaning it is advised to use a say 16V cap when dealing with an 8Volt signal. In the case of the filter caps on the digital board, Tek used 35V caps for the 8.6V rail, so quite an enormous derating ! Helps to make them last.He also explains how chemical reactions in the cap, make it so that the capacitor actually indeed heals itself. The thing to do is simply to slowly ramp up the voltage, to let the reaction take place progressively. If you apply the operating voltage all of a sudden, as when in power up the instrument, then of course it can be damaged because if it's too far gone, the dielectric/oxide layer is so thing, or completely eaten, that the cap is effectively shorted, so a current surge will damage it at power up. But if power it with a small voltage, and limit the current, then progressively increasing voltage, then you avoid current rush piercing the anode, and you allow enough time for the chemical reaction inside, to progressively re-grow the oxide layer to it's original thickness, and the cap is good to go again.Aside from that, I found it particularly interesting, among many other things he talked about, to understand how electrolytics managed to achieve such high capacitance. it actually kinda relates to why the caps "die" : the high capacitance is achieved by increasing the effective contact area by etching the oxide layer, which created hills and valleys in it, so to speak. This means that when in a "valley", the oxide layer is actually, locally, much thiner than in other places, meaning the electrolyte will be able to eat through it even faster.This talk is really packed with interesting and practical information, I find. Well at least to a newbie like me, I gleaned many interesting pieces of information throughout the video.Anyway, I checked the website of my local shop, and they have a broader choice of electrolytics than I imagined. I was able to find something suitable. Same package size, capacitance and voltage rating. Actually the package is slightly smaller, 5mm in diameter (same height). If I take the same diameter as the original part (6+ mm ), then I corresponds to a 47uF cap rather than the original 33uF. Can't be a bad thing for a filter cap, to be a big bigger, eh ? So I bought that... but also the small package so I can put the exact same value, should there be a reason not to use a larger value.The replacement will also be improved with regard to temperature rating : the Tek parts were rated only at 85° (despite being close to the power hungry acquisition chip, and zero airflow in the area.). Replacements parts will be rated at 105°. Tihng is, the shop ha da much borader choice of caps in the 105 rating. The appropriate cap was not available in their 85°C range.Obviously, I anticipate I will get some unknown/cheap brand, not the original Chemicon, but I guess a brand new "noname" 105°C cap is better than a 30 year old Chemicon.Since they are dirt cheap (10 cents a piece) I ordered 10 of each (33uF and 47uF), so I can replace all the electrolytics on the digital board while I am at it : only 6 of them, all filter caps, all the exact same specs.I should get them in the post in 2 or 3 days max, I would imagine. When I ordered Zener diodes from them the other day for the regulator, they were very quick.So hopefully in a few days the scope will be re-assembled.

author: tautech
author_rank: Super Contributor
timestamp: July 23, 2017, 03:00:07 am
content: Yep, seen the Kermet vid, it's very good.You and I have just a different terminology, for E caps most call it reforming where you call it healing, a term IMO moreso applies to Tants and Polyester caps.No biggie.For the prereg then some temp dependency could be attributed to old E caps............I've seen LV stuff warm considerably and then not trip overcurrent protection. You'll ESR tests to confirm this.

author: Vince
author_rank: Super Contributor
timestamp: July 23, 2017, 03:25:45 am
content: Quote from: tautech on July 23, 2017, 03:00:07 amOops yes, you might be right... I came across both terms and thought they were used interchangeably by people, but now you mention it yes "reforming" refers more to a human being actually putting the cap through a process, out of the board, whereas "healing" might refer more to the chemical reaction that happens naturally in the cap, during its normal operation, in circuit, without any particular intervention from outside. Thanks for clearing that up, I will be more accurate next time ;-)QuoteHey good idea ! Yes, I definitely need to get an ESR meter quit soon... I am working more and more on fixing old test gear so it's quite hard to do a good/serious job of it, without an ESR meter to check all the caps. I admit...I did have a quick look at the options on Ebay fairly recently, but not much I fancied. There were either things in kit form but I prefer a finished product. The cheap oens look ugly. There is a little one that looks real cute but costs 150 buck from memory, not gonna pay that for a simple ESR meter... come on.Of course, long term, I plan to get a proper bench RLC meter, but even old stuff costs 500 buck or more, so can't afford it at the moment.So I am thinking of a modern/new handheld RLC meter, as a compromise. I need to see what's available on the market at the moment, price/performance/feature wise... I am hoping I can get something decent for 100 to 150 bucks, will see.

author: tautech
author_rank: Super Contributor
timestamp: July 23, 2017, 03:47:19 am
content: The $20 LRC one is pretty popular, there's a thread on them in the TE board.This one:https://www.eevblog.com/forum/testgear/$20-lcr-esr-transistor-checker-project/There's a few variants of this on eBay and Aliexpress.

author: David Hess
author_rank: Super Contributor
timestamp: July 23, 2017, 05:43:56 pm
content: Quote from: Vince on July 22, 2017, 10:55:06 pmSince the transistors are operating in their linear region to regulate the output voltage, they are already "inefficient" and a fast switching time is not required. The slow switching time also contributes to a low noise output which is important in this application.Note that this is a saturable core inverter. Saturation of T944 is the only thing controlling the switching frequency. Normally the base control winding would come from output transformer T948 but by using a separate and smaller saturating transformer for controlling the transistors, the peak flux and losses are lower and flux leakage is minimized. Among other changes, later versions of this power supply did away with T944 and clocked the inverter synchronously with the preregulator.QuoteThe regulator controller is referenced to the output voltage so that no level shifting is necessary to control n-channel MOSFET Q9070 and the output current can be measured directly via R907. The output voltage however *is* referenced to ground via voltage divider R915 and R914 which connects to ground. The +5 volt reference creates a current of 5/10k=500uA through R915. This current is balanced by a current through R914 when 500uA*84.5k=42.25 volts exists between ground and the output.QuoteThe drive to Q9070 also requires considerable current.QuoteIt is much more efficient to power the regulator from a separate low voltage winding on the inductor and this also provides for safer operation by inherently shutting the regulator down if the output voltage cannot be maintained by for instance excessive output current. Practically all modern off-line switching power supplies work this way also for the same reasons and the really high efficiency ones disconnect R926 once they have started.QuoteNotice the phasing marks on the inductor windings. Tektronix calls it a transformer to to my mind, it is really an inductor. Pin 6 goes positive with respect to pin 7 only when pin 4 and 8 go positive with respect to pin 5. So CR920 only conducts when the transistor switch is off and the inductor is discharging into the output via CR907. Further, the voltage across pin 4 an 8 to pin 5 is a constant 42.25 volts (actually 42.25 volts plus the voltage drop of CR907) so the voltage across pin 6 to pin 7 is also constant and depends only on the winding ratio between the two windings. So when the output voltage is in regulation, the regulator's supply voltage is also in regulation. At startup, the output voltage has to reach 42.25 volts (or close enough) before C925 discharges.QuoteThat is right but CR920 is reverse biased the whole time so the winding is isolated while C925 charges.QuoteQ930 is actually just powering the entire regulator controller. Pin 12 is the power pin for U930.QuoteU930 draws more power supply current through pin 12 and pin 11 to drive Q9070 than R926 can supply so the voltage across C925 drops until the output voltage rises high enough for CR920 supply power to the regulator circuits.QuoteExactly, modern regulators have this winding also.QuoteThat is my understanding; this was a clever way to meet EMI requirements by driving an opposite current into the chassis for cancellation. I suspect this has to do with C903 and C902. Later versions of this power supply did not require this.QuoteThis power supply design is unusual and I have trouble visualizing it as well.If the output of Q9070 went to ground and the controller was referenced to ground, then this would be a flyback regulator and C907 would not exist. By referencing Q9070 to the output however, both phases of the switching cycle contribute to the output. Further, now windings 1-2 and 5-4,8 can be 1:1 and C907 added so that the leakage inductance spike from winding 1-2 get redirected by C907 to the output. This lowers the peak voltage seen by Q9070 so a lower voltage part may be used.It could be considered a floatingCUK converterwith the output shorted. I wonder if Tektronix might have done this to avoid the patents on the CUK converter which existed at the time.QuoteI think you ruled out damage to T906 with your current measurements across R907.How stable is the switching frequency? The lack of decoupling at pin 12 of U930 bothers me.Hissing is common with constant off time switching regulators but this one is not that and I suspect it could indicate a problem with unstable operation of U930. It will be a difficult measurement to make but I would try to take a close look at pin 3 of U930 to see what the error amplifiers are doing. Pin 3 is high impedance so it will need to be buffered and of course it is about 50 volts above ground further complicating things. Pins 5 and 10 might also reveal what is going on.Magnetostriction in inductors and transformers can make them noisy but they are often designed with dampening materials to make them quiet.QuoteExactly and the last evolution of this power supply design dropped primary side regulation for optocoupler feedback. It is worth noting that optocouplers are notoriously unreliable. Maybe newer ones are better but old ones have an appalling failure rate and they are one of the old parts I tend to replace spontaneously.QuoteThe regulation is still closed loop but feedback it is from the primary side of the inverter. CR946 and CR947 rectify the primary side waveform and that is compared to the 6.2 volt zener reference via differential pair Q938 and Q939. The forward voltage drop and temperature coefficient of CR946 and CR947 roughly compensate for the forward voltage drop and temperature coefficient of the rectifier diodes on the secondary side. Since the diodes roughly match and the peak voltage on the primary side of the transformer is proportional to the peak voltage on the secondary side, the secondary voltages can be regulated by sampling the primary side voltage.QuoteMaking a variable output voltage and current switching regulator adds a lot of complication.This power supply is so unusual because it requires lower output noise than normal and had to be made with the low performance switching transistors available at the time. There is probably something to be learned on this account by studying the designs it led to.QuoteI suspect you are talking about 33uF C9006. I would probably replace it with a 10uF 16V solid tantalum capacitor but if I did that, I would probably replace the other bulk decoupling capacitors on the storage board as well. If I had to order parts, I would definitely replace them all.ESR meters are very useful although I get by with an old style impedance bridge which makes measurements of capacitance and dissipation at 1000 Hz. It is great for evaluating capacitors but time consuming to use. I am inclined to believe that there is a market for a dedicated handheld capacitance meter which measures capacitance, ESR, D, and leakage at the operating voltage but I have never seen such a thing. Dielectric absorption might be fun also but is probably too specialized.

