Tektronix TDS 310 repair - Page 1
Forum section: electronics / repair

author: Vince
author_rank: Super Contributor
timestamp: August 14, 2017, 03:11:51 am
content: Hi,Well, third scope repair in a row : after my Tek 2232, and my TDS 544A, now I am turning to my TDS 310.The first two scopes were my main scopes so there was some pressure to get them fixed, I actually needed them.OTOH this 310 puts zero pressure on me, I am totally relaxed about it : I bought it very cheap (60 bucks ISTR) purely for training purposes, I bought it as my first repair "project". If I can fix it great, if not, well it will just sit on a shelve waiting for me to reuse part or all of it for god knows what project. If anything, it gives me a chassis with a nice CRT and power supply to play with. Its specs are so crap that it wouldn't exactly be a great loss, should it be impossible to repair : 2x50Mhz @ 200MSps.....Still, the aim is to work on it as much as much as possible, as long as I don't run out of ideas, and hopefully fix it ! We shall see !Symptoms ? They are multiple, and by the looks of it, are a mix of analog and digital, so that makes this repair particularly interesting to me, regardless of the usefulness/specs of the scope.1 ) To start with, powering the unit was/is unreliable : switched (pun intended) my attention to the SM power supply... first time I had a look at these... bad luck because it turns out its a third party unit made by Zytech and there are not schematics for it, never mind a service manual. Still, I did some digging in to it so learned a bit. In the end turned out the SMPS is fine, and the problem is just the switch on the front panel which is temperamental. These membrane switches I understand can wear out, the carbon pads on the flex PCB I mean, and you can buy kits to rejuvenate them, but it costs a little fortune for a few drops of teh precious liquid, and to the eye the carbon pads really don't look that bad to me.... but the ohmmeter says otherwise, and it fails to fire up the SMPS so... I guess I will have to buy this stuff anyway. In the meantime, I just ground the standby pin manually with a bit of wire, works just fine, I can now at least turn the scope on and off at will... which is a good beginning, eh ?!2) When I bought it 3+ years ago, the sellers said it was working fine except for channel 2. Well back then I played with it for a few hours, and it seemed that the problem was more to do with the front panel : some of the buttons did not work, was a well delimited group of buttons, in the top right part of the front panel. Coll I though, should be easy to figure out what's wrong. So took the front panel out, studied it using the schematics from the TDS 340/360/380 series because I could not find the schematics for the 310/320/350 Series. But they are the same basic design, close enough to be a good help in this repair at the very least.So to my disappointment I did not find anything obviously wrong on the front panel. Ended up replaced a couple chips anyway but wasn't holding much hope.Indeed it didn't fix anything. If nay thing, things gradually got worse and worse. So I left it at that, because at that point I didn't have a lab whatsoever, nor any experience in repairing anything this complex. But now that I have started building my lab at last, I could resume resume work on this thing. 6 months ago did some more work on the front panel. The state of it back then, and still today, was that the front panel is now 100% unresponsive to USER input, but not unresponsive to SCOPE/CPU input. That is : no matter what button I press, or what know I rotate, the scope will not react. However when the CPU want to drive LEDS ont the front panel, the front panel executes promptly. By that I mean : when you power up the scope, it does what it's supposed to do : lights up all th LEDs for a few seconds, then you hear the relays click in the attenuator section, then the UI is started and the front panel is taken to its default setting : CH1 LED is lit. Also, I can tell from the way the two LEDs in the trigger section of the front panel, react, that the CPU is able to drive them as expected.So I started probing around the panel. The local CPU on the front panel is clearly working. It's oscillator works, it generates the signal for the probe compensation just fine, and I can see activity on the many lines it uses to scan the buttons and what not.So it's like everything was fine in this front panel, expect that the scope does not react to user input. So the logical idea was to say well, it communicates with the main board vi a serial port/UART, so that just means that the receive line (from the front panel point of view) is working, but the transmit line must not be working, hence why the main board/scope can't "hear" the front panel shouting "Hey he pressed this button 200 times, will you budge or what ?!! ".That was 6/9 months ago but I remember probing the serial port and could not quite make sense of what I saw. ISTR I was intrigued by the fact that I DID see stuff coming OUT of the front panel.. EVEN when I was not operating any control !!! ... so was intrigued as to why the main CPU at the other end, would not respond to that...3) This scope has ONE thing going for it : it came with the optional interface board, so it does have GPIB at the back ! SAVED, I thought ! I will be able to operate the scope remotely via GPIB, I don't need a front panel anymore ! But.... yes you guess it, GPIB doesn't work !! ARRRGH !!! When I scan the bus, the scope does not show up /respond. Works just fine on my other scopes and instruments so I am pretty sure the problem is on the TDS 310 side of things sadly.... so, that's yet another problem to fix, and it just makes my life more difficult, with no way to operate the scope... not from the front, and not from the back ! Grrr...4) Now for the analog side of things : the problem is that the scope fails to display the input signal, on CH1. CH2 I don't know obviously since I have no way to enable it, being short of a front panel...So, it DOES display a trace, but it's just some undulating signal that's so minuscule in amplitude (a small division peak to peak, and the scope is on its 5mV scale !) that we may as well say that there is no signal. Also, this signal is strange : it moves about. At power up it starts at the bottom of the screen usually, then over time it moves higher and higher, and eventually goes off the top of the screen. OK I thought, some capacitive effect somewhere in the front end, getting charged somehow. But... when you look at it, you realize that it's not the signal actually getting higher and higher in amplitude, not it stays at zero, instead it's the GROUND reference/arrow on the left edge of the screen, that moves, as if someone had turned the vertical control on the front panel !See screen capture below.Obviously the vertical and horizontal position of the trace on the screen, are entirely controlled by the CPU, has nothing to do with the analog front-end. So it all looks like the front panel was kinda haunted, and actually telling the CPU that someone had moved these controls ?! Strange ! OTOH that would explain/be consistent with I said earlier about seeing activity on the transmit line of the front panel, even when I do NOT touch any knob or button !But obviously that does not quite explain why it fails to display any input signal I throw at it. Whether I short the input, or I apply a nice large signal that easy to trigger on (I applied the probe compensation signal, 5Vpp 1kHz), the trace on the screen does not budge the slightest bit !Come to think of it, there might be a simple explanation ? Maybe this low signal is just the input being grounded (via the input coupling relay), but then we would see the "ground" symbol somewhere on the screen, to remind us of this setting, and I don't see that anywhere.5) Believe it or not, the scope actually does NOT report any error at power-up ! The POST is CLEAR ! And hold on to your hat : if I power up the scope with the front panel DISCONNECT altogether, it still gives a clean POST !How can that be... the front panel is supposed to be tested during the POST...Well, ISTR that on the TDS 700 scopes, there is a dip switch that you can trip to make the scope boot faster : it will basically bypass/not perform any of test, and will just display a clean POST report no matter what ! Maybe there is something similar on this 310 scope...Soo.... that was for the presentation of this scope and where I am i the trouble shooting of it.Many problems to tackle. I must not chase red herrings all over the place or I will never see the end of it.I think my best call is to start with the front panel issue. Because 1) I need a way to operate the scope if I want to go any further and 2) I have already done a lot of work on it already so I might as well finish it off and 3) it's most likely something simple. This front panel board is small and not much on it.As you can see on the pictures I have soldered a ribbon cable on the front panel connector, so I can wire it on a bread board, to let me easily probe any signal while the scope is operating.So I will keep working on the front panel side of things. I don't expect progress to be made quickly though because... vacation is almost over, only 3 days left : grrr.... so I won't have much time to work on it during the week.The scope is extremely simple though, compared to my TDS 544A. The 310 has a monolithic/single board design. Attenuator section, acquisition section, CPU section... it's all on the same PCB, and it's a very tiny PCB as well, about the size f my hand ! Yeah I do have large hands but still... the entire scope basically is contained on that very small board !So now that this is all said... let's work on this thing ! Will post/update as I go...Oh, let's start with the fundamentals : this evening I checked all the power tails again, like I did ages ago, just to be sure it was till OK. This time I also looked at the ripple. All DC levels are good. Ripple I don't know because the service manual does specify anything, but they all seem OK to me if my TDS 544A and 2232 are anything to go by. Seems that basically, for the digital 5V we get around 20mVpp, and 5 to 10mVpp tops for the analog rails. I get that here, so I am happy enough. As we can see, from the ripple on al waveforms, looks like the SMPS is operating at 100kHz. Quite on the high side.. my old Tek 2232's inverter was running at only 20kHz... I guess the TDS 310 being more recent, they could use better components that could switch current efficiently at higher speeds. I don't know. This SMPS is not made by Tek at any rate.The power rails on this scope are as follow :- Digital +5V- Analog +/- 5V- Analog +/- 8.4VRipple waveforms in order of appearance (measured from edge to edge on the connector) :- +8.5V ana- +5V Digital- +5V ana- -8.5V ana- -5V ana

