HP 1727A oscilloscope with multiple issues. - Page 1
Forum section: electronics / repair

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: April 20, 2021, 09:01:38 pm
content: Hello people of the EEVBlog forum, I am new here, this is my first post. I should start by saying that I am a beginner, or just above beginner level so bear with me here.The background here is that a few months ago I bought a used HP 1727A dual channel storage scope on ebay with unknown issues, the seller had pictures of it powered on, but when I got it it the screen did not light up, long story short it arrived in the box with more issues than when it was put in the box. It turn out that the no screen issue was caused by power supply issues and a secondary issue on the "storage board" I fixed those problems, there were several shorted transistors, but a couple of major problems remain, both of which I cannot trouble shoot om my own, that where the forum comes in,The first thing I would like to tackle is the channel inputs, channel A works kind of (although V/dv is way off), and channel B does not work at all. The puzzling thing is that while inserting the test signal from the scope into channel A, a trace shows up as normal but then there is a second smaller trace in the middle of the screen in a arch shape. I attached a picture of it, I don't know how to insert pictures in here yet.I have done some probing around with my USB scope and I have some ideas on what the issue could be but I don't know enough to make any conclusions. The other problem I think has to do with the CRT, or at least it shows symptoms of another problem, I will explain it more once I get some pictures of it but for now I want to focus on the channel issue.All you more experienced people out there feel free to point out any mistakes I make, I am here to learn and to hopefully fix my scope along the way. I am a freshman in college, and I hope to major in electrical engineering.Thanks,Alex Baker

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: April 20, 2021, 11:04:44 pm
content: Common problem when one channel is not working is that the attenuators or the input transistor on the dead channel, probably a dual FET, is blown by someone putting too much power into the scope. Happens all the time.I would check that.Don't know what the goofy trace is. Maybe the "Delayed" trace??

author: cosenmarco
author_rank: Regular Contributor
timestamp: April 21, 2021, 12:17:57 pm
content: Congrats, I think you got yourself a very nice scope! You can read a bit about it here:https://www.hpmemoryproject.org/wb_pages/wall_b_page_02.htmReading the service manual will also be a great learning exercise. Read it thoroughly! Those engineers knew quite well what they were doing back in the days.Disclaimer: I don't own this scope, so I'm just giving some generic advice / guesses about what I would do.The scope is an analog storage scope so you have potentially 20+ signals and voltages coming into the CRT which is a challenge by itself.Also the usual stuff: This appliance has potentially lethal voltages in it. Before touching metal parts in it which are not connected to ground already, you should always discharge the capacitors and the CRT. Document yourself about how to safely do these things without damaging the instrument.I think you should focus first on getting channel A to work properly end-to-end so that you have something working to compare to when fixing channel B.The signal showing up multiple times could be as simple as an issue in the time-base or in the blanking circuit or could be as complicated as an issue with the storage mesh.I'd suggest to leave all the storage features turned off for the initial troubleshooting.The vertical stage in this scope is also somewhat complicated because the CRT features a transmission-line deflection plate.So in short: after ruling out all possible power supply issues I would look in this order at:- Try triggering from an external source, follow trigger signals inside the scope and try to compare with expected signals in the service manual- Look at the vertical output signal on the back on the instrument with another scope- Look at the signals before the final vertical and horizontal final stages and try to follow the signal back to its source- Look at the vertical delay line which drives the transmission-line vertical plate- CRT blanking signals (Page 8-16 and 8-17 on the service manual)- CRT storage meshes signals (even if turned off there may be faulty components / switches / whatnot which play a role in that area) after you ruled out other problems

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: April 21, 2021, 04:19:18 pm
content: Update: This morning I spent about 20min fiddling with stuff on the front panel and I discovered that channel B works fine, though it still has the extra funky trace. I just had to change the vertical display and trigger source buttons, i'm stupid.Also I just realized that the user manual that came with my scope is missing section 4, which has a bunch of trigger information in it, oh well, I can get it from online. I need to look up how to use the external trigger since I have not used a scope like this before.I got some pictures of the odd CRT behavior I am talking about, If you look at the pics, you can see a dark region in the upper right hand corner. It slowly grows from the corner until I assume it would cover the whole screen, although I have never had it on long enough to do that. Additionally you can see that the dark area covers up the good trace, only leaving the weird trace behind. I will focus on the second trace issue first but I figured I would post some pictures of the screen problem when I had them.

author: cosenmarco
author_rank: Regular Contributor
timestamp: April 21, 2021, 06:15:02 pm
content: Good news with channel B!What you captured in picture is most probably related to the storage mesh which is probably not driven correctly.I would solve the problem with the phantom trace first before looking at the storage functionality. Just make sure storage is disabled for now. I think you have to press the button at bottom left corner of the screen so that "display" led is lit.I also read the following from the manual: interesting

author: cosenmarco
author_rank: Regular Contributor
timestamp: April 21, 2021, 06:18:02 pm
content: Could the phantom trace be also be a malfunction of the "beam find" functionality? What do you see when you press the "beam find" button ?

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: April 22, 2021, 02:01:39 pm
content: When I press the beam find button it just scrunches everything down and in towards the center of the screen.I agree with your idea that there is an issue with storage mesh. When I was repairing the power supply I traced a short on the +120V rail to the storage mesh output amp on board A17, where I replaced three transisors that tested as resistors on one of those cheap component testers. The ones I replaced were Q4, Q5, and Q6.Through some more messing around, either I dint know how to use the storage function or the storage mesh is not working, both cases are quite likely.

author: cosenmarco
author_rank: Regular Contributor
timestamp: April 22, 2021, 07:44:01 pm
content: QuoteOk this is expected and fine.QuoteOk bear in mind that in general CRT are very hard to break. Provided one doesn't burn the filament or strips the cathode by applying excessive cathode current it's hard to cause a fault on the CRT (but not impossible).I would check (when you get to fix storage) in case you didn't already C15, R52, CR7, CR9 and R57.QuoteI think you have quite a lot to read and I as well... I will try fixing soon something similar: a HP 141T mainframe though way older than yours.Anyway, does the phantom track go away when storage is turned off ? If it doesn't, I would try to look at the blanking circuit straight away.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: April 23, 2021, 02:23:54 pm
content: I don't know what you mean by turning the storage mesh off, as far as I can tell, it's never off. If you mean "off" as in the normal operating mode of the scope, where it is not storing or displaying a stored trace, then my answer is that the phantom trace is always there, nothing I do can make it go away.I also should note that yesterday when I was messing around with a function generator, (until now I was just using the CAL signal) at some higher frequencies the trace would ghost a little. Could this be related to the blanking circuit as well?By "issue with the storage mesh" I meant the circuit that drives it, however with some of the erratic behavior the CRT has, physical damage is not out of the question in my opinion, but I will proceed as if there is not.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: April 24, 2021, 08:40:08 pm
content: is it possible to learn anything by directly looking at the signals on the horizontal and vertical plates? Like if the second trace was being produced by the vertical circuitry or if there is a problem with the blanking circuitry or something?