author: David Hess
author_rank: Super Contributor
timestamp: July 23, 2017, 05:48:50 pm
content: Quote from: Vince on July 23, 2017, 01:43:34 amThere are no good reasons to reform or attempt to repair a dinky inexpensive 20 cent 33uF low voltage aluminum electrolytic capacitor. For immediate replacement if I did not have one, I would just a part from my collection of parts that have been used for prototyping or pull one off of a parts donor. If I was refurbishing a 2232, I would replace them with solid tantalum capacitors.

author: Vince
author_rank: Super Contributor
timestamp: July 23, 2017, 11:39:06 pm
content: David, thanks a lot, again, for taking the time to write all this, was most interesting as always.Seems to me that SMPS design is really a very complex subject if one really needs/wants to take it seriously and really master it.For a "general purpose" design engineer, or a serious hobbyist, I think the best we can expect is to get a general knowledge of the various types and configurations that exist, principle of operation, most common tips and tricks, taking some basic measurements like I did, so that we are able to trouble-shoot/repair a defective unit, or maybe slightly modify an existing/well documented design, to be able to adapt it to slightly different requirements.But as far as mastering the thing in such fine detail as to be able to actually design a complete SMPS from scratch, make it reliable, make it meet all applicable regulations, take legal responsibility for it, and actually charge some other company for your design/product... I think that's an entire world altogether !I mean. Picture the average modern engineer, fresh from school. His first job will likely be to work on some modern digital stuff. He will spend all day long putting big digital chips together, like Lego bricks : FPGA, 32 bits MCU, RAM and ROM, and fight his compiler to try and get Linux to run on his embedded product.... doing this for 10 years. The guy is therefore doing more "computing" than real electronics. Then 10 years later his boss suddenly tells him : "hey kid, I decided I should not put all the eggs in the same basket. I decided the company should diversify and get into analog design as well. I have just secured a contract with Tektronix to design the SMPS for their next generation 40,000 USD fancy Spectrum analyser, how cool is that. So you better get reading fast on SMPS stuff because Tek gave us just 3 weeks to show them a working prototype. Here are the specs they want, get started kid !!! ".Nooooo... this just can't happen... I hope so anyway !I mean, sure, whacking tiny/simplistic/minimalistic local low voltage DC-DC converters on a digital board, with just a tiny well crafted 8 pins chip with its built-in high performance FET, a small coil and a couple filter caps at the input and output... no worries. Digital boards have these spread all over the placeBut designing the mains SMPS for the entire gear, from scratch, with zero prior experience ? No matter how clever or talented the guy might be, I don't see that as a reasonable expectation from his boss...So... I guess I understand why, starting with the early '90's by the looks of it, (when gear started to enter the highly digital age) just about any test gear manufacturer moved to third party power supplies. And it is my understanding that the company supplying these SMPSes, do nothing but SMPS design. It's become a highly specialized job, so requires highly specialized engineers, that do this and only this, alllll dayyyy loooong...Quote from: David Hess on July 23, 2017, 05:43:56 pmHard to give numbers, but from what I recall when I looked at it on the scope, it was perfectly stable, signal was not jerky at all.QuoteI guess it wouldn't be too much work whacking some cap in there just to see if it makes any difference...QuoteMight try to probe for that, while waiting for the new filter caps to arrive, for the digital board.QuoteAlmost, I am talking about his brother, C9007 !QuoteI sticked to old style electrolytics because it was the original part and I don't know enough to feel confident replacing it with a different type of cap. But since you are confident, then yes I am all for a change, if it means better performance and/or long term reliability : no leaky electrolyte corroding the PCB traces (what ruined my TDS 544A !) , to start with...Come to think of it, yes it should be alright : just about every modern digital board I see now, professional/expensive stuff at least, now have boards populated with numerous yellow tantalum caps (SMD type of course), the industry must have moved to this new "standard". I still do see bunch of electrolytics in modern stuff but only in cheap consumer gear, for example in that little 'DVB' TV tuner I opened up the other day. It was full of them.OK, so let's go with tantalums ! Will have to be the "dipped" /leaded type so that I can re-use the existing through-hole pads. I can't solder SMD tantalums on that board. Not without it looking ugly, at least.Luckily my local shop has some dipped tantalum available, that was not a given...However they cost an absolute fortune ! The same votlage and capacitance rating, 33uF / 35V then, costs a whooping 3,50 Euros ! The electrolytics I have ordered from them yesterday cost only 10 cents ! That's 35 times more, oh !However if we lower the voltage rating to 16 volts as you suggest, which is half what Tek did, but which is still x2 the operating voltage (it filtering the 8V rail) so still comfortable... then prices drop significantly ! A 33uF 16V is only 70 cents. Still 7 times more expensive than the electrolytic mind you, but at least it's back to the tens of cents range, phew...So OK, let's got tantalum, and let's derate less than Tek did, but what I don't understand is why you want to replace their 33uF caps with 10uF ones ? Surely they must have determined that they needed 33uF not 10. OK maybe just like the voltage derating, they played it extremely safe, too much maybe, so we can afford to lower the value and still have a perfectly reliable scope... but the point is : we don't KNOW that, it's a gamble, for what benefit ? The 10uF tantalum is only 30 cents cheaper (40cents a piece ) than than 33uF one (70 cents)... and shipping is 3,50 Euros ! So why gamble ?QuoteYep, I later posted (when chatting with Tautech last night) that I would just as well replace all the filters caps on that digital board, especially since there are only 6 of them and all the same specs, so money is no concern here.QuoteYep would love to be able to buy such a thing indeed !Failing that, I will get a cheap ESR meter to get me going short term, then invest some money later on when I can afford it, for a good/proper bench RLC meter. Will most likely be again an old '90s unit so might need some repairing of its own too ! LOLQuoteYes sure ! LOLAs I said (I think ?) the people who do that, I gather (might be wring again) are generally owners of vintage/classic tube/valve Tektronix oscilloscope, who are trying to revive the existing caps so as to be able to preserve the "originality" of the scope as much as possible. So in this case it's not guided by pure/cold technical reasoning as much as it is driven by a will to preserve a historical piece of equipment for posterity. Well, I can understand that, no worries.But in the case of my "modern" 2232, for which replacement caps are still available and for cheap, then yes, no reason whatsoever to be dicking around, it's a no brainer.