author: Vince
author_rank: Super Contributor
timestamp: August 15, 2017, 12:52:04 am
content: OK, making good progress today !Spent a few hours fiddling with the front panel as expected. I concentrated on the serial link that connects the little 8 bit MCU of the front panel (Motorola 68HC705), with the main MCU (Motorola 68K based, like all '90s TDS scopes it seems... don't know about more recent models).It's a full duplex link with CTS/RTS hardware flow control. I attached below the schematics for the front panel.Started with the main board side, looked alright : sends stuff on the line only when it needs to (to drive the two LEDs indicating the trigger status). I checked the corresponding control line CTS, which goes to the Interrupt input of the front panel, and I did see a short 1.5us pulse on that line, every time the main board would send a few bytes to command a change in the LED states. So I was happy with that.Then checked the front panel side : indeed my memory served me right, it spits out stuff constantly on its TxD line, even though I am not touching anything on the front panel. It seemed random. Captureds dozens of events to try to see a pattern, but couldn't see any : packets of data are sent at random intervals ranging from a few packets per seconds, down to a few seconds par packet. But I could never hope for more than 5 seconds of rest before seeing a packet surge onthe data line !Packets contained anywhere between one to 5 bytes maximum.I was not very happy with that... I just could not see why the hell it would bother the main board with constant chatter, when there no event whatsoever to tell him about ? The whole point of having a dedicate micro on the front panel is to let the main CPU alone, so it can concentrate on more important stuff !Then I measured the frequency of the crappy ceramic resonator of the MCU... these things aren't supposed to be ultra accurate, so I thought maybe this results in some speed mismatch between the front panel and the main CPU. It was clear that the front panel was understanding what the main CPU was saying, but maybe the other way around was not true. A possibility any how. The resonator, specified at 2.45MHz, measured at slightly less, 2.42MHz or so, but cursor measurement on my 2232 aren't the most precise thing in the world, so I would call it acceptable. The timing on a serial line is not that critical anyway, the receiver basically syncs at the start of every byte, so the clocks don't have to match very accurately for it to work... IIRC the tolerance is 5% ? So we are still within spec...But to be more accurate, I measured the bit time that both MCU's would produce, to have a definitive and more accurate answer on a possible mismatch.Measured first on the front panel MCU, did my best, it falls between 26.1us and 26.3us or so. That means a baud rate about between 38,025 and 38,300. Looked at the baud rate table in the MCU data sheet. The 2.45MHz clock is a standard one. Closest baud rate available is 38400, so indeed the resonator is a tad on the low side, but the gap is still below 1% so plenty within specs to get the UART work reliably, I thought.Then on the main CPU, it's going a bit faster, I measured the bit time between 25.25us and 25.45us, ie a baud rate somewhere between 39,300 and 39,680.So if we take the worse case scenario, i.e. 39,680 on the main CPU side, trying to talk with a front panel running at 38,025, that's 4.3% variation. Getting to the limit, but not quite there yet, and that's a worse case scenario, probably less in reality... if I had a proper/accurate counter, that would help...So I decided that the main board CPU had no excuse for not listening to the front panel.Next in line : the RTS line. Does the front panel assert its line before sending stuff to the main CPU ? Answer : NO it doesn't !!!I was overjoyed, thinking I had found my problem, there must be an open trace or something, just need to find it, should be quick !This was measured on the breadboard, so I rushed and probed that RTS signal directly on the pin of the MCU... oh.... nooooo..... it's not there either !What ? The MCU is not driving his RTS line ?!Started depressing at this point, because well... that could only mean that the MCU was defective, and finding a replacement might be tough, and not economically viable, given the very poor specs of this scope !But, I thought... well this hardware flow control isn't even necessary, after all. I mean when either MCU receives a byte, they get notified internally by an interrupt, so they already aware that data is available, why would they NEED, a control line. So I thought.. maybe the front panel doesn't uses this line in fact, though one would then have to wonder why Tek bothered wiring it up on the PCB... but these things happen.OK... so what to do next ?I thought... regardless of this handshaking protocol, there is still one thing that's not normal and that must be investigated ! The fact the front panel keeps sending stuff all the time, even though it's not supposed to ! How could this be explained, quite a mystery ! I thought it's worse investigating because maybe this RTS line is not necessary/used at all, maybe the main CPU is INDEED getting all the stuff that the front panel ius sending... and maybe this IS the problem : maybe the main CPU is OVERWHELMED by this constant chatter, especially if it's crap/non sense that drives him crazy. Maybe if the front panel stopped sending stuff, the scope would behave less strangely...So I decided I had to investigate this. I came with one theory/possible explanation : the front panel basically handles 3 different types of things :1) it drives LEDs, through a latched 3 to 8 decoder.2) it scans all switches, i.e. all the push buttons + the 2 range rotary (detented) switches for vertical gain and time base settings.3) it reads a dozen different analog voltages, corresponding to the non-detented rotary knobs (vertical and horizontal position, trigger level, general purpose knob) + the probe sense rings on the BNC input connectors. This is done by a couple 8 channel analog multiplexers, the usual 4051, which then go to 2 ADC inputs on the MCU.My idea was: what if.... what if, say, for some reason, any one (or more !) of these numerous analog input was "floating"... this would mean that the ADC in the MCU is seeing a constantly changing input, which in turn would fool the MCU into thinking that there is huge activity going on at the front panel, and that it needs to report that to the scope.So, I looked at the analog portion of the board, and noticed that on one of the two multiplexers, one of the pins looked like there was very little solder on it, if not no solder at all...These two chips where the ones I changed 3/4 years ago when I first got the scope. It was my first attempt at repairing it, that failed so I gave up at the time. I had no lab, equipment, and not much experience back then. It was my first attempt at replacing an SMD chip, ever. I followed advice I saw from Dave Jones in his video at the time, where he proved on camera that you didn't need a PhD in electronics nor a millions dollar worth of gear, to solder an SMD chip... so I had a go at it !But when I looked at these solder jobs this evening, clearly I didn't do that good a job ! So I fired up the hot air station to remove both chips cleanly, cleaned the pads, soldered the chips much better this time... and what do you know, the front panel works now !I probed the data and CTS lines again, and oh miracle, all is fine now : the front panel stopped sending data all the time. The data line is now perfectly silent, sits quietly at 5V, not even a glitch, perfectly silent. It now only sends data when, and only when, I press a button ! And the scope now can display a signal trace on the screen, no front-end problem it seems ! YES !!!Not everything on the front panel works, though all the analog/position knobs fail to responds : when I move any of them, the front panel does not send any data. But that should be easy to figure out, just the two analog MUXES to probe and that should do it I guess !I am slighhhtly worried about them, though : when I (re)soldered them today, I could not find the mark on them to figure out which way around to put them on the board ! So I put them in the way that they were originally. When I replaced them back then, I really don't remember having any trouble figuring out the orientation... but I sure do now ! Maybe my eye sight has degraded since then...So I need to figure out a way, by probing them somehow, to make sure they are oriented properly. If you have suggestions....Worst case, if they are indeed somehow soldered the wrong way around, I think I may have some chips in stock. I mean when I order cheap parts like these, I usually order a few more of them, just in case.. and because the minimum at Farnell is 30 Euros (to get free shipping). So I am pretty sure I must have some spares somewhere.... just need to find them... because I moved home since then, things get lost sometimes...Anyway, quite happy with that !!!The mystery in this front panel has been solved, the remaining issue with it is very minor I think, not worried the slightest bit about it.I might try GPIB again, maybe it will start working, now that the main CPU is not spending all of its time trying to digest the tons of crap the front panel used to flood him with ?! Worth a try anyway...

author: tautech
author_rank: Super Contributor
timestamp: August 15, 2017, 01:03:15 am
content: Quote from: Vince on August 15, 2017, 12:52:04 amSo YOU broke it.Reminds me of my early repair attempts.Good progress, you'll be a detective before you know it.BTW, love your running commentary, a great asset for those that will read this thread.