author: cosenmarco
author_rank: Regular Contributor
timestamp: April 25, 2021, 07:59:18 am
content: QuoteOK as far as I know, the scope can work in 2 modes: storage on/off. When storage is on, even if the scope doesn't seem to store, it's driving the storage mesh and erasing it quickly and it's ready for you to freeze the trace.When it's working with storage "off" then the storage mesh is not actively driven and behaves as if it's "transparent" to the electron beam.This is the mode I would concentrate on at first. That said, maybe I'm also oversimplifying and the reality is more complicated. I'll have to read the details.QuoteYeah I'm thinking in the direction that you may have "re-traces" meaning that the blanking circuit doesn't work properly, generating the ghost trace during the re-trace cycle of the horizontal.The horizontal should look like a tooth saw with some gaps between the teeth depending on the current horizontal settings. If the signal is not blanked correctly during re-traces, then you get artefacts on the screen.There may also be issues with the horizontal stage itself, that's why you have to confirm triggering and horizontal work fine. Blanking circuit is usually slightly more complicated because drives some high voltages involving galvanic isolation.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: April 25, 2021, 03:20:05 pm
content: Looking through the section of the manual I am missing in my hard copy I found the section called "service", It details the theory of operation for all the boards and functions. In there it shows some expected waveforms, yesterday I compared a bunch of the examples to mine. The horizontal output looks exactly as it should, and most of the others I compared on board A8 do as well.If I remember right the CRT "blanks" during retrace by turning the electron gun off(or something to prevent a picture), correct? I don't know if there is a specific section in the schematic for blanking or if its part of another circuit.I just looked at your post saying to look at the blanking signals on page 8-16, I will do that now.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: April 25, 2021, 04:22:10 pm
content: Update: I quickly checked the three waveforms shown on page 8-16 with my USB scope and the one called delayed gate is missing. I also found a waveform called "chop blanking" on board A3 page 8-26 that goes to board A13, this signal was also missing.I haven't had a chance to look more into this yet but hopefully I will later today.

author: cosenmarco
author_rank: Regular Contributor
timestamp: April 25, 2021, 08:09:46 pm
content: QuoteYeah, you definitely need thatQuoteDelayed gate: I think this is a signal which is present only if you set "Detla-T STOP" at something different than 0. It allows to wait a certain time after a a trigger event before triggering again. Useful for slightly more complex signals where the trigger may go nuts. If you want to see that signal then set the scope as described in the "Waveform measurement conditions service sheet 5". If the signal is not there, then it may be some problem but maybe not the one which generates the re-traces.The "chop blanking" I think is visible only when you select channels A and B simultaneously. The scope quickly alternates (in some scopes is called "alt" or "chop") between the two channels at a certain frequency giving the impression that the two channels are visible at the same time even though the scope has only one electron gun in the CRT. If you want to see that signal you have to set the scope as described on page8-29(EDIT: I mean page 8-26) on paragraph 5 of "waveform measurement conditions"

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: April 25, 2021, 08:17:39 pm
content: Ok, that makes sense, I always forget to look at the measurement conditions.I'm still not finding a blanking circuit though.

author: cosenmarco
author_rank: Regular Contributor
timestamp: April 25, 2021, 08:24:37 pm
content: QuoteOh... sorry in these scopes it's called "gate" so it's the circuit on section 5

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: April 26, 2021, 01:32:42 pm
content: ohh ok, yeah that makes sense, thanks.I should mention that this week is finals week in college, so if I am not very active on here for at least the first half of this week you know why.

author: cosenmarco
author_rank: Regular Contributor
timestamp: April 26, 2021, 06:16:52 pm
content: No problem at all. Focus on the finals and you'll work on the scope with less accumulated stress (which is in general bad when operating with high voltage equipment).

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: April 28, 2021, 06:11:55 pm
content: Whew, finished my finals this morning, now I have about two weeks before summer classes start so I should have all the time I want to work on this, It takes up soo much space on my little workbench!I took a brief look at A13, the gate amplifier the other day, and everything lines up with what is in the service manual as far as waveforms go. I am not sure where to go from here unfortunately. I think I saw a block diagram somewhere in the manual, If I can find it I will take a better look at it and see if it gives me a direction to go.The extra trace is an exact copy of the normal trace, but half the size, does that give any clues as to the cause?

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: April 28, 2021, 06:18:53 pm
content: Update, I was fiddling with a couple of adjustment pots and I found that the HV colimator pot R42 on A16 LVPS does not affect the brightness of the extra trace, whereas the normal one is affected. Looking at the diagram of the CRT tube itself, this would suggest that whatever is producing the extra trace is past the HV collimator? is that possible. if it is true then that would point to an issue in either the lenses or the storage mesh, but please correct me If I am misunderstanding how these tubes work.

author: cosenmarco
author_rank: Regular Contributor
timestamp: April 28, 2021, 08:20:40 pm
content: Ok I read page 8-52 and it seems to me this CRT is even weirder than the usual storage CRTsIt seems to me you have to check all storage elements even though you're not using storage. If I get it right, the writing gun never writes directly onto the phosphor layer but rather writes on the storage mesh and then the flood guns will transfer the image onto the phosphor...This means all storage elements must work properly in order to have correct image on the screen, so forget what I said about the "display" mode not making use of the storage elements...I should have done my homework betterYou have on page 8-53 a recap of the various signals / voltages you should have on the various elements of the CRT. I suppose you may want to check those for the various modes and make sure everything is ok (especially everything related to lenses, collimators and meshes).I would also try to go through the "adjustments" section and see whether you can go through the adjustment procedure successfully for things related to your issue (everything storage).