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2017, 01:35:05 am
content: Quote from: Vince on July 23, 2017, 11:39:06 pmAs if a switching regulator was not complex enough already, there is definitely a step up in difficulty when a switching regulator has to operate directly off of the power line which typically means either 150 to 370 volts for a universal input or 305 to 370 volts for a voltage doubler input. The 2225 series of oscilloscopes which followed the 2235/2232 series use a 60 Hz transformer to provide isolation followed by a switching regulator. The 2335 series of ruggedized oscilloscopes which preceded the 2235/2232 series used a linear power supply.The 2235/2232 switching power supply design accumulated various improvements when it moved to the 4 channel 22xx series and the 24xx series of analog and digital oscilloscopes and these later designs were more reliable. Based on various changes over the 2235/2232 series production time and the alternate design in the 2225 series power supply, I get the feeling that Tektronix was not completely happy with the 2235/2232 series power supply.QuoteOff-line switching regulator design really is a specialized field but most products these days use a third party power supply brick or module.QuoteThis is common enough to be a known joke. After the digital guys finish, they throw the design over the wall to the guy responsible for the power supply and regulators with a tiny bare area of the board left over for him. One of the lessons is *not* to hand the power supply design over to the new engineer assuming that it will be the easiest part.QuoteWe also saw a lot of "cookbook" designs coming out of the semiconductor parts industry from companies like National and Linear Technology including some cookbook off-line designs using standardized magnetics.QuoteMaybe but on the other hand, messing with it risks causing damage and you have it apparently working at this point except for the hissing.QuoteThe general rule that I use is that a good aluminum electrolytic capacitor needs to have 2 to 4 times the capacitance of a solid tantalum capacitor that it replaces. This is to keep the ESRs roughly equal.QuoteI have a local shop with inexpensive NOS (new old stock) dipped tantalum capacitors so if they are that much more expensive for you, just use aluminum electrolytic replacements.QuoteSee above about aluminum electrolytic versus solid tantalum ESR. These capacitors were sized for ESR and not capacitance.

author: Vince
author_rank: Super Contributor
timestamp: July 24, 2017, 02:05:54 am
content: Quote from: David Hess on July 24, 2017, 01:35:05 amWell that's quite interesting ! So the sizing of filter caps, as I learned it at school, is mostly relevant when used behind the main bridge rectifier in a linear power supply.. but for "local" decoupling on individual boards, ESR is the most important factor. I thought that's why there was this common practice of associating a big electrolytic cap with a small 100nF "plastic" cap, for better high-frequency/transient response ?I guess using a tantalum kinda make for an "all in one" compromise : ESR low enough to be able to get rid of the extra 100nF cap, along with a capacitance high enough to be able replace the old electrolytics. OK, so basically in modern designs they use tantalum to kill two birds with one stone, as well as increasing long term reliability I guess.OK so no worries, I will order some dipped tantalum right away ! Might even e-mail the shop to see if they can modify my previous order (which they have not yet processed, of course, as I placed it during the week-end...) to add the tantalum caps to it, so that I don't have to pay for shipping twice.Well, come to think of it. Since you said the ESR decreases as the capacitance increases, does that also hold true for tantalum ? In this case wouldn't it be worth using 22uF or 33uF caps instead of 10uF ? I mean, the price difference is not that great, it's not a factor... so no need to be cheap here. The 35V ones are expensive, yes, but the 16V ones, that we are going to use here, are much cheaper :http://composants.e44.com/composants-passifs/condensateurs/condensateurs-tantale/16-v/#toplisteSo if we can make that precious ESR that little lower still, why not !

author: Vince
author_rank: Super Contributor
timestamp: July 24, 2017, 07:41:02 am
content: I think I may have (?) unintentionally mixed two different things above, ES-R and ES-inductance, maybe. Was 5AM when I posted that so I hope I will be excused...

author: David Hess
author_rank: Super Contributor
timestamp: July 25, 2017, 03:11:00 am
content: Quote from: Vince on July 24, 2017, 02:05:54 amThese are very different capacitor applications. Input capacitors are selected for energy storage to produce enough holdup time between power line cycles. Decoupling capacitors are selected for impedance which primarily depends on ESR. Output capacitors need low (and often controlled) ESR but may need to be selected for ripple current in high current applications.QuoteSolid tantalum capacitors have better high frequency characteristics than aluminum electrolytic capacitors however they are still usually used in combination with ceramic or film decoupling capacitors and in practical designs, the distance between the decoupling capacitor and circuit is very important anyway. Notice on the storage board that only one bulk decoupling capacitor is used per supply voltage while lots of small ceramic decoupling capacitors are distributed around the board.Tektronix used aluminum electrolytic capacitors here because they were cheaper and sufficient.QuoteThere is no reason you cannot do that except for the increased cost. The larger capacitance part will also have a higher ripple and surge current rating leading to higher reliability in some specific cases but probably not in this case where the voltage derating is enough. There might be some performance benefit since this board is a mixed signal design but I doubt it. In my experience, the 2232 achieves unusually low noise in digital storage mode anyway; it is not unusual to see +/-0.5 LSB or better performance.

author: Vince
author_rank: Super Contributor
timestamp: July 25, 2017, 11:45:56 pm
content: Quote from: David Hess on July 25, 2017, 03:11:00 amI was goingto say they were miniature tantalum, being yellow and in a dipped package, but of course it would not make sense. BOM is there to remind me...So, these tiny cute yellow dipped are ceramic not tantalum. I always thought Tantalum had the monopoly for yellow dipped packages, somehow ! LOL [hiding in the corner]QuoteI guess even expensive gear need to be pragmatic and count beans if the company making them is to make a profit...QuoteI just received the news caps in the mail. That was quick, happy with their service...I actually ordered both 10uF and 33uF, not knowing what your reply might be...In the end I ended up using the 10uF ones, simply because they had the correct 5.08mm pitch, and had a lower profile which made them unlikely to ever kiss the case while opening the scope, which looked like what caused the death of that electrolytic. The 33uF ones had their legs spread too far apart, and bent outward at sharp angles. Bending them back to a narrower pitch proved to be ugly and stressed the leads. So, 10uF it would be. Replaced all 6 of them of course.Then powered up the the scope, gave it a quick sanity check in both analog and storage mode... had somehow to again tweak a couple trimmers there, nothing that worried me that much (vertical a tad, and horizontal offset a fair more bit, not sure why). All was fine. Then screwed the back plate to the underside of the digital board, and secured the digital board back down to the chassis, quick check again, still fine, then put the scope cover back on, been a long time ! Getting there... I thought. what a fool. Then checked the scope again, for the 3rd time in 5 minutes... still fine in analog mode... then switched to store mode, uh what's going on ?! No ! Moving the trace up and down produces a funny result ! It does move no worries but there is some "ghosting" for want of a better term. It produces a vertical "comb" pattern, like a paint running on a wall or something ! Weird.Made a clip of it to illustrate :https://youtu.be/YKeMm_kueGkSuddenly it reminded me seeing a similar behavior a year ago, when I acquired that scope and played with the store mode to get a feel for it.Then a couple minutes later, the "comb" was gone and the trace was fine again, phew... then 2 more minutes later.... back to troubles : the trace this time had completely vanished/disappeared from teh screen ! Shows up perfectly fine in analog mode, no worries, but as soon as I switch to storage mode, the readout show up just fine no problem there, but the signal trace is nowhere to be found ! As if it's brightness control were turned all the way down, and/or the scope was not triggering properly... except I don't think it's any of that since the "trigger'd" LED on the front panel is appropriately lit (and it just uses the same settings as the analog mode anyway, which does work/display the trace just fine).So... I think... this scope.... despite all the time and effort I have put in it in the last 6 weeks... does NOT like me ?! Can a scope ever show GRATITUDE to its loving and caring owner ? .... grrr.... or maybe I am misinterpreting his wil... maybe he does love me, so much, soooo much that he is afraid that I might stop looking at him the moment he is healthy again ?! I didn't think psychology had a place in electronics...Anyway, I am puzzled ! Will I ever see the end of this repair ?I am trying to see the bright side of things (scope SMPS fixed, "working" again)... but I am finding it hard presently...Sooooo.... back to school again. What could be wrong ?Logic has it that the problem is related to the last thing I touched... the caps ? Hmmm... don't think so, because of everything you said, and also of course because as I said I tested storage mode right after replacing the caps, and all was fine.So my second best hypothesis would relate to that X offset trimmer. I had to adjust it, quite a bit (was a full division off) but I didn't get WHY it was that I had to touch it... why would it have moved during my repair ? I didn't touch it or even think about him, and I don't quite see how I could have moved it by accident !I also found it strange/worrying that I had to rotate that trimmer all the way to the left in order to adjust the readout&trace, and even then it was still a tad not quite there yet, so had to finish it off by slightly adjusting the analog control counter part on the front panel, which I didn't judge quite normal...So me thinks, maybe the trace disappearing might be the X offset circuitry going bad. Maybe that trimmer is defective, or has bad solder joints, goes open circuit and that makes the trace fly off the screen all the way to the left or right.Will have to open the scope again and check the trimmer up close. Will also check the schematic to see how the X offset circuitry is put together, see if my theory would be possible at all.While I am doing that, if anyone has any ideas about this strange "comb"/ghost pattern when I move the trace up and down ? Maybe a know problem, or a common issue in digital scopes at large, of that era, all brands included ?So again.... NEVER a dull moment with that scope !!!