author: Vince
author_rank: Super Contributor
timestamp: August 15, 2017, 04:22:04 am
content: Quote from: tautech on August 15, 2017, 01:03:15 amNo !I merely put it back in the same state that it was in when I bought it : just did some more checking... not all push buttons work, which was already an issue back then.Bad news about the analog controls : just finished probing around, and strangely enough the MCU receives perfectly good signals to me ! I mean I checked each and every pin of both MUXES, power pins OK, Chip Select and all 3 input selection pins are "scanned" continuously as you would expect. Checked all the analog inputs, they are what they should be : the inputs vary from 0 to 5V as you rotate the knobs, as you would expect. All these voltage are clean and stable, no worries. Then checked the analog outputs of these MUXES... see below. You can see the "stepped" waveform taht you would expect, and if I turn the knobs I can see the corresponding step in the waveform, move up and down, so all is well. At first I got scared because as you can see their is some slow transition between steps, which I did not see on the TDS 544A when I looked at the stepped output of the DAC in the analog section of the board, if you remember. On the 544, the output of the DAC was perfectly clean, perfect/fast transitions between each step. So I was worried at first. But then I looked closely and I maybe understand what's going on : unlike the 544A, we can see that there is some dead/"OFF" time between two "steps". The MCU allows for some dead time between two input selections. The slow falling edge looks like an exponential, and is steeper when the voltage difference between too steps is greater. There is no external components in the signal path : the outputs of the MUXES are routed directly to the ADC inputs of the MCU. So maybe it's just the sample/hold input stage of the ADC that leaks badly, it can't hold the charge when the MUX is in between steps and all it's outputs are "OFF". Or maybe it's the MUX leaking, I really don't know !They are brand new... and I remember that the original chips were producing exactly the same weird output (which was why I decided to replace them just to see). So I don't think the MUXES are at fault. I welcome any knowledgeable input on this matter ! Is it normal that the ADC of MCU can't hold the charge ? Should I care ? .... if it's the source of my problem then I am back square one : I need a replacement MCU from some donor board....The timing is not state of the art either : it's all jittery : The "ON" and "OFF" time are not always exactly the same length, can vary quite a bit. Still, it appears taht the overall duration of a "sweep", however, is perfectly stable/constant : around 3.2ms. So that's alright I suppose...So I am starting to run out of ideas here.... or almost ! One last hope :Now that (most) of the push buttons work on the front panel, I when in the UTILITY menu to check the GPIB settings, just in case I got lucky and that it not working was just due to it not being activated, somehow ! Not easy to navigate in there with a front panel dangling upside down on its cable, the scope/CRT upside down too, not to mention the lack of soft buttons, the front bezel being absent of course ! But I had to try... was worth it :1) GPIB was indeed disabled (was set to "HardCopy" only, not as "Listener/Talker" ), now it WORKS ! The computer can detect it just fine, yes ! :-D So I can now have complete control of the scope !2) I could check the error log : I can see countless errors relating the lack of front panel... but why did it never complain about it during POST ?! I am lost. Anyway, this log is better in some way than the one in my TDS 544A ! The 310 tells you how many errors there are in all, and how many pages of errors there are, and you can swift through them, instead of the down slow scrolling on the 544A ! However the messages are much less detailed, and don"t bear an accurate time stamp, that's a fail.3) While setting up GPIB, just for the sake of it... I tried using the general purpose knob to modify the address of the scope on the bus, even if I didn't need to. Result is interesting : the knob is NOT totally unresponsive : if you turn it fast enough, for a few seconds, it will eventually register some changes. It will be totally erratic/uncontrollable, but I did see it change values a few times. So it's not totally dead, there is some activity going on... just not what I want !So, it's looking bad indeed, but there are still some little things I can do :1) check the data sheet of the MCU, ADC section, to see how the S/H is put together exactly, and how it's supposed to perform, its specs, if it can make sense of the discharge during the OFF time of the MUXES nu between steps. Maybe the ADC is trying to sample during this unstable period of the signal, which might explain the erratic behavior I witness.2) There are still a few push buttons that don't work. Will try to see what's going on. Maybe it's just the membrane (like the Power on switch that stopped working), but that seems unlikely. MAybe it's more an electronic problem. There are lots of diodes in involved in scanning the switches matrix, maybe some of them are bad. Also, some of the signals used to scan the switches, are also common to the analog MUXES. So maybe there is some relations between the two. Maybe if I fix the remaining buttons, I will see some change of some kind in the analog department as well.3) Keep playing with the buttons while observing the serial port, maybe I will see a pattern in there, somewhere, that might lead me somewhere...So it's not totally hopeless just yet, but it's not looking too good either, I am starting to be pessimistic....