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: April 28, 2021, 09:05:30 pm
content: I agree that now would probably be a good time for me to go through the adjustments section, especially after fiddling with a bunch of stuff for a while. The adjustment procedure is very long and takes a considerable amount of time, but what may be good for me to do is to slowly follow the procedure and take notes on what doesn't work or seems off. I am missing some of the recommended equipment for the adjustments procedure, the one I most likely can't substitute is the High voltage 1000:1 multimeter probe, so the CRT anode voltage will remain unknown until I either get one or find a different solution.I an mot sure if you have ever looked at other people repairing the 17xx series scopes or not but I have seen at least two other instances of bridge rectifier failure on the LVPS, causing intermittent of weird faults, and I think my scope is no different. At one point when I was working on the LVPS it failed such that the unregulated 15V rail was a lot higher than it was supposed to be, which in turn affected the regulated side of the +15V supply, I cant remember hoe much it was affected but my fear is that if the rail was high enough it could have damaged several other circuits unrelated to the original issue that the scope had, which at this point I don't know what it was since the thing didn't work when I got it.Long story short I would really like to replace the bridge rectifiers, but I don't know where to get any that would just drop in, the best I could do is somehow retrofit others. Anyways I'm rambling now,I will start the adjustments procedure this evening.About the scope CRT being weird, it seems like most of the devices I work on have something weird about them that makes fixing them harder, especially vacuum tube stuff, that's just my luck!

author: cosenmarco
author_rank: Regular Contributor
timestamp: April 29, 2021, 12:14:14 pm
content: QuoteYou see stuff on the screen so don't worry about the high voltage: it's fine.QuoteYeah, that's why you always start from the power supply. You check the voltages, ripple, you check the capacitors and diodes one by one. Ideally for the caps you check ESR for all of them and leakage for the caps starting from 50V using an isolation tester or some sort of capacitor tester.In general for regulated and unregulated voltages you want to measure DC value and AC ripple. Ideally you put the scope on it (using appropriate probing technique) and check the noise figure. Experience in knowing the design of power supplies usually will guide you for what's acceptable in your application and what's not if the manual doesn't provide information.Quoteif you have a suitable replacementthis shouldn't cause any harm buta) if you just replace becauseothers in other situationshad issues with that I would suggest not to touch it, just test it in operation. Desoldering stuff is messy and you get a lot of chance to damage the PCB (...how many pads I lifted...)b) if you decide for replacing, check the performance of the supply before and after the replacement to see if the replacement brought you somethingQuoteI always do my research before acquiring some equipment on the chances I may have to fix it... of course then I ignore the findings and aquire irrepairable stuff anyway

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: April 29, 2021, 02:21:29 pm
content: Thanks for the info, I did decide to go ahead and replace all the large filter caps on the LVPS board because one had white fluffy stuff oozing out of the end, so I figured it was just best to replace all of them if one was bad.You may not hear much from me today because I got my second covid vaccine dose yesterday, and this morning I feel like I was ran over by a truck.

author: cosenmarco
author_rank: Regular Contributor
timestamp: April 29, 2021, 05:34:01 pm
content: Caps are ok to replace, especially in 40+ years old equipment and especially if they show sign of deterioration.QuoteOh boy... get well soon!

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 02, 2021, 06:00:36 pm
content: I know it has been a few days but I have been working on the scope bit by bit. I am focusing on the the storage board (A17) right now, using the waveforms given in the manual. Everything is OK except for the storage mesh output from the output amp, this is the circuit that I mentioned had issues and shorts from the beginning.I did find one thing that was causing some erratic behavior on the screen, and that was a switch on the back panel of the scope that had failed so that it was open all the time except for when you were putting pressure on the lever. Interestingly this switch is not in the schematic, and the manual is a little different layout wise as it pertains to A17. The switch is labeled Brightness; maximum/normal, and as far as I can tell it just joins or opens a connection between pins 6 and 8 on U4, which is just a trace in the schematic. Sadly this did not fix all the random behavior, the trace still flashes randomly sometimes.I do have a question about something on the schematic, on page 8-59, what is Q10 doing? I can see that its a PNP, so with the base grounded it would be on if the collector was more positive than ground or vice versa? that looks odd to me, what's going on there?

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 02, 2021, 07:26:18 pm
content: I have identified another potential issue on A17, there are two analog switches, U8 and U7, and U8 plays a big role in the storage mesh control. The issue I see, and I could be overlooking something, is that the pinout of U8 does not correspond with the schematic or the PCB circuit. The manual gives part number 1826-0502 for the IC, which crosses to an ON-semi MC14066BCP, here is the datasheet I found;https://pdf1.alldatasheet.com/datasheet-pdf/view/174197/ONSEMI/MC14066BCP.htmlI find it hard to believe that there is a mistake in the HP manual, so is it possible that this is not the right datasheet? U8 and U7 are the only ICs on this board that do not have a HP part number on them, and although they have the same P/N, one is a plastic package and one is a ceramic, leading me to believe that they have been replaced? I will keep looking for a different datasheet but until then U8 is confusing.