author: Vince
author_rank: Super Contributor
timestamp: July 26, 2017, 12:36:15 am
content: Below an extract of the circuitry around the X_OFFSET trimmer (sheet #16). Indeed looks like an open circuit/nad connection there, would send it off scale, as it's connected simply across a 5V reference. Simple enough to trouble shoot then... I should be able to get a voltage ranging from 0 to 5V as I move the wiper.Then that voltage goes to sheet #19 (attached below), to the "X VECTOR" input circuitry. Basically it just goes to an op-amp input so I guess the troubleshooting ends there !Might check a bit further down that vector chain though, while I am at it, as later in that pipeline there is yet another trimmer : "X VECTOR".I I have not fixed it by that point, then I could unfold the story a bit further : after the XOFFSET, after the X VECTOR, there is the X amplifier section, with a differential pair which I guess produce the actual signals driving the CRT deflection plates, or quite close to that anyway (going back to the analog/main board at least, which we know work fine). So I guess that's my whole "X"/horizontal chain right there... just need to follow it until I find where my trace is hiding !

author: Vince
author_rank: Super Contributor
timestamp: July 26, 2017, 03:27:04 am
content: OK I did it again.... 5AM ! This scope just won't let me go to bed, what a selfish piece of .... gear.Made some progress I think. X offset trimmer seems fine in fact, red herring I think. Now I think the problem is more to do with the vertical side of things :I have got the trace BACK ! I got it by turning the ANALOG vertical position control almost all the way ! The Vertical offset trimmer on the digital board won't do anything, probably because the trace is so far away ! Way out of adjustment range. So, I fed the scope with a sine wave (5Vp-p, 1KHz). Then probed the input signal on the digital board, with my old Hameg scope (which I isolated of course), right at the ribbon cable connector/input of the front-end. I put the alligator clip of the ground lead, near the connector, on the chassis. Then probed one of the signals in the differential pair... any of them should show my sine wave anyhow. Not too clued about a potential DC offset the differential lines might sit on, with respect to ground, but I figured it did not matter : I should still see a bloody sine wave in there. I set the Hameg to AC coupling to make my life easier, no sine wave to be seen. So switched back to DC and set vertical gain so I could see the entire/absolute maximum voltage range the differential line might present, ie +/- 8,6V, since that's what the analog rails are on the scope.So, result ? Still no sine wave. Instead, a DC signal sitting at +4V, with random square pulses (impossible to trigger/stabilize the trace) going about 1 or 2V (can't remember) negative relative to that 4V base level. So, no sine wave, but DC with some (presumably, for now) digital crap finding its way on top of it !So if the front end of the digital board sees basically 4V DC, I guess that's way out of range and why the trace was way of the screen, and why the offset trimmer on that digital board, couldn't get the trace back in sight. So it's starting to make some sense now, what do you think ?!....So I am going to bed now, but tomorrow I guess the next course of action will be to figure out if the problem comes from the digital board/front end, or if it's the main analog board sending crap though that ribbon cable ! Some service manual reading is on the cards I think...This scope is showing it's age I think, it's like an elderly.... no matter how easy he takes it, it always hurts one way or another. A joint aching, legs suddenly not responding without warning and falling in the stairs... it's just a random succession of misery no matter how much you baby it, no matter how careful you are with it...Still, I don't give up, I don't give up.... not sure now if I will ever manage to get it to work 100% ever again, getting depressed a bit, but I will keep trying, if just because it's a good for developing trouble-shooting / repair skills which will be useful to fix other gear I plan to buy later on... and because I can't afford to buy another 2232... which most probably would need fixing as well anyway, so might as well fix mine !Oh boy.....

author: Vince
author_rank: Super Contributor
timestamp: July 26, 2017, 05:20:53 am
content: 7 AM, still up and thinking of that scope... this thing will kill me no doubt.Just had a quick look at the schematic for the analog front end of the digital board, attached below.I see that there is a 4.3V Zener diode at the input, reversed biased between ground and the 8,6V rail. So by the looks of it, presumably, it's there to provide a DC bias for the differential signals to ride on. So, that would explain the 4V DC I am seeing on my Hameg when I looked at the differential lines at the connector. It's normal then. So the crap/negative random pulses that ride on top of that 4V DC level... is supposed to be my sinewave ? Gee... takes smore than imagination to believe these square looking pulses could be my sien wave. OK, I guess if the sine got amplified and/or clipped, somehow, it could make the sine look squarish. But at least it would remain a periodic signal and I should able to trigger on it.. but that's not the case, these pulses look random and jump all over the place.

author: tautech
author_rank: Super Contributor
timestamp: July 26, 2017, 05:26:37 am
content: Get some sleep man or you'll be no good to anyone.

author: David Hess
author_rank: Super Contributor
timestamp: July 26, 2017, 11:23:28 am
content: Quote from: Vince on July 25, 2017, 11:45:56 pmThe "comb" is completely normal when the oscilloscope is using equivalent time sampling at faster sweep rates like 0.1us/div. Instead of waiting for a full display record, the oscilloscope updates the display on every acquisition.QuoteIt has to be something which affect the storage display and not the readout. See if the reference display works; you will have to save something to it though if the reference memories are empty.Under the Advanced Functions menu, there is a diagnostic menu for testing various functions.

author: Vince
author_rank: Super Contributor
timestamp: July 26, 2017, 09:24:29 pm
content: SUCCESS !!!!After some sleep... things became clearer, I guess...The more I fiddled with the scope, trying to make sense of its behavior, the more crazy it acted ! It got to the point where the readout itself, not just the trace, would get shifted upward by several divisions, and at some point the readout would form a CIRCLE and dance all over the screen, psychedelic ! Has to be seen to be believed ! LOL The time base/sweeping sometimes also decided to go haywire.Anyway, at this point it was acting sooooo weird, that I decided to stop wasting my time chasing red herrings all over the place... I thought it must be something stupid simple, a bad connection somewhere. I eventually found the faulty bugger : J9011 !It's the big connector near the hinges in the middle, with nothing on it but 8 black beefy wires. One side of the top part of it, was cracked and so was not making a firm/good contact anymore. Still, on that type of connector it should not matter this much, since the top part is mostly useful during the assembly of the connector, as it pushes the wires into the terminal so they can cut through the wire insulation and get to the copper. But once this is done, contact is made and the top part could as well be discarded ! Well, exaggerating a bit of course, but that's the idea. So I still wasn't sure that this connector was the problem, especially since all these thick black wires sure had to be ground, so why would it make the scope go crazy if a wire or two, out of 8, were not making perfect contact ? Hmmm. So, removed the top part of the connector altogether. Then realized that the wires were NOT at all firmly in place ! Even though they were sitting inside the terminals, the insulation of the wire had become quite hard/stiff, due to the heat generated by the board I guess. So although the wires were in place... it didn't take much from my part for them to lift out of the connector !!!STILL ! If these were all ground, it should still be making enough of a contact for the board to work, I thought ! So... could it be that, maybe.... just maybe.... these black wires were NOT all grounds, could there be actual SIGNALS in there as well ?! Highly unlikely, but at this point it was my best lead so I followed it... looked up the schematic, it's connector J9011. Still no signal there, but MORE than ground though ! this connector carries all 3 the power rails ! Why would Tek make them ALL black ?! Just so people would make mistakes like I did ? Was color THAT expensive an option ?!Anyway, now knowing that all the supplies were in there, of course all was starting to make sense at long last !I could not reassemble the connector of course, and I didn't want to anyway, as I had seen that the stiff wires were the real root cause of the problem, more than the connector itself. So I didn't want to be dicking around, I needed to make a reliable connection to all these wires if I wanted to make further trouble-shooting any meaningful. So, I didn't cut corners : I pulled all the wires from the connector, and soldered them straight to the connector terminals, on the underside of the board.That fixed the problem, scope works like a charm ! And with such a good connection, I now have the peace of mind that I am no longer at the mercy of some random glitch in the future ! At least not due to this crappy connector...HAPPY I AM, fixed a last !!!!!!I would of course like to replace that connector if it's a standard item one can buy off the shelf, but I can't find J9011 in the parts list! Not in the A10 section any how. Maybe in some other place but god knows where....But for now, it works just fine. The original connector was NOT removable any how (the removable part is on the main/analog board, not on the digital board), so having the wires soldered to the board actually doesn't change a thing to the original setup really, from a practical point of view....So if you know/recognize this type of connector and know where to get one, I would replace it, but for now I will call it done and put the cover back on ! Crossing fingers that this time, THIS TIME, putting the cover back on will not give rise to yet another weird problem !