author: Vince
author_rank: Super Contributor
timestamp: August 17, 2017, 11:04:39 pm
content: SUCCESS !!!Can't believe it, just when I was starting to be real pessimistic about this repair, suddenly fixed it !Was all due to corrosion, surprise surprise....There are a couple small (2.2uF) SMD electrolytics near the ribbon cable connector. During my first attempt back then, I remember their terminals looked crusty, so I removed them and replaced them with what I had at hand at the time : my big collection of leaded/through hole electrolytics. Found a couple 2.2uF, in a tiny package.... as tiny as this kind of package can get anyway, but of course was still huge compared to the original SMD parts... but still, I though better non-corroded parts, than no part at all. Was hoping maybe there was some kind of decoupling issue because of these bad caps, but that didn't fix the problem so I gave up at the time.So I removed this ugly hack and soldered a couple nice Tantalums of the same value that I had salvaged recently while I was trying to get the hang of my new little hot air station. On one of the caps, one of the pads came off as I removed the leaded cap. Oh well. Scrapped the adjacent trace to reveal some fresh copper I could solder to, as that did it. The cap package was a little small to bridge the gap so had to install it sideways and had not much pad an trace to solder its terminals too, but that did it OK.Then next to one of the caps, I noticed a crusty looking resistor. Probed it... hmm... an open trace on the pad nearest the capacitor ? Am I dreaming ? Have I just found a practical/real world example of this electrolyte corrosion that plagued my TDS 544A ?? My first encounter with corrosion damage ?? Couldn't see anything with naked eyes, and the solder mask doesn't help..So I scraped the trace by the solder joint. removed the resistor, some flux and solder wick, cleaned the pads... then a high resolution macro pic of the pads, then zoom on the pic afterward.. OH ! Taht becomes quite obvious now ! Yes indeed a cut trace ! Right by the pad, and the pad itself is eaten all around, slowly but surely. The PCB itself looks brand new, the cut show to the naked eye ! Not mine anyway.Tried to repair it with just a blob of solder but it would not bridge, so had to put a wire, but even my AWG 30 "mod" wire looked quite big and would not stick to the thin trace. So had to run a much longer wire to the nearest big pad so I could more easily solder to. That turned out to be the tantalum cap I had just put in place.Then looked this resistor up in the schematic... was the pull-down for the MCU's Reset line. Hmm I thought, sure enough the MCU does run, so clearly this floating reset line never stopped it from initializing, however maybe some internal peripheral might require a "cleaner"/more predictable Reset... something like maybe the ADC ?! So I thought why not, maybe this will fix my problem with the analog knobs... no joy, no improvement what soeverOh well, at least it was worth a try, and this pull-down definitely had to be fixed anyway, couldn't possibly let it flap in the breeze eh ?!......Original was 10K, closest I found was 3.3K, good enough, it's just a pull-down...So I had quite mixed feelings at this point : "happy" I had now gained experience of how to find and recognize and fix a cut trace... but quite frustrated that it did not actually fix anything and I was back to square one !But, at leas tit showed that there WAS some corrosion damage on the board, so why not some more ? Maybe if I could find & fix some more corrosion it might improve things.So I started probing around the switches area. I tired all push buttons and figured only two of them were not working : one in the bottom right of the front panel : " Force Trigger ", and one in the top right corner : the "Acquire" menu. According to the schematics they shared a line.Was probing a long line that went from right behind the "Force Trigger" button, straight to the MCU area, near one of the two leaky caps.... there was a via. Bingo, that trace was cut, right by the via next to the capacitor. Soldered a long jump wire: easy, just stick the ends of the wire straight into the holes of the via !Hey what do you know, my two buttons are now working ! Yes ! Making progress, 100% of the (push) buttons work now ! And that was due to corrosion.So... probed some more, hoping to find another cut somewhere, that would fix the remaining, big issue with the analog rotary knobs, but no joy. Was depressing again, grrr.... So tried again to make some sense of the behavior of these buttons, actually managed to find a very distinct pattern but I just had no idea what cold cause this behavior.. because this circuit is quite straight-forward and I had already probed/tested 10 times every signal involved in this circuit and never found anything wrong ! So was out of idea on this front sadly !So I thought well, I have run out of ideas here, so I might as well probe agaiiiin for cut traces... I maybe have missed one or two, you never know.So at this point I was not trying to figure out with naked eyes which solder joints looked dull/susceptible of hiding corrosion damage. No, I turned my brain off at this point, and just used brute force : I probed each and every pad and via one by one, no questions asked, methodically, without holding much hope but well what else could I do. And what do you know : the very last thing I checked, was defective ! A via that was hard to see and a pain to look at/work on, buried half-way below a corner of the MCU package. Was looking like a pain to deal with, so avoided looking at it, hoping I would find a cut somewhere else that would be more easy to work on... but no, the cut was there ! So fixed that. I thought oh no, if this doesn't fix it, I will probably have to remove the MCU chip so I can inspect underneath it, might be more vias under there. So in order to assess my chances of this via fixing my problem, I looked it up in the schematics to see what it did. Via is connected to pin #7 of the MCU.... where is it... pin seeeeeeven..... ah, there it is. So, what does it do exactly....... eh ? OHHHHH !!!! Its only purpose in life is to connect to ground, the lower voltage reference of the ADC !!! Yes, that's highly likely to fix my problem !!!!Couldn't wait to power up the scope see how it would react ! BINGO ! Problem fixed, all analog rotary knobs now work perfectly !Scope is now 100% fixed ! No weird problem with the front end in the end, was 100% due to the defective front panel driving the scope nuts !I did a quick sanity check and seems OK : fed a sign wave, tried all the basic controls, on both channels, all fine. Also ran an SPC calibration routine, came clear !so I call this a fix !Then I hooked GPIB and tried to clear the error log using the same little software that I used the other day on my TDS 544A. It's advertised as being able to handle pretty much all the old TDS scopes, a bunch of them any way. Worked like a charm ! The 130+ errors are now gone, clean bill of health !I noticed that this also cleared the "PowerUp" counter.... was 2500+, now zero. I think this must be a bug/unintentional... Clearing the log is one thing, it's perfectly normal/acceptable to do it if you know that you have fixed the problem. However clearing the Power Up counter is just plain lying ! Might send an e-mail to the author of this software, maybe nobody ever told him about this unfortunate side effect.Anyway, I am so happy that I was finally able to fix what looked at first like a really, really sick little scope that I was having a hard time to figure out !Also real happy that I now have practical experience dealing with electrolyte corrosion, will no doubt come to good use in future repairs !Lessons I learned while doing this :1) even if the PCB looks brand new, there can still be corrosion. Don't rely on the cleanliness of it.. Maybe in some severe cases, with very big caps leaking, you can see deposits/gunk on the solder mask, but in my case with just 2 tiny 2.2uF low voltage caps, there were nothing to see.2) don't waste time looking all around the board... if corrosion there is, it will be only in the vicinity of an electrolytic cap. In my case it looked like a radius of an inch around a given cap, was a generously sized area. No need to look 3 or 6 inches away from the nearest cap.3) don't waste time/bother inspecting closely each and every trace on their full length : the electrolyte doesn't seem to be able to eat though the solder mask. It will "run" and end up getting trapped around vias and pads, which it will eat because it's the only thing not protected the solder mask.4) the trace is being usually much narrower than the pad or via it connect to, the cut will will be right thre, between the end of the solder mask layer and the edge of the pad/via.. hence here to see by eye.5) Look/probe first for the pads/via that are closer to the caps, and those that look the more dull... but if nothing stands out and/or your problem is not fixed, then check all vias/pads because even not so dull looking ones can still have a cut trace. So check all of them one by one, methodically, making sure not to miss any.6) once you have found a possible candidate, scrape the trace with a sharp blade to reveal fresh copper you can solder to, and this also makes inspection easier. Remove the component (and replace it if looking to crusty/not trust worthy anymore) to reveal the pads, clean them with some flux and fine solder wick. Then use a microscope to confirm, or if like me you don't have one : you can make do by taking a high-resolution pic (8M pixel or so) or so of the pad, then using the "macro" mode of your still camera. Move the board around so as to get the best possible lighting on the suspect. Then zoom on the pic big time. See below for the pics I took, I think it's decent enough to be able to clearly see where there is a cut and where there is not.I was able to get away with this method because the area to check was small and the components are quite big and comfortably spaced, compared to modern SMD "standards"... but next time I have to search for corrosion (ie, next time I work on any thing that uses electrolytics ...) here are the things I wold like to improve, based on this first experience :1) a "proper" good quality scalpel/sharp blade.2) better probes more suited to small SMD components and corrosion : gold plated tips, thin, very pointy, sharp and durable, so that with minimal pressure, you can pierce though surface oxidation of solder joints and component terminals. This way you the search is made quicker and more reliable.3) a different wire : this wire wrapping/ 30AWG mod wire is not only too big, it's also very fragile and breaks very easily : I would solder it, then brush the area LIGHTLY with a soft ESD safe brush, and even though I would of course take care not to stress the wire joint too much... sometimes i would break anyway.Looking at Louis' Rossman Youtube Channel, where does it these things for a living all day long repairing densely packed modern Apple Mac Book mother board, it seems that enameled wire salvaged from a some small SMD coil (from whatever DC-DC converter for example), works just fine.4) Kits of SMD passive parts of all kinds and package sizes.... so you can easily/quickly replace any crusty little component. I was lucky to happen to have these tantalum SMD caps of the appropriate value and package size, but for the pull-down resistor of the Reset line, a simple 10K thing, well I didn't have that. glad it was just a pull down so the value didn't matter much, but most of the time the value WILL matter of course... so one needs a decent collection of values to chose from, eventually combining several resistors to achieve the required value.5) A bloody microscope !!! This very low-scale/modest first experience on this little front panel was eye opening, pun intended.... it was a pain ! I just can't imagine using a similar process on a large scale, never mind do it multiple times on several instruments. And since I do plan on repairing more old instruments in the future, because I like it and because I don't enough money to buy more recent stuff anyway.... then yes I will definitely buy a microscope. Looking at Louis' channel, his scope is good enough to my taste, and not too much money IIRC. Still some money though, of course ! So if my next repair happens before I have the money to get a microscope, then AT LEAST I will buy some cheap(er) way to magnify the image... ANYthing will be better and more comfortable than naked eyes.So... what's left to fix this scope ?A couple little things still need my attention before I can put back together for good :1) the membrane keypad for the soft buttons : the POWER/Standyby button is completely worn out, impossible to start the scope from the front panel. Yeah I could just wire a push button on the back panel and call it done... I guess, but I would rather try to fix the keypad. I think there exists kits to restore the carbon conductive surface on the contacts... but what I see on Ebay costs 50 bucks, which is what I paid for the scope ! Still, I would be prepared to pay this amount if and only if 1) It's quality stuff that actually makes a good and durable job. 2) there is enough of the stuff to repair many keys, so I can reuse it for future repairs and 3) the stuff once opened, doesn't dry out to fast and has a shelf life of several years so I can reasonably expect it to still be usable in my future test gear repairs. Might post on the forum for buying advice from people who have actually been there done that...2) Unit is missing its cooling fan !!!! I tossed it back then, can remember why exactly. Would have done such a thing lightly obviously, so either its was plain not working, or very, very, very loud/worn out. Looking at the shroud on on the case, looks like a standard 120mm unit, and from memory it runs from 12V DC from the third-party power supply board. I have a dozen computer power supplies, these typically have either an 80mm or 120mm fan I think, so no doubt I have one of those at home. Not sure if these are usually power from the mains AC or from the local 12V rail though, will have to check. Worse case, might shell out a few Euros for a brand new one... can't be that expensive, and I intend to keep this scope anyway, so might as well do things well.Then might try hacking it. The TDS 310 is the bottom of the line. 50Mhz 200MS/s. It's biggest brother the TDS 360 can do 200MHz and 1GS/s, much better !!!! Now if I could figure that out, I would be a happy camper ! Will post on here, you never know your luck, someone might have done it already...Obviously the schematics aren't available for this 310/320/350, but the schematics I have been using here, for the 340/360/380, which are so incredible close to my 310, are nice enough to clearly show you what the differences are ! At least on the hardware... I guess the NVRAM and/or FW would need updating and of course that can be show stopper, how to figure this out. So I am not holding my breath on this !Anyway, 3 scopes fixed in the lab in just 2 months, most of which was spent on the 2232 repair ! Not bad for a first dive into instrument repair, I am starting to enjoy this stuff as much I imagined I would !Can't wait to have some money and buy a new old instrument for repair and add functionality to my lab !Yeah, no point in buying yet another scope, even if I would love to ! Hard to resist , when I find something I like on Ebay ! LOL But it's more reasonable to concentrate my modest income on getting new types of instruments, to make the lab more useful, and more pleasurable.