author: cosenmarco
author_rank: Regular Contributor
timestamp: May 05, 2021, 07:17:36 am
content: QuoteI think you're on something.QuoteAll of this seems quite strange. It seems this is controlling the flood gun grid (TP3) and it seems that it would inhibit signals which turn off and on the flood guns which I believe may result in the weird things you've been seeing because they are a very important part of the behaviour of this CRT. Can you tell if such a switch is "original" or was added later? its function seems more like a hack or a debug tool rather than something you would really need to use...QuoteSo you're saying that the ghost image is now flashing rather than being all the time there? I think this may be an improvement.... then you have to trace the timing of the signals which may cause the flickering. Or perhaps it's still the switch which is not connecting properly... honestly I would bypass the switch for the time being.Quotehttps://en.wikipedia.org/wiki/Common_baseEDITED :U4 is an Emitter Coupled Logic (ECL) gate. In ECL you don't couple gates by voltage but rather by current.TTL is a current-sinking logic since a current must be drawn from inputs to bring them to a logic 0 voltage level. I think by connecting the two gates output together, they are realizing a AND logic gate here.That transistor I think is (EDIT: besides providing the logic function "AND") a current buffer which interfaces the result to a voltage-coupled device (the CRT flood gun grid) by means of a load resistor (R46 and R47) which would be too hard for the gate to drive directly. Note how theemittercollector is referenced to -100V supply.QuoteMistakes may be much more common than what you may think. I would just really trust the PCB because at some point it must have worked. Looking at the datasheet I see only pin 3 and 4 might have been swapped in the schematic... I don't have the PCB to confirm and trace where the pins go.QuoteNow if you think somebody has had a go, keep you eyes more open than usual around those signals and check them all bu t I wouldn't be surprise if they used different packaging or slightly different ICs at the factory.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 06, 2021, 01:56:17 pm
content: I want ahead and bypassed the switch for now, even though it was working properly. As far as I can tell this switch is original, there is labeling printed on the back panel that looks the same as everything else.I am not sure if I mentioned it before but I have noticed a few other differences between the storage board I have and the HP manual. The differences are mostly in the board layout , but I suppose it is possible that U7 and U8 are not the right ones for this particular board. I did verify that the circuit board matches the schematic, so pins 3 and 4 are swapped on the actual IC.As for the flickering its not just the ghost image, its everything, and not always everything at the same time, I will look into it more later but for now there is some work I need to do on the storage board before anything else.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 07, 2021, 09:20:13 pm
content: Ok, I have an update and a major discovery, hopefullyAs of right now everything on the storage board A17 works fine, I compared what I have to the table on page 8-53 of the manual and everything looks good as far as the storage stuff goes. However In tracing and measuring the floodgun stuff in that same table I found that the floodgun accelerator was nearly zero volts, when as you can see it should be about +16. So I did what I always do first and started measuring power rails, and the one involved with the particular section I am looking at is the +53V rail. Low and behold there is not +53V, but 15V.This is where it gets interesting, I attached two pictures of A13, gate assembly/amplifier, one of the top and one of the bottom. As you can see on the big plastic connector at the top, somebody has disconnected the +53V rail and ran a jumper wire to the +15V rail. This it where I stopped for now because I am not sure how to proceed, should leave it how it is? or should I put it back to factory configuration? The puzzling thing here is that the wire used looks like factory wire, it is the same hard plastic with a colored stripe, in this case its green.This is interesting for sure but it raises more questions as to the history of this scope.

author: 0culus
author_rank: Super Contributor
timestamp: May 08, 2021, 04:50:14 am
content: I have a 1727A that works perfectly fine, and has this ghost trace as well. I'm interested to see what you find out about it. This isn't a very common model at all. It is also huge...I'd say it's stretching the definition of "portable oscilloscope".

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 08, 2021, 02:11:03 pm
content: I am slowly working through this thing bit by bit, and hopefully I will eventually find the source of the problemQuote from: 0culus on May 08, 2021, 04:50:14 amI attached a picture of how much space this thing takes up on my bench, it's massive and heavy!EDIT: I don't know why my pictures are always upside down on forums I post to.

author: 0culus
author_rank: Super Contributor
timestamp: May 09, 2021, 06:47:06 pm
content: Quote from: Alex_Baker on May 08, 2021, 02:11:03 pmYeah, mainly due to that storage tube. Like I can't seriously see it being portable due to the length and weight. A tek 454 is more what I'd call portable.I'm definitely interested to see the result of your troubleshooting, since mine has that ghost trace too. Other than that it works perfectly, so I had written it off as a characteristic of the storage tube.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 10, 2021, 01:12:49 am
content: Quote from: 0culus on May 09, 2021, 06:47:06 pmI suppose it is possible, I never really thought about that. I really should do some more research into storage scopes since I have no prior experience working with a scope like this. I doubt something like a second trace would be a normal characteristic, but you never know!

author: cosenmarco
author_rank: Regular Contributor
timestamp: May 12, 2021, 06:36:21 am
content: QuoteI'd be interested in what you find out!! Please keep me posted.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 12, 2021, 04:00:44 pm
content: I did a little research and it seems that there are not too many of these things out there, I did find two other examples of scoped with the same second smaller trace, or at least it appears to have it. The first link is an ebay listing for a 1742A not a 1727A, however it looks like it has a smaller trace in the middle(the bright part), however it doesn't look like the 1742A is a storage scope, so perhaps the second trace is not caused by the storage bits? The second link is actually an old thread from this forum that was about a 1727a with the same problem, unfortunately that thread did not get anywhere and it went dead over 7 years ago. with that being said the only picture I found of a 1727a without a second trace was in the third link, which seems to be a history of HP scopes, its actually kind of interesting.a good point mentioned in the other forum thread is that with storage scopes you could possibly get reflections at high intensities, so it is not out of the question that a reflected trace is normal to a degree. Because of that fact it may be a better idea for me to look at a couple of other issues that this scope has, and then once they are fixed I can go back to the ghost trace problem.https://www.ebay.com/itm/254966332232https://www.eevblog.com/forum/testgear/hp-1727a-beam-light-when-retrace/msg419172/#msg419172https://www.hpmemoryproject.org/wb_pages/wall_b_page_02.htmearlier I made post about a weird modification on the gate board, and I decided to rewire it back to the way it is on the schematic with a jumper wire. low and behold it did not change a thing, so either there is still a problem with that circuit or it has no visible effect on the screen. I also managed to blow the little 18.2V zener while testing itso I had to replace it with one that I had that is close enough to 18V.

author: dzseki
author_rank: Frequent Contributor
timestamp: May 12, 2021, 07:21:22 pm
content: HP 1720A (non storage) owner here. I can't add much to troubleshoot the storage part, but once done I would be curious to see how well it fares with bandwidth? My unit strugles to meet it for sure, in fact mine barely goes beyond 250MHz...

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 12, 2021, 08:28:19 pm
content: I don't have any kind of test equipment that can go anywhere near 275MHz. I have a PC scope that can only do 10MHz bandwidth and an old function generator that can barely do 1MHz, so unless I have an opportunity to test this scope with a piece of equipment at my college I probably will never be able to stretch its legs. would be neat to see though.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 12, 2021, 09:30:46 pm
content: Small update on the scope, I found the source of the random flickering of the screen. R42 (a potentiometer) had gone bad on the LVPS, so the HV collimator voltage was bouncing all over the place. I have to order the right pot but I threw in a random 50k pot I had and it works fine.