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 26, 2017, 09:31:24 pm
content: will you promise to get some sleep now?

author: Vince
author_rank: Super Contributor
timestamp: July 26, 2017, 10:52:18 pm
content: Yes I will !This 2232 put me under a lot of pressure because it s my main scope, it was vital for me to fix it really as fast as possible.But now that this is done, what a relief !!!I will just need to buy 4 little plastic knobs for the front panel, to replace the ones that disintegrated when I tried to pull them off, and I will be done I thinkI can now move on to the next repair : moving up league in complexity : the TDS 544A ! This is my second scope, the big boy, so I definitely need to fix it too, but the pressure is not as great as the 2232. I will start working on it in the coming days (another thread soon to come then), but this time I will not loose sleep of over it. I will take it easy. Will work actively on it because I am in vacation so have lots of time, but I won't let it pressure me. If I see that it's starting to drive me nuts I am starting to loose sanity... I will just take a little break and switch to some other gear repair or project, have a few on-going things, always something to do in the lab

author: Vince
author_rank: Super Contributor
timestamp: July 27, 2017, 02:14:01 am
content: Well I am stupid... I just said the problem was with the wires, so replacing the connector won't get me very far. I would need to replace all the wires so that the new connector would have a good "bite" on some fresh/softer wire insulation. But replacing the wires would mean replacing the connector at the other end of the cable, on the main board... this one I don't know how I could replace it, tooling wise...So I would have to re-use this connector, by cutting all the wires and soldering new/fresh wires onto it... might look ugly ! Or maybe not, especially now I have bought plenty of heat shrink tubes. Color as well ! hmm.... replacing the wire would also allow me to use color wires to show clearly where the grounds are, and differentiate the various rails. That would be a nice upgrade for future servicing... me thinks ?!So well yes, might do that at some point... but not now. Finding a new connector might proved challenging I don't know. I don't want to spend my holly days on this ! So I will put the cover back on for now, move to the next repair/project, and when/if I can find a replacement connector, I will re-open the scope to do the upgrade. Will be quick any way, once the cover is off, there isn't really much that needs to be disassembled to perform this little job, if anything. Just removed a few screws to be able to get the underside side of the digital board to do the soldering, and that's it.

author: tautech
author_rank: Super Contributor
timestamp: July 27, 2017, 02:31:16 am
content: Be sure to take measurements before you put the cover on.Pin pitch, height, OD, ID etc.And pics.....if your memory is anything like mine.

author: Vince
author_rank: Super Contributor
timestamp: July 27, 2017, 02:44:26 am
content: Yes good idea !I was relying on the fact that I would eventually find the technical details of this connector somewhere in the parts list, but if that fails then yes I guess that if I take all necessary measurements maybe I could find a replacement if I spend enough hours reading dozens of connector datasheets...Either that or... I could just remove the connector from that board altogether, so I have it handy ! After all it's not even being used anymore now ! Just sitting there on the board doing nothing...

author: Vince
author_rank: Super Contributor
timestamp: July 27, 2017, 08:08:37 pm
content: OK found it ! The block diagram and schematics all talk about 'P' or 'J' 9010.... but on the actual connector itself is printed "W" 9011... "W" that's how they refer to "W"ires ; they don't supply the connector itself, but only the cable sub-assembly, made in-house. This one I could at last find int he parts list ! :It's an 'IDC' type connector, made by 3M, and wire gauge is 18. Pitch is 0.156 .... I guess unit is imperial/inch, meaning it would be 3,96mm which appears indeed to be a standard pitch, and consistent with what I measured with the calipers.Problem ? Spent some time on Farnel and Mouser but I can not find anything that looks close to that ! Looks like these types of connectors are put in the "Wire to Board" category. In this category there really isn't that many 8 way 3,96mm pitch connectors...Surely it must be somewhere, I just can"t find it that's all... might post on the forum to get help identifying/finding this little bugger. I can't possibly be defeated by a bloody connector eh ?! How sad !So looks like I have no choice but indeed leave these wires soldered directly to the board and put the cover back on to the scope. At least I tried...

author: Vince
author_rank: Super Contributor
timestamp: July 28, 2017, 07:55:09 pm
content: Oh no !NOOOOOOOOOOOOO !!!Yes, you guessed it ! Put the cover back on, and the problem in store mode (only) are back again ! Weird behavior involving vertical offset on way or another, and signal trace not showing up, just a flat line.At that point I was close to either committing suicide.. or murder, couldn't make my mind up so I decided instead to get back to work...Again I voted for a bad contact somewhere. At least I knew the power supply wires were fine since I soldered them straight to the board... but what about the connector on the other end of the cable, on the bottom/analog board side of things ? Looked in perfect condition, but wiggled the wires anyway, no joy.Unplugged/plugged all the ribbon cables from the hinge side of the board, no joy.So I thought hell, some manage to fix expensive gear that display weird faults, just by re-seating some chip... so I thought why not, worth a try at this point, I am so upset with this thing that I am willing to try anything. So did that. Started with the 2 EPROM chips for the CPU... just for heck of it, because obviously the CPU side of things was working just fine anyway. Next on the list, and more likely to be involved in this analog related series of events/problems : the highly sensitive stuff... the 2 ADC hybrids, and the all mighty acquisition chip where all the black magic happens. All 3 in fragile/brittle ceramic packages, all with with lots of pins that hold them firmly in place... in short, a recipe for disaster ! But I had to try something ! So as you can imagine I used extreme precaution, went very progressively and slowly, and lost a couple pounds worth of sweat in the process ! But... all went well, miracle, what a relief !And it was worth it because the problem is fixed (again), back to life !... until I put the cover back on I guess ....But before I do that, there is still a little problem to fix, something new. Not too worried about it since the scope works fine basically, but still I would like to try and fix it if possible.Problem : at power up, the scope always displays an error message:- - - - - - - - - - - - - - - - - -POWER UP FAILUREPRESS SETUP BUTTONS TO CONTINUECMOS: recovered- - - - - - - - - - - - - - - - - -Now I am not sure about the CMOS thing... will have to check the manual.The backup battery is still good, 3 Votls no problem, so the SRAM should be alright even when the scope is powered off.I get that message each and every time I power up the scope, even though I have now fixed all the previous problems.No fault code shows up on the 7 segments display on the digital board.So I ran the various self tests. See below. It reports an error about the trigger system, but maybe it's just because you need to apply a signal to the inputs of the scope so it has actually something to trigger from ! LOL So I will check the manual to verify this and see what signal I am supposed to feed exactly, so that the trigger test can be performed.That being said... when I selected the A/D tests, it DOES tell you that it requires an "appropriate" (their wording not mine...) ramp signal... so if they take the time to ask for it in the A/D test, why wouldn't they also ask for it in the Trigger test, hmmm....Anyway, I am off to read the service manual to check for that.Oh, and the memory tests are funny : there are 3 tests : ROM0 and ROM1, both passed (I does not much more than scanning them quickly to verify the checksum) and an NV RAM test, which report : "sorry, NV RAM not implemented" . You bet ! No NVRAM in my scope, just a stand alone battery ! Does that mean that some revisions of the 2232 did come with a fancy NVRAM ? I guess so...I notice there sme jumpers on the CPU side of things :- WDT : "Watch Dog Timer " I assume.- RESET : I suspect I might have to play with this one to clear that bloody CMOS error... will have to read the details in the manual- NORM : Normal running mode ?- DIAG : DIAGNOSTICS mode ?The latter does not have a jumper on its pins, so I assume it works in conjunction with the "NORM" jumper. Jumper must be set to either setting.I wonder what magical things I may have access to to if I power up the scope with the jumper moved to the DIAG position, mmmm.....Any way, all thoughts are welcome, as usual !