author: Vince
author_rank: Super Contributor
timestamp: August 17, 2017, 11:06:51 pm
content: ...

author: Vince
author_rank: Super Contributor
timestamp: August 17, 2017, 11:41:20 pm
content: OK just searched a bit on-line for repair kits for this membrane keypads. Lots of dubious no-name stuff on ebay, yet still expensive. So checked what the big names in the PCB products have to offer. I looked the website from Chemtronics/CicrcuitWorks : found that, special purpose :https://www.chemtronics.com/circuitworks-rubber-keypad-repair-kitThe retail price they indicate matches up with prices I see on Ebay ads, so no rip-off there, phew. Typed in the part number on EBay to get all ads for that particular product... turns out a little humble computer shop just 30 miles from me sells the stuff, for as good a price as any, so ordered from them, should be here real quick, one less problem to worry about !Now just need to sort out a fan, and this scope will back in the game !

author: Vince
author_rank: Super Contributor
timestamp: August 18, 2017, 12:05:21 am
content: Looking up fans... can't believe : only 2 bucks for a brand new 12V DC 120m fan, delivered ?!www.ebay.fr/itm/120mm-x-25mm-12V-2Pin-Sleeve-Bearing-Cooling-Fan-for-Computer-Case-LW/182304419940OK, that's the cheap stuff from China direct, but still...So for 10/15 bucks I should be able to find something decent...

author: Old Don
author_rank: Regular Contributor
timestamp: August 18, 2017, 11:30:42 am
content: Quote from: Vince on August 18, 2017, 12:05:21 amJust about all new fans these days are coming from China. Sometimes if the only problem with an old fan is that it is running dry and you can lift the label on the stationary side and add a drop or two of light weight oil to the bearing. Depends upon the brand/model fan. Doesn't need much at all.

author: Vince
author_rank: Super Contributor
timestamp: August 18, 2017, 03:43:29 pm
content: Quote from: Old DonYes of course, just meant Chinese designed, and designed be crap/lowest possible cost. Chine crap, not chinese made but designed elsewhere.An 80,000 USD spectrum analyzer from Agilent/Keysight is probably also made in china, just like another chinese 500 bucks spectrum analyzer, still ,they are not realty the same. Even the Chinese themselves don't want to buy their own stuff, and as soon as they hit the middle class and have some money, they all rush to buy American and European stuff, and are all in awe and super proud to exhibit the stuff they bought from abroad.As for fans, looking at the specs in the Tek parts list, they specified a 39CFM fan. Looking at the offering on Farnell, at least the subset I have access to here in France, 39cfm doesn't exist anymore. Most of them output over twice this flow. The slower fans make 40+ CFM, so will take that. Bonus is that obviously they are much quiet than the more common 90cfm ones. downside is, much less choice, and over twice the price... 30bucks instead of 15, then add VAT. Still, an acceptable price to pay to put this scope in top shape.http://fr.farnell.com/ebm-papst/4412f-2gll-515/ventilateur-pc-120mm-12vdc-avec/dp/1875295Oh, lucky me, just found a brand new one of these on Ebay Germany for only 10/15 Euros, ordering right now...

author: Vince
author_rank: Super Contributor
timestamp: August 27, 2017, 06:51:02 pm
content: OK..... received the kit to repair the keypad... 25 bucks down the drain. Tried my best (first time I attempt such a thing), but clerly that was not good enough. Next time I will be much better prepared for sure, but for now the bottom/horizontal part of the bezel buttons is a total mess, I screwed up big time.Now most of the tracks that run above and below the carbon pads, which constitute the matrix... are ct in many places, at every key location or pretty much.Theses tracks are sandwiched between the two transparent layers of the flex PCB, so I am not even sure how I could get to them, to attempt a repair with yet another expensive repair kit. I can't use a sharp knifE/blade or I risk doing even more damage I guess. Maybe I could locally melt the plastic with a soldering iron and a fine tip, locally, to reveal enough of each and every cut trace, so that the conductive repair varnish/compound could have a chance of sticking to the area.It sucks... I did all the hard work, figured out what was wrong with the scope, fixed it, and now all that effort ius ruined by some cut traces on a bloody keypad, grrr...Had a look on Ebay, couldn't find any keypad PCB for sale. Will keep looking. In the meantime I will move on to something else, I need a break... kinda pissed to be honestIf anyone knows where to find a keypad flex PCB for the bezel buttons, put your hand up... will keep looking on Ebay as part of my daily search for anything "Tektronix" related, might get lucky in few weeks or months. In the mean time I will put it aside. It's basically diagnosed and fixed, so not worried... no mystery fault any more, unit works fine... "just" needs a bezel keypad....As for the cooling fan, brand new unit is in the mail, should be here any time now. It's a 3 wire job not 2 wire like the OEM one, so will need to figure it out, don't know who 3 wire fans are put together. Hopefully the third wire is just a pick-up used by the speed controller to adjust the voltage supply, so basically I could just ignore it and just run the fan on 2 wires without even caring about this third wire. If it's more involved than this then that would need to be figured out. Not too worried though... this subject has probably been covered a billions times all over countless forums, a quick search on Google would sort me out, I would think.. hopefully.

author: Old Don
author_rank: Regular Contributor
timestamp: August 27, 2017, 08:43:38 pm
content: I'm not sure what failure you have with the membrane, but if it is a break in the traces then you might look at an auto parts store for a rear window heater repair kit. The kit is used to repair breaks in the wires that heat the rear window to defrost the window.

author: Vince
author_rank: Super Contributor
timestamp: August 27, 2017, 09:00:58 pm
content: Hi Don,The problem is described in previous posts, cut traces on the flex PCB, causing keys to stop responding. I already bought the conductive touch up stuff, as said earlier. bought the stuff specialized for PCB work, purpose designed for this type of problem.Problem is that it was manageable to start with, but one tihng leading to another.. I eventually severely aggravatedd the problem, instead of fixing it... now instead of having a single key to fix, with nicely exposed traces, I now have MANY, many cut traces to fix, which are buried inside the flex PCB itself. So multiply the number of traces to fix by the lack of access to said traces... and you find Vince with a MUCH, MUCH bigger problem to fix than before !Basically : the repair compound is impossible to apply with any decent accuracy, even with the tiny brush that's supplied with the kit. So it's impossible to follow gthe complex shape of the carbon traces that make up a pad, see pictures above. It's looks likes crossed fingers so to speak.. trying to "repaint" the fingers individually ends up invariably in shorting the fingers/contacts together. So only solution I foudn was to cover the entire pad with the stuff, then make a slit right in the middle, to break electrical continuity where the carbon pad inside the membrane, which it would then bridge, just like in cheap remote controls of all sorts, where the pads simply consists of two moon halves so to speak, instead of the complex intermeshed combs that I see no the Tektronix PCB, presumably for added reliability. So, the question was : how to perform this slit ? It had to be very narrow, because the carbon pad inside the rubber membrane, is tiny, 3mm in diameter, so you must position the slit very precisely, as well as making it as "sharp"/neatly defined, and narrow as possible. Ideally a sharp blade would do it, but the risk of cutting through the PCB (which is what I did ultimately anyway...) was too great. So I looked around the work bench to find something a bit less sharp, yet still sharp enough. My eyes stopped on my pair of tweezers. They crap and hardly razor sharp. So I used that to gently, and progressively, cut a slit into the compound, where required. I did that on all the keys of the bottom section of the keypad, not just the power button, because I cleaned them with of IPA before hand, as required... and I to my horror, the IPA would not just clean the carbon traces... it would take them off the PCB as well !No, I was not rubbing hard at all, not by any stretch of the imagination, I swear. Looks like carbon just doesn't quite stick to this flex PCB all that well to start with !So I was screwed, had to cover all the pads/keys with compound, rather than just the power button. So.... I ended up cutting all the traces not just above &top of the Power on pads, but also above and top of all the other keys.... yeah.....As as right now, I am patiently and carefully cleaning/scraping all the excess compound so I can get a better view of the numerous cut traces... first step in assessing the extent of the damage and figuring out if I can attempt a repair of some kind.... Problem is that I don't see how to access the cut traces other than using a sharp knife to scrape the top layer of the flex PCB, and sharp stuff is precisely what caused all this trouble to start with ! So I fear I will just aggravate things even further.Only positive note ? Well, I don't have anything to loose, so might as well try anything and everything to repair it ! If I leave it as it is, the scope is unusable....