author: cosenmarco
author_rank: Regular Contributor
timestamp: May 13, 2021, 07:28:30 am
content: I think some ghost trace at high brightness is to be expected. The HP manual even reports it as I pointed out on 21st of AprilIn your case though I think it's not normal that the ghost trace has higher brightness than the legit trace.@dzsekiQuoteConsider those units are 40+ years old. Think of what has become of all capacitors and resistors along the very complicated signal path to the CRT... So I wouldn't complain. Bandwidth is tricky to debug.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 13, 2021, 02:47:04 pm
content: The more I work on this scope and dig through every corner of it for problems, I am noticing more small differences between my scope and the manual, including modifications that seem like they were done at the factory so to speak. I have no way of knowing if the hardcopy of the manual I have is original to the scope, but it has this page; (see picture), which seems to suggest that this manual and scope was shipped out to the customer before the manual was complete. This would explain the lack of the chapter on service in my manual, since the service section was probably not done yet.It seems to me that it is a real possibility that my particular scope was produced and sold very early on in the 1727A production, and thus is a little bit more prototype than most. further lending to this theory is the fact that the serial number is as follows; 1947A 00075, does that mean that my scope is the 75th unit?If Oculus is still watching this thread, I would be interested to know what the serial number of his scope is

author: 0culus
author_rank: Super Contributor
timestamp: May 14, 2021, 03:37:12 am
content: Quote from: Alex_Baker on May 13, 2021, 02:47:04 pmOK, this is real interesting. Mine is 2452A1208. the first two digits generally indicate years since 1960 for instruments newer than 1960 or so. The date, however, indicates the engineering revision date. So yours is a 1979 revision and mine is another revision that was done in 1984. Here's where it gets fun. I consulted my -hp- catalog collection and the 1727A does _not_ appear in the 1979 or 1980 catalog. I don't have a 1981 catalog, but it's definitely in the 1982 catalog. You, sir, may have a prototype on your hands. This might explain the weird bodges and stuff.

author: dzseki
author_rank: Frequent Contributor
timestamp: May 14, 2021, 08:26:19 am
content: Quote from: 0culus on May 14, 2021, 03:37:12 amThis is indeed strange, as the 1727A was introduced in the April 1982 issue of HP Journal:https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1982-04.pdfThough HP journal not neccesarily articled the newest instruments, but there was no point to advertise a years old tool either...BTW in that article they went into highlight the strengths of the new tube (and less the drawbacks obviously).

author: 0culus
author_rank: Super Contributor
timestamp: May 14, 2021, 03:18:48 pm
content: Quote from: dzseki on May 14, 2021, 08:26:19 amThanks, I knew there was an article about it in the journal, but I couldn't find it last night. I'd say it was formally released for 1981 at the earliest.It is a very interesting and rare scope today...cue my excitement about seeing this thread. I use mine occasionally but it's size makes keeping it out impossible because of the amount of bench space it uses.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 14, 2021, 06:00:31 pm
content: Thanks for the interesting information, I was looking at board A13 and it is very different than the layout in the manual, I know it was slightly different but now that I looked at it harder Its layout is completely different. This of course makes it difficult to trace signals If I don't know which component is which!I am focusing on A13 right now, just to make sure everything there is working since the beam intensity does not respond to the knob on the front panel.Just my luck to get a prototype scope without knowing it,

author: cosenmarco
author_rank: Regular Contributor
timestamp: May 15, 2021, 07:35:07 pm
content: I think you're lucky, you may now try to sell it to collectors for 3X what you paid for it

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 15, 2021, 08:24:44 pm
content: Ha Ha,that would be funny.In all seriousness my ultimate goal is to fully repair and refurbish this scope, hopefully learning as I goThanks to you people on the forum for being here for the ride!The point I am at right now is troubleshooting A13, There is nothing on TP1 that resembles what is shown in the service section, so I need to figure out how to trace that signal back without an accurate board layout. I am not sure how accurate the schematic is but I presume it is pretty close if not the same.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 17, 2021, 09:04:13 pm
content: Ok, I found a dead transistor on A13 in the output amp section, Q14. Although it is in a metal can It does not seem to be anything special except that in the parts list it lists a frequency FT=600MHz. Unfortunately the manufacturer part number it gives in the manual is not helpful, as it's just the HP number. I should stop here and note that this particular transistor is not the original one, since according to the manual Q14 is the same as Q1,3,4,5, and 6, which are all plastic. I was not able to find a part that matched the P/N given in a cross reference table for HP numbers either.I found this part on mouser.com just by filtering BJTs by 600MHz, I don't know if that is the best way to do it but it at least got me a result:https://www.mouser.com/ProductDetail/Central-Semiconductor/PN918-PBFREE?qs=l7cgNqFNU1iR8p7FRKw7wg%3D%3D. The collector emitter voltage is only 15V but it is all 5V stuff anyway, and with my limited knowledge I would assume that this circuit would not be too picky about the transistor specs, provided that it met the required transition frequency, but if anybody has any other input or an alternative transistor choice let me know!

author: dzseki
author_rank: Frequent Contributor
timestamp: May 17, 2021, 09:45:50 pm
content: NPN or PNP? A popular NPN TO can transistor was the 2N2369 at that time. Can you insert the schematic?

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 17, 2021, 09:59:51 pm
content: It's NPN, I didn't think about the schematic.

author: 0culus
author_rank: Super Contributor
timestamp: May 18, 2021, 11:53:45 pm
content: https://www.sphere.bc.ca/download/hp_xref-free.pdf

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 19, 2021, 12:03:13 am
content: That's the cross reference sheet I used. I found no references to a transistor when I searched PH3563-5, google only came up with other cross reference sheets!my gut wants to tell me that either 2N2369 as suggested by dzseki or PN918 will do the job.I don't think the original transistor is a metal can one, because according to the parts list there are several other of the same transistor on A13, and they are plastic. Although since my scope seems to be a prototype all bets are off.I am sure a metal can will work though.

author: artag
author_rank: Super Contributor
timestamp: May 19, 2021, 12:42:22 am
content: Quote from: Alex_Baker on May 12, 2021, 04:00:44 pmI have a 1725A, a non-storage scope that's also 275MHz. On that one, there is sometimes a small bright region on the trace. It's due to the delayed timebase : unlike a Tek 475 where the dot suddenly accelerates to the second timebase speed, the HP way is to generate a full-width trace that acts like another channel (you can shift it vertically) and brightens the main trace to show the zoomed section. I guess they do this by alternating the trace.If you want to disable it, turn the delayed timebase to the same speed as the main timebase. On my scope, once you've done this the two knobs are locked together and have to be intentionally unlocked to get a delayed trace. You should also find the delay setting (the vernier control) will change the position of the bright-up within the main trace.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 19, 2021, 12:44:13 am
content: Thanks Artag, that's good to know

author: artag
author_rank: Super Contributor
timestamp: May 19, 2021, 12:48:58 am
content: Quote from: 0culus on May 08, 2021, 04:50:14 amOf course it's portable. It's got a handle, hasn't it ?Non-portable scopes are a two-man lift onto their cartI swear engineers are getting weaker !