author: David Hess
author_rank: Super Contributor
timestamp: July 28, 2017, 08:56:52 pm
content: Quote from: Vince on July 28, 2017, 07:55:09 pmPage 6-17 of the service manual says that the "NV Ram" test was not implemented. On the 2232, the NVRAM is implemented on the storage board and all 2232s include it. On the 2230, the NVRAM was included as part of the RS-232 and GPIB option boards.The same area of the manual discusses the power up tests and what display they produced. I think yours means that an error was found in the NVRAM but recovered. You could try erasing the contents of the NVRAM at the Reference - NVMem menu. Also go to Advanced Functions - Save Setup menu to configure the startup as default or power down.Or just disconnect the battery for a bit so that the oscilloscope powers up and reformats the memory.

author: Vince
author_rank: Super Contributor
timestamp: July 28, 2017, 11:22:17 pm
content: Hi Dave,Scope is fixed, errors cleared. As expected it was minor.Unplugged the battery as you suggested, worked for the CMOS error.For the Trigger test, it was just me not understanding the instructions given on the CRT : set trigger MODE to "BOTH". There is no such thing as a "MODE" control in the Trigger section of the front panel, and if there was, then a "BOTH" option would have me puzzled, how could you possibly trigger from both channels at the same time... I guess with some imagination, we could trigger on some logical combination of the signals: "Ch1 OR Ch2", but that's far stretched and best left to digital power houses like the TDS 500/700 series... they have this kind of stuff.So, had to be either "BOTH" Input channels activated, or "BOTH" time bases. Tried both options (pun intended), turned out I satisfied it by enabling both time bases. No trigger error anymore. Read the manual section 6 as you did, and it's not clearer there either. Also what's not clear is the A/D test. CRT instructions ask for an "appropriate" ramp signal, without giving actual amplitude and offset nor frequency.... and the service manual doesn't give any more detail either. So... I would love to feed you with whatever signal it is that you require for this A/D test, my dear little scope.... IF ONLY you would be so kind as to tell me WHAT IT IS exactly that you want ! LOLAnyway, I put the cover back on.... and it STILL works !!!...though it did scare me a little (he likes to scare me, you must have figured this by now...), because before putting the cover back on, I checked/adjusted one last time the Y_OFFSET trimmer of the analog front-end, as a better safe than sorry measure, so that the "ground" levels were matching perfectly between analog and storage mode. But once the cover was back on and I gave the scope and quick test, horror.. the trace in store mode was a couple small divisions below zero ! NOOO !!! But... after a couple minutes warming up (?), it progressively settled back to where it should be !So that's one fixed scope, I say !About the damaged connector, I posted on the forum about it, and no luck ! So indeed looks like it's hard to find... I took close up pictures, and took all necessary measurements... in case I come across something suitable in the future.And still 4 little knobs to replace on the front panel and that will be it.On to the next project... and a BIG thank you to all who helped me through this 7 weeks, 6 pages long repair !Learned a lot in the process, and even got to build a cute little dim bulb tester !

author: tautech
author_rank: Super Contributor
timestamp: July 28, 2017, 11:59:17 pm
content: Quote from: Vince on July 28, 2017, 11:22:17 pmDo a Google search for the Tek part # as there's a few sources for the common Tek knobs.

author: Vince
author_rank: Super Contributor
timestamp: July 29, 2017, 12:44:37 am
content: Parts list says 366-2049-01 .Checked the Sphere and Qservice. The latter has it listed but none in stock, as for the former, that have a different part number for what looks like the same knob, but at 15 USD EACH? and I need 4 of them, plus expensive shipping to France + duty taxes..... that's gonna be for X-mas....So I will get some knobs, now I know the can be found.... but not right now..... not at this price

author: tautech
author_rank: Super Contributor
timestamp: July 29, 2017, 01:06:27 am
content: These ?http://www.repair--parts.com/Industrial-Metal-Machines-Parts-/CNC-Equipment-Parts-/CNC-Wood-Router-Parts-/6-pcs-tektronix-tek-knob-cover-366204103-366-2041-03.php5Email them.

author: Vince
author_rank: Super Contributor
timestamp: July 29, 2017, 01:31:43 am
content: Yes that looks like it... will e-mail. Weird site though, looks like a '90s website lost in cyber space... the guy at the other end is probably dead by now.... or am I just being a bad mouth... will see !

author: Vince
author_rank: Super Contributor
timestamp: July 29, 2017, 01:38:47 am
content: E-mail sent... felt like sending a bottle to the sea !!! ....

author: Vince
author_rank: Super Contributor
timestamp: July 29, 2017, 01:49:47 am
content: Just read the ad more closely... it mentions "QService" !!! So, this mailing-list ad must have been how tehy started teir business 20 years ago.... but now they have there own website... can't believe ther server hosting this mailiing-list still has not shut it down, must have been inactive for many many years now, probably !Still, was worth a try....So I know now that I can search for 3 different part numbers which all correspond to that same knob :366-2049-01 or -03 or 366-2041-03On Qserice I see there is also the this knob :http://www.qservice.tv/vpasp/shopexd.asp?id=1902Looks like the same one I need, but without the "bar". I guess I could use that as a fallback if need be. I mean I don't care about the bar, I have never looked at it in practice ! I couldn't care less. When I grab these knobs I hardly look for the bar.. I just get hold of the knob and turn it in whatever direction, and whatever amount, is required to get the trace where I want it to be ! I don't care about the bar...Better no bar than no knob at all, IMHO !

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2017, 02:04:42 am
content: Quote from: Vince on July 28, 2017, 11:22:17 pmPage 6-20 describes the test signal. 25 kHz 6 divisions and then double the amplitude to overscan the display. Set the timebase to 10us/div.

author: Vince
author_rank: Super Contributor
timestamp: July 29, 2017, 03:08:12 am
content: Oh no, typical ! That was on the last page ! The only one I did not read ! All teh others tests were described one by one, foloowing the order in the Diag menu, so when I saw that the A/D one was skipped, I thought well too bad, and that's it, I did not read the last page thinking maybe it may talk about it !Bad boy Vince, bad boy....I just printed the procedure and rushed to the bench to try it out, of course !Procedure says you need a x10 attenuator which I don't have, a 50 ohm terminator which I don't have either, and a Function Generator with minimum 99% linearity, which I probably don't have since my Generator is an old 1970's basic analog Philips unit ! Never had a very stable amplitude nor Frequency. As for linearity well....Still, it was either that or nothing. So I fed its 50ohm output straight to the scope, no attenuator, no 50ohm load. Adjusted the frequency as close I could to 25kHz, using the 2232 cursor measurements because I don't have a counter either ! LOL Yes, still lots of gear to get for my lab...At least I could adjust the amplitude just right.Well I guess the attenuator they ask for might be because the FuncGen tehy ask to use, a Tek FG503, might not have a built-in attenuator to output the required amplitude. But my Philips unit has both 20 and 40 dB attenuators available, so no worries, I switched the x10/20dB one, that will do.Can't do much about the 50ohms terminator though... I just don't have one.Then I ran the A/D test and hoped for the best ! Worked just fine ! On both channels, I get either 0 missing code, or one single missing code, about 50/50% distribution. I put that on the horrible output of my generator. Even by eye, the waveform was jumping all over the place (frequency not stable) and signal was very noisy (due to the required very low amplitude I assume). I turned on the averaging which cured both problems, giving a stable and clean signal.. but I guess the A/D test processes the "raw" input data, not the averaged signal... So in these conditions I find it quite excellent to only have one missing code, and only part of the time !So looks like this scope is working like new !I am so glad to have it back, and feeling better than ever !BTW Dave, I gather from your previous posts here, that you have several 2XXX scopes ? My 2232 is my only one of this era so I can't compare, but I am wondering about the big time base knob : I find it extremely "stiff"/hard to turn.. especially compared to that of my old Hameg scope which is absolutely feather light in comparison, like your typical detented rotary encoder, say, to give an idea. So the Tek being so stiff, makes the knob feel a bit "spongy", every time I turn it, I am scared that it will (again) slide on its shaft and the dial loose track of things again ! Admittedly that knob has not one but TWO screws, 90° apart, to secure it to the shaft... but still, even though I tighten them quite firmly, it still doesn't feel "right". Is this stiffness normal on these scopes ? Or is it just the switch getting stiffer due to age, and I could make it a bit smoother with some lubricant/contact cleaner spread liberally in it ? Or is it just the knob that could need to be replaced altogether ?... now that I know of Qservice and Sphere, I guess if need be I could order a "new" one.I just wish Tek had machined a key on the shaft, or splines, or something.. ANY thing.... but not relied on a smooth/slick shaft5 AM again ! LOL but this time it was not because I HAD to, it's because I CHOSE too, quite a difference !Going to bed anyway...