author: carl_lab
author_rank: Frequent Contributor
timestamp: August 28, 2017, 06:07:29 am
content: Have you tried conductive silver varnish?I've repaired dozens of PCBs, including flex PCB keyboards, using similar stuff like this:https://www.kemo-electronic.de/en/Components/Consumables/Soldering/L100-Conductive-Silver.phphttp://www.cmr-direct.com/en/cmr-rs186

author: Vince
author_rank: Super Contributor
timestamp: August 28, 2017, 10:11:31 am
content: Yes, as I said earlier, I did use what you just showed. Maybe I was not clear... my bad.I bought this kit, what whatever the brand I guess.https://www.chemtronics.com/circuitworks-rubber-keypad-repair-kitProblems are1) Getting to the cut traces (buried inside the flex PCB, not exposed) and2) The sheer number of cut traces to fix/bridge (my mistake, sure), and3) The difficulty to apply the varnish onto one trace without covering the adjacent ones. The supplied brush is tiny but still way too big/inaccurate for the job.I need to first expose all the traces by patiently and gently scraping the top plastic layer of the flex PCB so as to expose all the cut traces... am halfway dong this now, talk about tedious... and I could used some better eyes and/or magnification...The traces are so thin (nothing like copper traces on a "real" PCB, sadly), so it's a challenge to expose them without actually thining them down to the point that they start to vanish.Then once all the numerous cut traces will be be exposed... I will have to make some kind of a stencil so I can apply the conductive varnish accurately without bridging traces all the time...It's a lot of fun... lesson learned, will be more careful next time !

author: tautech
author_rank: Super Contributor
timestamp: August 28, 2017, 11:04:00 am
content: Bit of a long shot but have you checked Sphere ?http://www.sphere.bc.ca/test/tekparts.html

author: Vince
author_rank: Super Contributor
timestamp: August 29, 2017, 12:48:13 am
content: To be honest I didn't even think of checking them ! I simply checked Ebay, quickly.I will keep working at the mess I made of that poor keypad, and if/when I get to the point where I declare it totally hopeless... I will start searching for a replacement a bit more seriously !But I am not quite there yet... not far from it, but not quite ! Last night I managed to scrape the top plastic of the flex PCB to reveal cut traces, more "easily" (hum...) than I feared, as this plastic layer is not as thick as I thought it might be. I have managed to reveal many traces and they survived, pretty much. So I will keep doing that. Once all cut traces, 3 dozens of them, will be exposed and clean... I will have a second chance at fixing this thing... I won't have third chance so I must take my time and do it right/properly, this time !I can't stand the idea of giving up on that scope just because of this keypad, when I have done already all the hard work of trouble-shooting and fixing it ! Taht would be awful... so, I will keep working at it.... you gotta persevere in electronics if you want to achieve anything valuable, don't you.. just like in many things in life eh ?!Good things/rewards rarely come for free ! ....

author: tautech
author_rank: Super Contributor
timestamp: August 29, 2017, 12:57:03 am
content: Couldn't think of them last night but can nowQservice should be checked too:http://www.qservice.tv/vpasp/default.asp

author: Vince
author_rank: Super Contributor
timestamp: August 29, 2017, 01:43:48 am
content: Yep ! QService and The Sphere, since you guys told me about these two mine golds in my first repair, on the 2232, I have these two places well in mind now !Just looked up the part number in the service manual: 259–0086–00QService has them listed !http://www.qservice.eu/shopexd.asp?id=4235... but none in stock at the moment.I liked their comment : "The problem with these Flex PCBs is usually the "Power On" button which wears out" ! Sure, exactly what happened on mine !It's the only button on the bezel that's guaranteed to be pressed every time the scope is used, so I guess it's logical that it goes first...Still, crap design IMHO... the buttons on the front panel hold up just fine, so they could just have put a "proper" PCB for the soft buttons, instead of a Flex, and then used a flex only to link the bezel board to the front panel board. Anyway, what it is done is done, not much point rewriting history I guess. I screwed the repair, not Tek... I am the one to blame ultimately !

author: tautech
author_rank: Super Contributor
timestamp: August 29, 2017, 04:09:02 am
content: Do a Google search for "Tektronix 259–0086–00", there's more suppliers to check.

author: Vince
author_rank: Super Contributor
timestamp: August 29, 2017, 09:28:01 pm
content: Really ? The only helpful link that Google served me, was that page on QService but they are out of stock. No other useful link...ISTR that Google returns different results for different people, based on their geographical location... so as to give people more relevant information. Maybe Goggle is trying to be too smart on my end, and in the case of this particular search, it's playing against me... bummer...

author: tautech
author_rank: Super Contributor
timestamp: August 29, 2017, 09:35:27 pm
content: Quote from: Vince on August 29, 2017, 09:28:01 pmLet's see if you find anything with this:https://www.google.co.nz/search?q=Tektronix+259%E2%80%930086%E2%80%9300&rlz=1C1ASUM_enNZ497NZ497&oq=Tektronix+259%E2%80%930086%E2%80%9300&aqs=chrome..69i57&sourceid=chrome&ie=UTF-8

author: Vince
author_rank: Super Contributor
timestamp: August 29, 2017, 10:26:24 pm
content: Thanks.I does give more interesting resutls indeed... though still no help.Mainly 2 new links I don't have on my end of the world :1) "Tek-Parts"... but when you click on it, it redirects you to... QService again ! LOL Go fogure.2) "http://www.vernonianorthern.com" ... they do have a few Tek parts, but not that many, and mostly IC's.. not keypads, and ... on their site they do say that they want to sell their TEk parts operation, and concentrate on their core business which is to do with model trains...Thanks for the effort though, I appreciate !For the time being I think I will just keep scraping the old flex PCB and order another expensive repair kit... and this time I will try and do a better job....I need to figure out what kind of glue I need to use to glue the flex back onto metal work of the chassis... any suggestion ? On QService website they say NOT to use "super glue"...Maybe a simple double-sided tape might do !In order to inspect the back side of the Flex PCB and better assess the extent of the damage I caused, I pulled the Flex PCB off of the metal work, then used some alcohol to removed the old glue from the back side of the Flex. So... I need to glue it back. Can't repair the cut traces until the PCB is laying perfectly flat and secure, on the metal work.

author: tautech
author_rank: Super Contributor
timestamp: August 29, 2017, 10:56:58 pm
content: Other than a good double sided tape some modelers glue is what I'd try.....you know, that clear rubbery stuff.

author: Vince
author_rank: Super Contributor
timestamp: August 29, 2017, 11:24:17 pm
content: Thanks for the suggestion Tautech ! I am not at all into model things, but thanks for the pointer, I will investigate !

author: carl_lab
author_rank: Frequent Contributor
timestamp: August 30, 2017, 12:22:08 pm
content: Quote from: Vince on August 29, 2017, 01:43:48 amThat's not a Tektronix specific problem, HP also made similar designs in 8647A, 8648A-D signal generators for example.

author: Vince
author_rank: Super Contributor
timestamp: August 30, 2017, 09:57:33 pm
content: Good to know... I do plan on buying an RF sig gen (for repair/cheap, probably... money is tight) at some point, and these particular units are in my short list, I am quite fond of them. Well, I mean purely on a look & feel perspective... don't know how they perform technically, not competent enough to judge, but since they are fairly "modern" HP stuff, and I am just doing this as a hobby with no particular tight specs to meet, I guess any top brand model from this era will do just fine !OK, Just bought a new, yet another, repair kit/conductive "varnish"/compound, might be able to make it in time for this week-end. Today I also bought some double-sided tape, to stick the flex PCB back into place, and today again, I received the brand new cooling fan for the cabinet. So looks like I am all set.. with some luck the scope might be fixed for good, completely, during the week-end... crossing fingers.At any rate, I have already a new repair project lined up as soon as this TDS 310 is done, so stay tuned !I expect to receive this mystery unit in the next few days...