author: cosenmarco
author_rank: Regular Contributor
timestamp: May 19, 2021, 05:58:55 am
content: QuoteWhat abouthttps://www.mouser.com/datasheet/2/149/PN3563-889953.pdf?This transistor is used as pre-driver in the push-pull output amplifier. As long as max current, ft, power and hfe range are comparable or better you can replace it.Being that in a sort of push-pull amplifier, you may also want to replace with the complementary of Q13 (Q13 is according to manual a 2n4917 -https://alltransistors.com/transistor.php?transistor=4299) I can't find right now what transistor is supposed to be the complementary.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 19, 2021, 02:29:04 pm
content: Does Q14 have to have a ft of 600MHz when Q13 only is rated at 200MHz in the manual? could you get away with a 200MHz replacement for Q14?

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 19, 2021, 03:22:30 pm
content: Ok now I'm confusedThe schematic for A13 shows Q13 as PNP and Q14 as NPN right? I just noticed that the parts list for A13 lists both transistors as NPN. I am going to go off of the schematic and assume that they meant PNP for Q13 though.I found 2N5400 as a possible replacement Q13, which is PNP.https://www.mouser.com/ProductDetail/610-2N5400-PB,Do you think it will work?

author: cosenmarco
author_rank: Regular Contributor
timestamp: May 20, 2021, 09:15:33 am
content: QuoteI do see in the parts listA13Q13 1853 0089 Transistor PNP SI PD 200mW 07263 (mfr. code) 2N4917 (mfr part n.)A13Q14 1854 0092 Transistor NPN SI PD 200mW FT 600MHZ 28480 (mfr. code) 1854 0092 (mfr part n.)Although it's very badly scanned and not so readable in my case. I'm using official service manual from keysight site.Quoteare you maybe confusing with PD ? 200Mhz would be too low I suppose.Code:2N4917 Datasheet, Equivalent, Cross Reference SearchType Designator: 2N4917Material of Transistor: SiPolarity: PNPMaximum Collector Power Dissipation (Pc): 0.2 WMaximum Collector-Base Voltage |Vcb|: 30 VMaximum Collector-Emitter Voltage |Vce|: 30 VMaximum Emitter-Base Voltage |Veb|: 5 VMaximum Collector Current |Ic max|: 0.1 AMax. Operating Junction Temperature (Tj): 125 °CTransition Frequency (ft): 450 MHzCollector Capacitance (Cc): 5 pFForward Current Transfer Ratio (hFE), MIN: 150I think the 2N5400 is going to be fine. At least it won't blow up.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 20, 2021, 02:47:38 pm
content: PD for Q13 is listed as 1W in the manual, and FT at 200MHz.PD for Q13 is listed as 200mW in the manual, and FT at 600MHzI attached a picture of my manual for that section.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 20, 2021, 03:01:20 pm
content: Mouser doesn't have anything that matches 2N4917, and filtering by the specs you gave 2N5400 is the only one I can buy small number of, unless I want 5000 little transistors laying around.I think 2N5400 is the best choice for me at this point.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 20, 2021, 03:48:16 pm
content: one other question I have is if I can replace U1 on A13, which is a CA3140T op-amp, with a LF356H op-amp. I still need to test U1 but I have a hunch that I popped it when I switched its +15V to the +53V like it should be in the schematic. It is most likely dead because it is supposed to have a 30V zener (VR3) to reduce the voltage across it to just 23V, but that Zener was replaced with a wire link in my unit, so the op-amp got the full 53V across it. The reason I want to replace U1 with LF356H specifically is because I already have then.On that subject, do you think I should undo all the little hacks/mods on A13? or should I leave it the way it was when I got it. The problem is that I have no idea if the original setup with all the little mods even works.CA3140T:https://pdf1.alldatasheet.com/datasheet-pdf/view/66338/INTERSIL/CA3140T.htmlLF356H:https://pdf1.alldatasheet.com/datasheet-pdf/view/557688/TI/LF356H.html

author: cosenmarco
author_rank: Regular Contributor
timestamp: May 21, 2021, 06:24:23 pm
content: QuoteI think you can try to replace it with the LF356HQuoteNot really sure... I think you should compare your manual with the one from keysight which hopefully is the last version, also have a look at included manual modifications and errata which may be included in that more recent manual. Also I would keep it as last resort and maybe do that in a controlled way like have the equivalent of that portion made on a protoboard and try replacing only that portion of the PCB while keeping the original components, comparing signals, measuring in a systematic way, trying to compare the measurement with a model I have in my mind of what the circuit should do or better with a simulated circuit.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 21, 2021, 07:49:26 pm
content: I ended up testing the op-amp on a breadboard and surprisingly it seems like it survived having 53V put across it! impressive.As for the mods I got parts to undo them If I want to in the future but it begs the question why it was modified in the first place. I am tempted to reverse engineer that part of the circuit is EasyEDA and compare it to my manual and the online manual.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 21, 2021, 08:37:48 pm
content: Ok, here is what I drew out in EasyEDA, on the left is what I have and on the right is what the manual has. (refer to the manual for resistor/diode/cap numbers)I don't know enough to tell if this circuit would work as it is, but I do know that with a supply voltage of 15V VR4 would not be doing anything, which seems like it would be a problem. VR3 is replaced with a wire link on my board, but I don't think that would keep the op-amp from working since there is still 15V across it, and the base of Q7 is at 0V referenced to the op-amp anyways.

author: andy2000
author_rank: Frequent Contributor
timestamp: May 22, 2021, 02:16:29 am
content: I recently repaired and sold one of these. It's an unusual oscilloscope to use. Unlike the Tektronix storage scopes, this one doesn't have a non-storage mode. The setting of the brightness control is critical. Even a little too bright, and you can see a shrunken phantom image. A little more too bright, and the whole screen becomes a mess. I didn't find it to be a very easy to use scope. Definitely read the manual if you haven't already because if you treat it like a normal scope, it won't work well.Mine had a leaking cap in the HV supply that caused HV regulation problems.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 22, 2021, 04:19:57 pm
content: HI andy2000, what would bad HV regulation look like? just curious in case I have a similar problem. I don't have the proper multimeter probe to measure 2700V though.intensity settings causing the shrunken trace makes sense, as the intensity control doesn't seem to work at the moment. The brightness control does something, although not very much. Once I get the parts in from mouser for A13 I think it is time for a project recap so everybody on the forum an myself knows where I am at as far as what issues have been fixed and what issues still remain.I honestly do not know what half of the controls on the front of this scope do.