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2017, 09:48:47 am
content: Quote from: Vince on July 29, 2017, 03:08:12 amThe display is 25 counts per division so the digitizer has a range of about 10.2 divisions. The setup procedure is to start with a centered 6 division triangle wave which you can see and then double the amplitude to 12 divisions so it covers the entire digitizer range.So for an informal test, the attenuators and terminations and 10mV/div and 20 mV/div ranges are not really needed.QuoteI only have 2232s and 2230s but I've worked one some of the others.QuoteIt is not the knob but the cam switch which is causing the problem. When the timebases are locked together, the horizontal control should be stiffer than the vertical controls but not spongy. Individually the timebases on mine seem about the same stiffness as the vertical controls.The cam switch bushings are just plastic and can be oiled but getting to them is not trivial. They might have split in which case they can be sanded down a little bit and then lubricated but that requires removing the cam.

author: Vince
author_rank: Super Contributor
timestamp: July 29, 2017, 04:05:08 pm
content: Quote from: David Hess on July 29, 2017, 09:48:47 amHi Dave,Terminators definitely not needed indeed, since I don't have any, but the amplifier range most definitely needs to be at teh requested 10mV/div. I took the picture at 20mV/div so the waveforms fits on the screen, but running the tests like this results invariably in 100 missing codes or so. See screen capture below. Looking at the pattern the test returns (with flat lines at the start and finish), I guess these correspond to the top and bottom of the screen where no signal is present at 20mV/div. Switching to 10mV makes sure that the signal covers the entire height of the screen, and some more, so that the test doesn't risk missing codes because a signal that might not span completely the viewing area.QuoteYes you are right, if I pull the knob to unlock the second time base, the knob feels perfectly fine, same as the vertical amplifiers.OK so the stiffness in single time base mode is mechanically normal/explainable, I will stop worrying then, it's not going to fall apart any time soon...QuoteAh thanks, so I do indeed have problem there... see the video clip I just made a minute ago to make myself clear (hopefully) :https://youtu.be/Pi-mr1mysAoThere appears to be some play in the knob, like 2 or 3 of the splines that are molded into the button. When you grab it and go rotate it, the shaft does not move immediately, it does not feel connected to the button... even though I have tightened the two screws really well.So when you turn the knob it doesn't inspire a lot of confidence, it feels like every time you go to turn it, it might suddenly spin round !So maybe the button is actually well secured to the shaft, and it's the shaft itself which some play inside the switch mechanism. In other word the mechanical assembly is worn out and it's not repairable.... or with difficulty.Oh and the scope did it again : when I turned it on it was already in store mode and... the triangle was again not centered properly, again like yesterday, it started 2 small divisions below zero/ground level... and just as yesterday, after a couple minutes it progressively climbed to end up spot on in the middle, where I set it to with the trimmer before putting the cover back on.So it must indeed be a warm up "issue", in quotes... A practical real world example that yes, when test gear manufacturers tell you all the time to let the instrument warm-up before attempting any measurements.... it's not all bullshit, they designed the thing so they know a thing or two ;-)Anyway, was nice to witness a practical example of this, something you can actually easily see.So, to sum it up, I would say this repair is done for now, but that there are still some things I might do later on, to polish it up even further :- Take a deep breath and get to the time base switch assembly to see where the play comes from exactly, and if there is any way/hope of fixing it, so that it feels "just right" and become even more of a pleasure to use.- Replace that bloody power supply connector on the digital board, if one day I come across that type of connector- Hissing from the regulator coil : didn't do the measurements Dave asked for to see if the hissing was inherent to the coil, or if it could be a symptom of something not working quite right... so might come back to the SMPS later on to do that- Have a spare coil now (and still the project of winding my own !) so could try it to see if it makes any difference.- Then I could replace each and every electrolytic cap in this SMPS, not just the big ones on the secondaries, but all the the smaller ones in the regulator and inverter section. One of them might be bad and contribute to the hissing by some mechanism yet to be determined. The smaller value ones I could replace with tantalum capacitors maybe, for better performance and reliability, to even further reduce that hissing noise maybe.- I could record that hissing noise and run an FFT on it with the TDS 544A, once I have fixed it, and fixed the firmware issue which causes it NOT to have FFT enabled even though Tek's catalogs of the day clearly say that the 544A had FFT as standard. Could be cool to see the spectrum of that hissing noise, might give a clue as to what is happening in the regulator, and we could maybe correlate it to some ripple voltages probed here and there. I don't know... could be fun and educational.Of course the "soft" FFT on scopes suck big time compared to a proper 'spectrum analyzer, but since we are in the audible range here/low frequency range, and we don't expect or require ultra high performance in any way shape or form.... well maybe the soft FFT of the scope will be good enough to actually see something useful. If not, I might repeat the experiment later on, once I have enough money to buy a real/proper spectrum analyzer. I have gathered by now that the "big" ones, that go up to 1 or 2 or 22 GHz, suck at low frequency, and that for very low frequencies they made dedicated "audio" spectrum analyzers. I guess they are bound to be cheaper.. so maybe I could hunt for one of those to start with, then get a "big one" later, to cover the higher frequencies, when I have saved some more money.So in short... there is ALWAYS something you can do, always something to fiddle with and learn from, I like that, brain never ceases to be stimulated !

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2017, 05:15:01 pm
content: Quote from: Vince on July 29, 2017, 04:05:08 pmMy point is that you could do the test at any vertical deflection sensitivity as long as you can produce 6 divisions of amplitude and then double it. Switching from 200mV/div to 100mV/div or 2V/div to 1V/div would work. Or start with a properly terminated x2 coaxial attenuator and then remove it.Tektronix could have recommended using a 5 division peak-to-peak signal and then switching from 50mV/div to 20mV/div or removing a x2.5 coaxial attenuator. What really matters is that the waveform exceed the entire ADC range of 10.2 divisions of which only 8 are visible by some amount.QuoteThat is way more play than it should have. On all of mine, the horizontal timebase controls are stiffer when locked together but each have the same play whether locked or unlocked.Note that when the controls are unlocked, you can still turn the A sweep control by reaching around the B control knob and using the transparent skirt which conveniently has that raised thingy to push.It may be best not to mess with the cam switches but I would undo the set screws and remove the knob to find out if something is going on there. Perhaps look for a cheap 2232 timebase board online to mess with and get an idea of how the mechanics work.QuoteIs the warm up offset different in analog and storage modes? The worst of my 22xx oscilloscopes has a warm up drift of about 1 or 2 minor divisions in the vertical signal path and for the others it is smaller. Also the entire CRT display shifts by a similar amount during warm up but that is not unexpected.

author: Vince
author_rank: Super Contributor
timestamp: July 29, 2017, 05:54:31 pm
content: Quote from: David Hess linkSorry for the misunderstanding, we were thinking alike in the endQuoteAh thanks, so there is definitely room for improvement there !QuoteHey that's an idea ! Sure would be nice to do that, will try and keep an eye open for such a thing on Ebay... if I can find something cheap enough, will go for it.QuoteYep noticed that. it did save my beacon at the beginning, when the timebase was behaving erratically... I somehow didn't even think of readjusting the knob and tightening its screws, and jumped straight to the "the mechanism itself must be to blame" conclusion... grabbing the skirt was then the only way I had to actually use the scope at all ! Eventually got tired of it and decided to open the scope to have a close look at the switch assembly, then realized only the screws needed tightening, then put the cover back on, turned ti on and.... the SMPS blew and here I was starting this thread !QuoteI have not noticed any offset in analog mode. At power up the trace is centered and stay there. It's only in digital storage where it starts one to tow small division below the center line, before settling to correct level after 2 or 3 minutes. I guess the thermals of the scope maybe at play. I mean, the analog front-end on the digital board is in a place which I guess must be much hotter than analog circuitry at the bottom front of the scope. So I guess it makes sens that any thermal induced offset would show up quicker and more prominently on the digital board than on the analog board ?!QuoteHave not noticed that so either it must be minor on my scope and/or, most probably... I just have not been using the scope often enough and for long enough for me to realize this ! .... built the lab only 9 months ago, still building it so haven't really had time/opportunity to start using any of my gear for any amount of time, really. For now I am constantly hunting for old gear that might be of interest to me, then I dismantle them to seee how they are put together, learn from that, clean them thoroughly, refurbish all the little things that need attention, and trouble-shoot/repair them when need be. It's time consuming (and I have other things to do as well). I am getting there progressively, though ! I think in a year or so, the lab should be in a decent shape and I could start actually doing something with it, plenty of mod or design projects on my to do list.