author: carl_lab
author_rank: Frequent Contributor
timestamp: August 30, 2017, 11:07:24 pm
content: Quote from: Vince on August 30, 2017, 09:57:33 pmHP 8647/8648 are not bad, I like them very much.A few years ago I had some with partial dissolved silver tracks on the keyboard flex. I assume the silver reacted with sulfurous evaporation from some rubber parts to silver sulfide (black), that bunk off and left some local disconnections. I bypassed them by conductive silver varnish and the generators worked without problems since that time.

author: shakalnokturn
author_rank: Super Contributor
timestamp: September 03, 2017, 09:58:20 pm
content: Congrats on the fix! You'll be saving TDS scopes by the dozen soon.When fixing broken tacks/vias a single strand of multi-stranded wire can be handy, they come in different diameters too.For your next attempt on a flex print, try either a needle point or toothpick to apply the paint. The blue insulation coating cansometimesbe scraped/chipped off with a good blade but you need to get the pressure and angle right to not remove the track with it.The other option (it would probably be mine at this point) is to scan the remains of the flex print and rebuild the as a stiff version. Of course that gives you an excuse to invest in some etching equipment for your lab.I see a Hameg scope made its way to your bench, is that in need of attention too?

author: Vince
author_rank: Super Contributor
timestamp: September 07, 2017, 01:17:38 am
content: Hi there, le Chacal Nocturne, welcome aboard EEVBlog !Quote from: shakalnokturn on September 03, 2017, 09:58:20 pmI have to say that these repairs are quickly becoming an addiction !No more TDS to fix at the moment, but I DO have another Tek scope repair lined up ! ... found a lovely piece of Tek gear, cheap, just could not resist...Hint #1 : model number is "TDS 300 like" .. but it's not a TDS.Hint #2 : fixing this one will most likely require getting some help and advice from the gray beards at TekScope !QuoteHmm thanks, will look into that...QuoteYea, that's my intention too ! No more brushes for me, no matter how tiny they are... they are still way too big for the job !QuoteYep, that's what I have started doing a couple weeks ago... see pictures below. Looks like it's working, the top plastic layer of the flex is quite thin in fact, so it's not that hard to get rid of so as to expose the underlying traces. It's a long and tedious though.. so I still need some motivation to resume work on it, and scrape some more, scrape, scrape, scrape....... with naked eyes... it's a form of torture I say ! .... I think I will teat myself with a microscope for X-mas....QuoteYes this TDS 310 repair is truning more and more like an excuse to get this and get that, learn this and learn that...Not sure I want to get back into etching my own boards.... I sure did 25/30 years ago, when I was a kid... quite messy !These days I would just outsource the job to some PCB service ! Price is affordable as long as the surface area of the board is small, which would be the case here. I would have to find a company that offers thin 0.8mm PCBs though, as I don't think a standard 1.6mm board would work : I fear it would raise the flex too much, ie have the same effect as pressing the buttons ! Not fully of course, but half-way... that's why I am giving up on double-sided tape to secure the flex on the frame work : it comes on a pretty thick foam substrate... Glue is really the only thing that would work I think...Anyway, I really need to move my butt and finish the repair on this flex, so I can put this scope back together : I need/want to move to move on to the next repair !Quote"made it's way" ?! It's one of my very first/early scopes !My old man gave it to me 20 years ago or so. So not quite my very first scope but almost.All the other scopes in the lab were acquired very recently, since I built my lab last fall.QuoteIt is now !!Worked just fine when I used it a couple months ago when I used it to help me fix my Tek 2232, but yesterday I noticed taht the vertical position control knob on the Channel one, is borken, spins round and round, doesn't work any more !See pics below. I had this same problem on this scope, a few years back, on some other button, delayed trigger fine adjust I think. There is a plastic coupling that links the long rod to whatever is on the board (a pot, a trimmer, a switch...), and these develop a crack along one of the tiny screw that secures the couping to the shaft. The screw therefore becomes ineffective and fails to grab the shaft. Anyway, this first coupling was easy to fix because it was very easy to get to, as you can see. However this second broken coupling, CH2 vertical position control... appears to be buried deep inside the guts of the scope, between two riveted metal plates forming part of the framework/chassis. I don't even know how I can get to it to attempt a repair of some kind ! Looks like the scope was literally built around this coupling !I will have to take this bloody scope completely apart just so I can get access !

author: tautech
author_rank: Super Contributor
timestamp: September 07, 2017, 02:25:05 am
content: Vince, get yourself a head mounted magnifying visor for the small work, I've had one for years and wouldn't be without it.For the Hameg shaft couplers, member Specmaster solved that one in this thread.https://www.eevblog.com/forum/testgear/hameg-408-1a/The shafts might have retainers just inside the panel and once loosened will allow the shaft to be removed....check for them.

author: Vince
author_rank: Super Contributor
timestamp: September 09, 2017, 09:13:03 pm
content: Quote from: tautech on September 07, 2017, 02:25:05 amYeah I think you might be right ! Anything is better than nothing at all... and this kind of things I guess should be pretty affordable, will check that out !QuoteOh, looks like a common problem, some one has been there done that !Looking at the huge number of views, 10,000 in 4 months.... I take it it's not just me and Specmaster having shaft coupler problems on these scopes !

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2017, 09:45:18 pm
content: Quote from: Vince on September 09, 2017, 09:13:03 pmYou don't need anything very flash depending on your preexisting eyesight.I only got a set when I started SMD work but my eyes are worse now and I now use them on minimum magnification for most work.2nd from the left in the images at the top of the page but I didn't pay that price.https://www.google.co.nz/search?q=head+magnifier+visor&rlz=1C1ASUM_enNZ497NZ497&oq=head+magnifier&aqs=chrome.2.69i57j0l5.11825j0j8&sourceid=chrome&ie=UTF-8Some prefer the ones with LED's.....but they are heavier and a little bulkier.Don't be tempted by anything other than those with one piece acrylic lenses....they have have all sorts of field of view and alignment issues.Quote from: Vince on September 09, 2017, 09:13:03 pmI don't think it's at all common as I haven't seen mention of it before but it was a interesting problem to solve and gathered a following of sorts. Even your threads will gather huge numbers of views over months/years as viewers just browse to gather info from others experiences.It's always good to share info.

author: Vince
author_rank: Super Contributor
timestamp: September 09, 2017, 10:10:22 pm
content: Thanks... looks like the ones on the first picture, left of yours then, is only 12 bucks shipped from China, and not so bad... dixit this 90 year old half blind gentlemen !Even comes with different lenses for different magnifications factors.Worth a try at such low price... not sure I wanna wait a month for them to come, though... don't want to wait this long to fix my flex PCB on the TDS. Will be handy for all future work though, obviously...