author: andy2000
author_rank: Frequent Contributor
timestamp: May 23, 2021, 02:42:37 pm
content: It caused brightness issues that got worse as it warmed up until it was completely unusable. Check the electroytic caps on the HV board since I wouldn't be surprised if it was a common problem. Even when it was working properly, the settings of the brightness and intensity controls was extremely critical.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 23, 2021, 07:49:07 pm
content: I have heard that brightness and intensity is important.The HV board only has a couple of little axial electrolytics, the rest are polyester or something similar it looks like, I doubt the polyester ones are leaky but I will still test them.

author: cosenmarco
author_rank: Regular Contributor
timestamp: May 24, 2021, 09:54:45 am
content: So, I see a couple of things with this circuit that make me not want to have to do with it. I don't even know if I want to comment because I may give you wrong advice so take all of this with a grain of salt.QuoteHere I believe that the diode is there for protection. The "Accel" signal comes directly from CRT so it may float due to strain charge to levels which would fry the opamp. ... at leat I think... the opamp input should never go lower than V- minus 0.5 but there it seems it may go as low as -3 V... soperhaps they did just see no harm in leaving the Zener there and didn't want to take the time / effort needed to remove it.I think this circuit works in a broader context. The feedback loop of the opamp involves some elements of the CRT itself and additionally it has some components (R18, C3) which are factory picked, so they must be selected depending on the CRT and / or other components.It seems to me the additional zener you don't have was used to float the circuit to a certain level so that the input signal coming from CRT "Accel" could be referenced to the level set by the "Current limit" pot and at the same time integrated.The output of this integrator sums up in current at the junction between CR3 and CR4 with another current which depends on Time/Div setting making up the amount of current which is sunk by Q12 which ultimately modulates the output gate amplifier.The goal of this would be to put a limit on the writing gun current. I think they would choose at factory components by measuring the voltage at teh "Accel" grid given a certain cathode current for the specific CRT and in this case they perhaps saw that the mod was necessary to keep the circuit working within tolerance.As a bonus you get a signal which compensates the focus at various intensity levels (thus "auto focus").I think messing up with this circuit poses the risk that this very same circuit tries to mitigate: burning the storage mesh with excessive current: If I understand correctly the storage mesh is made of non-conductive material so it could be burned by a beam which is too strong. As of which value the current should be limited to... I have no clue and must be some parameter on the datasheet of the CRT which of course is not easy to obtain I guess. Even though you have the data, measuring the relation beween Accel grid voltage and cathode current is tricky due to the high voltages involved (but not impossible).

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 26, 2021, 08:46:21 pm
content: I got my parts in today to replace the transistor on A13, along with some new zeners and rectifiers for the LVPS board just as preventative maintenance.I still need to do some testing to see if replacing the transistor fixed anything but I figured I would give you an update since I have been quiet for the last couple of days, I was just waiting on parts.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 26, 2021, 10:09:53 pm
content: Update, there is a recurring thing that has happened multiple times today but once I get out my camera I cant reproduce it. what it will do is it will start up normally, and then make a quiet "pop" noise toward the front of the CRT, at which point it will lose horizontal drive and the greed background glow.I don't think replacing Q14 on A13 did any thing, there is still no signal on TP1. Another thing is that now when I turn it on the main trace is not visible at all, only the second one(first picture). I think what is happening is what you can see in the pictures I posted on the first page of this topic, so I think the trace is still there it's just not visible. Too add onto that though I observed that if I unplug the connector J3 on the LVPS (upper right corner), which is the coliminators and lenses, the green background goes away and you can see the main trace(see second picture). I did just replace the rectifiers and R54 on A16, so I need to make sure the coliminators and lenses are adjusted.I know its a lot to take in!but any ideas about some of these symptoms would be helpful.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 26, 2021, 10:27:25 pm
content: I got a picture!I dont know exactly where the "snap" comes from since it is kind of random.Another thing I should mention is that the collector mesh on my scope runs at 150V, instead of 120V like you see in the online manual.

author: cosenmarco
author_rank: Regular Contributor
timestamp: May 28, 2021, 05:02:30 pm
content: Hi AlexQuotebut you got a signal on the base of Q13,14 right ? So it's just the amplifier stage not working correctly? I still think that you have to get the gate signal up and running before troubleshooting further but of course now...QuoteI think you may have an issue somewhere related to some capacitor becoming closer and closer to a short and one of the U1-U3 in the A16 LVPS detecting over current condition and cutting some voltage off. It is interesting to see, that even then you have 2 dots on the screen instead than just one. You still apply the input signal? If yes, then it might be the vertical still working so if there is one of the low voltages in the horizontal section which is not in the vertical, taht might be your voltage.... or just measure all of them when it happen. Also the dots are at maximum brightness.. this is compatible with my theory that gate signal is very relevant and needs to be fixed.Also... don't leave the scope running for long time with such bright traces... the tube may get damaged.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 29, 2021, 07:20:29 pm
content: I don't think there is a problem is the LVPS.I figured out that the "snap" noise was coming from the two series neon bulbs on the output of the storage mesh. I also learned that when the snap happens and the screen goes dark, it just put itself into "store" mode, so the store LED lights up. I can then get the screen back by pressing the display button. This almost sounds as if something is arcing within the CRT, but I hope not!The gate output looks as it should according to the manual, it just lacks the 70V peak amplitude that the adjustments section specifies, so yeah probably the amplifier stage. I tested the output transistors in circuit with a meter but I guess I will just have to take them out and test them properly. One of the output transistors gets pretty hot while the other does not.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 29, 2021, 08:44:28 pm
content: OK, so it seems that the gate output is intermittent, most of the time when I was testing it today it was just sitting at 4V on the scope. The two input signals comp gate and delayed gate are fine, so its being lost somewhere on A13. I am not very good at visualizing circuits so I don't know how the signal path works, and it is very hard to get a scope probe onto most of A13 to trace things.