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2017, 06:55:40 pm
content: Quote from: Vince on July 29, 2017, 05:54:31 pm1 or 2 minor divisions is not enough to worry about.QuoteI noticed it from the shift in the readout position which is more apparent in storage mode because there are more readout elements. Sometimes the text lines up perfectly with the graticule and sometimes it does not.I forgot about it until just messing with it but the secret menu has an option to clear all of memory and restore defaults. To get to the secret menu:Press the “ADV FUNCT” button once.Press the “SAVE REF 3? two times.Use the cursor control.One thing I miss on the 2232 is having the little "Tek" logo in the lower right hand corner of the readout like the 2230 has. I thought the secret menu might have an option to turn it on but it does not.

author: Vince
author_rank: Super Contributor
timestamp: July 29, 2017, 07:28:42 pm
content: Quote from: David Hess on July 29, 2017, 06:55:40 pmOK so the scope is perfectly fine then, I am just being a bit too critical I guess ! LOL Still, after adjusting the trim pot, it's now spot on once warmed-up, so quite happyQuoteQuoteHmm... yes, this I might have noticed indeed... I thought it was being temperamental, but well if your examples do it too then I will consider normal and not loose sleep over it.QuoteOh yeah I know about this menu, entered it 3 days ago when I posted here a picture of the Tek bug and wizard... but I did not pay attention to the other two options avaialble in thie menu ! Clear all MEM as tyou say, and also some option to set a straing/label for a "plot" something... not sure what it refers to. Maybe some text add to the waveform, when you ask for an XY plotter output. I am very find of XY plotter and it was one of the reason why I ghot attracted to the 2232 model, am I not ashamed of admitting !So I plan on hunting a little/cheap XY plotter just so I can hook it to the 2232 !I saw videos of this on youtube, works reasonably well if you don't ask the 2232 to plot the waveform too fast. Luckily you one can adjust the speed at will, ISTR.QuoteWell that's one project for you then : read the contents of the two EPROM containing the firmware, disassemble it, reverse engineer it, and I guess with sufficient time (10 years of hard work ?) it might be possible to modify the firmware to add what you desire, without ruining the scope. What do you mean, it's not worth the time and effort ?!In these days of open software and hardware, how great would that be, if Tek and other major brands, would be so kind as to make all their documented source code available on their website, for all their out of warranty products !

author: Vince
author_rank: Super Contributor
timestamp: July 29, 2017, 08:00:04 pm
content: Bench was a bit of a mess after nearly 2 months working on that scope ! Now that the repair is over, did some cleaning/house keeping.Put all the remains from the scope repair in a box (fans, news components for the regulator and caps for the storage board, remains of the old coil, plus the 2 new ones), along with all the notes I wrote during the repair, some data sheets. So, everything is being archived... will come handy next time I have to put my nose in there !Obviously this very topic is an archive in its own right ! Along with all the pics and notes I wrote on the computers, and lots of datasheets.The "Tek 2232" Chapter is now closed, until further notice !Was quite a ride, my first repair of a complex/professional piece of test gear was not without its ups and downs ! But I sure learned a lot. Most important thing is likely to be : PERSEVERANCE ! Good things/outcomes rarely come easy... you have to earn them one way or another...

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2017, 08:05:22 pm
content: Quote from: Vince on July 29, 2017, 07:28:42 pmThe hidden plot label function adds a custom line of text to the plotter output.At least twice online, I have run across hardware HP plotter emulators which accepted RS-232 and produced graphics files with a specified resolution on a memory card which could then loaded into a computer. A software application could do this very easily as well and I used to use one which accepted HPGL.QuoteI wonder though why they removed it. Maybe they ran out of ROM space?

author: Vince
author_rank: Super Contributor
timestamp: July 29, 2017, 08:55:30 pm
content: Quote from: David Hess on July 29, 2017, 08:05:22 pmThat sounds quite clever !I won"t be needing this on the 2232 though, seeing as it has a GPIB port and waveforms can be retrieved this way.I am presently trying to setup the GPIB stuff in the lab, hoping to get something working while I am in vacation. Zero prior experience so the learning curve takes some time.QuoteEPROM space sure would have been at a premium ! Now that said, by teh sound of it the bitmap picture was probably very small so maybe they could managed to cram it in to the EPROM(s) if really tehy cared to. Never seen this logo in action, care to power up one of your 2230 and post a picture for me to see it ?

author: Vince
author_rank: Super Contributor
timestamp: July 30, 2017, 02:00:30 am
content: Come to think of it... GPIB sure allows to read stored waveforms but ... the "plot" function is different since it also includes the readouts. I am not sure GPIB can get that... would have to read the manual closely to figure out what GPIB can do exactly....

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2017, 07:02:13 pm
content: Quote from: Vince on July 29, 2017, 08:55:30 pmThe text is written using vectors although I have no idea how they are encoded on the 2230/2232. With some cleverness, each vector can be implemented with 3 bits. (1) I think the late 7000 series digital vector readouts and the 2230/2232 readouts have the best look.I just replaced the batteries in my camera so here are 2230 screen photographs of analog and digital mode.Differences between the 2230 and 2232 include:1. The 2230 has no graticule illumination.2. The 2230 readout does not display the trigger level.3. The 2230 has the Tek logo in the lower right corner.4. The 2230 does not implement 20 MHz bandwidth limiting in DSO mode. (2)5. The 2230 does not store its configuration in NVRAM. Settings are always restored from defaults on power up.6. The 2230 lacks the rather clever "smooth" mode available on the 2232 when using peak detection. Tektronix might not have thought of it at the time but also the 2230 has a slower processor than the 2232 likely making it infeasible. I usually leave smooth mode on the 2232 disabled for performance reasons.7. The 2230 has a very different DSO interface. The 2232 interface is recognizably like more modern DSOs. Part of this is simply because the 2230 lacks the 5 buttons under its CRT.(1) See the change notice for the readout on the 7904A oscilloscope for details on 3 bit vector encoding.(2) Like most analog oscilloscopes, the 2230 20 MHz bandwidth limit is implemented after the vertical channel switch and this is after the trigger and digital pickoffs. Tektronix recognized this as a problem in a combined analog and digital storage oscilloscope so the 2232 fixed this by implementing the 20 MHz bandwidth limit in the vertical preamplifiers before the trigger and digital pickoffs. After studying the various ways to implement switchable bandwidth limiting, I get the feeling this this was considered a hard problem up until the 2235 series including the 2230 and 2232 which make it look trivial and that the secret was later lost. The modern designs I have seen are worse.

author: Vince
author_rank: Super Contributor
timestamp: July 30, 2017, 07:34:57 pm
content: Thanks for the piccies Dave... and for thje good into as usual !I always wondered what exactly differenciated the 2230 form the 223é, aside from the missing bezel buttons which was the only obvious thing I could see on pictures. Now I know !Strange about the missing Tek logo, maybe it's just some random marketing decision they wanted to try out, then every body complained and they put it back...Maybe they thought it was too intrusive, was interfering with the trace/signal too much, so they removed it. Then later marketing said No, we need to have a logo present at all times, so later in the TDS series they put it back, but this time in the upper left corner where it doesn't get in the way so to speak.I don't know, there must be reason, but we will never know !