author: cosenmarco
author_rank: Regular Contributor
timestamp: May 31, 2021, 06:50:24 am
content: QuoteI don't think either. Perhaps I wasn't clear enough: I think the LVPS is going into protection mode after the "snap" event but the cause is somewhere. Perhaps the LVPS going in protection mode causes the "snap" you hear. Sometime capacitors especially tantalums fail intermittently and unpredictably and becoming basically a quite low resistor, like 50Ohm, which trips overcurrent protection in the power supply making it hard to detect because they don't continue cooking off and releasing smoke.Quotethere you go... the one which is not getting hot is the suspicious one, but check all four Q13-16, the resistors and caps as well once the transistors are out of circuit.Quotereverse engineering is a crucial skill but before you get efficient at it you must learn forward engineering and the basic building blocks. It'll come with time and it's a muscle you have to excercise. I suggest you watch as many "back to the basics" videos (also the other are worth) as possible from w2aew and Mateo Aboy if you want to learn the basic building blocks of circuits and how they couple together. Taking a proper course would also helpIn the meantime, just ask... we can go through a section you don't understand. I can't promise accurate analysis: I'm just a hobbist but I have a degree in engineering and I learned some theory some years ago.QuoteThat can be hard. Take your time to set up probing eg. solder a lead wire to make a signal accessible. Probing and measuring is the only way to understand what's going on so don't dismiss it as a quick and easy thing to do. Also record the results, maybe take screenshots of the measurements, and take notes / record yourself (may seem silly but I do it all the time).

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 31, 2021, 12:53:13 pm
content: All the transistors I checked on A13 have been fine.I checked Q13-16 and several others but found no bad ones. I also did a quick test with my multimeter on all the other transistors but none had less than several Kohms between their pins, so I don't think there are any more bad ones.I would like to do some more testing today but I am quite busy, its my birthday,

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 02, 2021, 03:56:24 pm
content: I am now lucky enough to own a Flir thermal camera, I have wanted one for a while as a troubleshooting tool. Needless to say I pointed it at my scope and especially A13, but it didn't really show me much that I didn't already know. Q15 gets hot, as well as a couple of resistors next to it, but that is about it.Something on A13 is unhappy, I just need to figure out what!

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 04, 2021, 06:11:08 pm
content: I have new information! I got back to probing after a short break from this scope. I found that the gate signal on TP1 is fine (although half the amplitude it says in the manual) as long as the HV oscillator transistor is NOT plugged in.When I do plug it in the gate signal stays there for about 4 seconds, and then fades down to constant low level in about 2 seconds.So does this mean that the problem could be on the HVPS? of course I wont discount other issues on A13, like the output amplitude.I might see if there is a way I can measure the gate output current.

author: cosenmarco
author_rank: Regular Contributor
timestamp: June 07, 2021, 07:46:03 am
content: Hi Alex,I have a pretty bad scan here of the HVPS (clearly no quality control over those scans at HP... not worth the money) as you can see in the attached pic.If you can please share a pic where things are more readable I can try to figure out what may cause the problemsThe 4 seconds you mentioned seem to me compatible with the time constant of the little circuit which activates the optocoupler driven by Q4 so I suppose what you see is the soft start on the cathode bias.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 07, 2021, 02:05:22 pm
content: Here is a scan I took, I hope it is better.I measured the current passing through the gate output to the HVPS and it was on the order of microamps, I think the problem is still on A13. When I probe the base of Q13 and Q14 with my scope I have to set it to AC coupling and 50mV/div to even see a small noisy signal, so I think the gate amplifier is not being driven properly still. I just don't know where to look, Let me know what you think.I am kind of running out of steam on this repair project...

author: cosenmarco
author_rank: Regular Contributor
timestamp: June 07, 2021, 07:42:38 pm
content: QuoteI think you have to narrow things down. You have to establish if the single parts of the circuit work in isolation as expected. For that you need to power up the circuit separately, provide inputs and measure single sections of the circuit. And you have to understand what the various parts do in isolation (which is hard!!) The Service sheet gives you a lot of information about what the various parts should do. Learn it by heart. Seek for understanding how those different parts do what they are supposed to do and test them.QuoteQuoteI totally understand. My first serious repair project was and audio amp which I had in mind to repair since 15 years. I knew roughly which components were broken (some unobtanium diodes) and I did butcher it when I was a teenager but then picking it up at 33 I was able to understand it a bit better, then I started digging into it and after breaking it a couple of times, re-fixing it and reading a book about the design of audio amplifiers I was able to understand it completely and also pick up replacement parts for the diodes. The thing works: I listen music on it every day.Here you got yourself a very hard project to start with!! I know you want to see it working ASAP but it wouldn't be bad if you took a break from it and started getting some smaller easier projects and learn the theory behind. The scope can wait if you have some storage room.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 07, 2021, 10:38:18 pm
content: When I bought this scope I was looking for some simple old CRO that I could use in parallels with mu USB scope. I did not know that what I actually was getting was a rare and very powerful scope, which is not a bad thing in and of itself, but it is also complex and hard to use on some accounts, not to mention that is is broken.I have been looking for small repair projects. I have done a number of vacuum tube radio repairs/referbs recently as well as some amplifier stuff, you may have seen the topic I started about the Marantz 4070, I enjoy working on those older solid state amps.As for the future of the 1727, I think I need to give it a break. I don't know how long it will be, it could be a couple of months or it could be years down my electrical engineering education/career path.Sorry to disappoint everybody who was hoping for a solution to this scopes issues, I just don't have the time or expertise to tackle a project like this right now. I suppose I will just let this topic sink into the depths of the forum for the time being, but I will leave notifications on incase somebody replies.Thank you all,Alex

author: 0culus
author_rank: Super Contributor
timestamp: July 09, 2021, 04:14:52 am
content: Don't give up, take a long break and come back to it. I'm currently doing battle with a recalcitrant Tek 454 (got it for free with nasty arcing in the HV...solved that problem but it has more) that I've put down for a week or so. Symptom is low horizontal gain on the A sweep with no obvious cause (and I've been through it with a fine tooth comb).

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: July 09, 2021, 02:48:29 pm
content: That's how I feel with my scope, everything I have checked or tested is fine, and if its not I cannot find the cause.I will keep the scope in storage indefinitely, given its rarity and the history mine has, but until then I will work on different projects. I just finished building a cloud chamber (look it up), it's pretty cool, the hard part is finding something good to put inside it!