author: Vince
author_rank: Super Contributor
timestamp: September 24, 2017, 12:02:40 am
content: OK I worked on the thing today... not looking good !My second attempt at fixing the flex was a failure... it's too much of a mess, I officially give up and am now searching for a replacement flex... if anyone finds one before I do (Sphere, Qservice, Ebay, wherever... ) give me a shout !In the meantime I thought I would at least try and fit the new cooling fan I bought for this scope... should be straight forward I thought.... well not quite.Fan uses a 3 wire connector, luckily it can be used as a simple 2 wire fan without any drawbacks.However there was one apparent problem already... even though I bought the slow/quiet version of this fan.. I tried it on the bench supply and at its nominal 12V it's very loud and air flow is wayyyy more than this scope needs... and the scope's power supply actually output more like 13/14Volts not 12... not going to make things better/quieter ! So already some hack would be needed here. Either brute force/adding a beefy series resistor (at 12V the fan draws about 500mA), or the elegant way : modify the power supply so as to adjust the voltage to a lower setting... but that might not be possible because this "12V" is also I fear, also going to the CRT board... plus, for all I know it might also be used as a starting point to created one or more lower voltage rails. I wouldn't know because it's a third-party power supply so the schematics are nowhere to be found, as usual....So I thinks the series resistor is the sager way. I guess I were motivated, I could use a compact adjustable DC-DC converter to waste less energy and let me easily adjust the fan speed. Yes, that would be a better approach... not going to spend any money on this ultra low end scope though, so would have to be either a home made converter, or a cheap Chinese power brick from ebay... I guess for a bloody fan this would be perfectly acceptable...Also had to remove the connector from the power supply board so as to fit a connector that would match the one that came with the fan. Could salvage one from my pile of scrap boards, great.From this point.. things would start to g downhill !1) PSU powered off, I plug the fan and it SPINS !!! EhVery slowly mind you, but sill, how could the fan spin AT ALL, given that the PSU is not powered up ?!I unplugged the PSU then plugged it back, fan would not spin anymore, phew. Still, a mystery...2) PSU does not work any more !!!!NOOOOOO !!!! This scope is driving me crazy, I did the hard job at figuring out and fixing the real/difficult problem, and then I got stopped byt this stupid flex keypad and now by a bloody PSU ?! This thing is getting on my nerves now... clearly the educational value of this thing, through the repairs that it forces me to investigate/learn from, is way higher than the technical/performance worth of the machine !!!So I try to remain positive about it : I see it as an educational tool, not much more. This means that, though, there is no way I will spend any significant amount of money on this scope, it's just not worth it. If I can't fix it cheaply, then I will keep it for spares because at some point I would like to geta higher spec TDS 3000, a 380 or something, with 2GS/s and 400MHz of BW, it's much more appealing.So now I am having to trouble shoot this HV SMPS, with no schematics... a good exercise I guess, so will try to see if I can manage that. "Luckily" my Tek 2232 blew its SMPS recently so I learned the basics of HV SMPS while fixing it, thanks to the good souls on this forum. I do have some basic knowledge of these things now... at least the topology. And indeed when I look at this third-party SMPS, I am not totally in the dark, phew.I am not sure if I feel like working on this SMPS at the moment, will depend on my mood (and I have other gear that need fixing as well, more on that in future topics ! ). A few minutes reveal the following, as a starting point !- All power rails read 0V.- The standby voltage is gone : read ground, whereas it should be around volt whether the board is powered up or down.- Even though the standby voltage is missing, somehow when I ground this pin (as would happen it I pressed the power button on the instrument), teh board DOES react : I can hear the relay click as it should.. but still, all power rails read zero volt.- There is a smell from the "hot"/primary side of the SMPS : the heat sink carrying the power FET and another, progressively gets warm... then slightly hot, but one can still maintain permanent finger contact. Still, why would it get "that" hot if all power rails are down ?- more blatant, still on the primary side : there are two beefy resistors, which are literally BOILING hot !!!! It's a 100+ kohm resistor, maybe 2Watt. Reminded me of the 150K resistor in the 2232 SMPS that makes the mains rectified voltage trickle to the PWM chip. The other resistor is a big 5Watt ceramic 3,9k resistor.Obviously these resistors aren't at fault, they are just the poor victims of the real cause of the problem, somewhere downstream.... but what problem is it... that is the question !I don't have the schematics so I will have to reverse-engineer the thing to some extent I guess, armed with the little knowledge I gleaned during my 2232 repairThe main IC's on the primary side are (datasheets attached below)- PWM chip : 8 pin CS3842A- Power FET : a BUZ 355 ... not sure what all the "buz " is about though, because it's a pretty horrific unit : 1.5ohm Rds(on) !- TIP 50 : 1A BJT , mounted on the same H/S as the FET. Maybe its drives the gate of the FET, the 2232 had a similar arrangement, though using a tiny T0-92 packaged BJT, not a big TO220 like this TIP 50 ! So maybe it's doing something else entirely, who knows.- LM393 comparators... 4 of them on the board ! They must love them I guess. 2 of them on the primary side, 2 on the secondary. No idea what they do. I guess one of them must be tied to the standby line in some way.- MOC 8103 optocouplers for primary/secondary feedback isolation, 2 of them.So I guess I should turn my attention to semi-conductors first, power ones first (diode bridge, FET, TIP 50), then low power stuff (PWM chip and comparators), the passives last..At least the good thing is that I can easily work on the thing : unlike the 2232, the SMPS in the TDS is stand-alone so I can have it on the bench on its own, and super easy access to all the components. Working on it will be much easier then.So here I am, second stage of this scope repair.. front panel at first, and now the PSU.. you don't get to choose...I will try to repair it with my limited knowledge, and if that fails I might create a new topic for this particular issue, so as to get a better exposure/larger audience.Anyway, topic far from closed then....

author: tautech
author_rank: Super Contributor
timestamp: September 24, 2017, 12:15:59 am
content: With two PSU transformers is this not a pre-regulator configuration ?

author: Vince
author_rank: Super Contributor
timestamp: September 24, 2017, 12:47:35 am
content: Yes it looks like a similar topology as the 2232 PSU : common mode chocke, then mains is full wave rectified, then a big filter cap, then a PWM chip with a big FET on a H/S... just not sure yet where the coil is, though !Then the "big" transformer in the middle must be that of the inverter, whose outputs are associated with beefy diode pairs mounted on a H/S, just like the 2232. Then the last/second big transformer is on the secondary side of things, and delivers all the low voltages then regulated with simple 3 terminal regulators for most of them at least. The 12V for the fan and CRT board being less sensitive, I think it may be possible that they are regulated using simple discrete components rather than a "proper" regulator.So I am using the schematics from the 2232 PSU as a rough guide to help me figure this third party unit....At least we have all the datasheets for all the actives devices on the board, a good starting point to probe around...

author: Armadillo
author_rank: Super Contributor
timestamp: September 24, 2017, 12:58:45 am
content: What exactly is cut the flex cable? Maybe a closeup of the cut would be easy to understand.

author: Vince
author_rank: Super Contributor
timestamp: September 24, 2017, 01:17:06 am
content: Hi,I explained and illustrated the problem in detail in my previous messages ;-)I am not wasting any more time and money and effort on this thing. I ruined it despite having the best of intentions for it, my bad, will know better next time I have to work on these things...

author: Armadillo
author_rank: Super Contributor
timestamp: September 24, 2017, 01:44:29 am
content: Do you mean that single carbon key you partially rub the carbon away?

author: Vince
author_rank: Super Contributor
timestamp: September 24, 2017, 01:52:58 am
content: No.. this was the ORIGINAL problem... at this point it would have been easy enough to fix with the repair kit I bought... but if you read the following messages you will see that I made a complete mess of ALL the keys, basically cutting pretty much each and every track that runs on this flex ! I tried to fix all that, I did... but no more, I give up. You must know when to stop, sometimes !

author: Vince
author_rank: Super Contributor
timestamp: September 24, 2017, 01:08:15 pm
content: OK making progress on that PSU. I Googled the manufacturer and part number (Zytec 22917300) and got 4 matches. No, not 4 thousands pages of results like you always get with Google.. not 4 pages, no.. 4 matches... of which 3 are useless because they all point to the same site, which is no help whatsoever. So basically there is only one, onnnne single match on this PSU, which gives useful technical information. So I thought it's worth sharing !https://www.electronicspoint.com/threads/tektronix-tds350-psu.25635/Basically a forum where there is a poor soul like me in need of assistance with this PSU, then comes along a lad who fixed a LOT of these TDS 300 series scopes, and no less than 18 times, this bloody PSU was the issue ! Fixed them all as I understand it, and has some useful information... I will summarize :1) The big 5 Watt resistor that's boiling hot on the primary side... it's a common fault apparently. Eventually dies and goes open circuit after a while. Mine is not dead yet, but I might as well measure it to make sure it hasn't suffered/drifted significantly.2) Said resistor is part of the start-up circuitry... so that would be consistent with the standby voltage having disappeared on my board !3) Topology : the 5V rail is used by the feedback loop to adjust/correct the primary voltage. So if the 5V is not working, all other rails will be down as well even though they work just fine. So the 5V must be checked first... as long as it's not working, no point wasting time trouble-shooting any of the other rails.4) The two beefy double diodes packages (FEP16DT) mounted on the secondary side heat sink... are used to form the full wave bridge rectifier of the afore mentioned 5V rail. Sometimes these diodes are shorted. If shorted, then a NTE 6240 works fine as a substitute.OK, that's quite helpful, better than nothing at any rate, that's for sure... maybe I could try to see if this Zytek company still exists and ask them nicely about the schematics... you never know your luck ! Product is 20+ years old so maybe they won't mind...

author: Vince
author_rank: Super Contributor
timestamp: September 24, 2017, 08:23:08 pm
content: OK I think I sussed it... in the end it's no coincidence that this PSU decided to "fail" just when I so happened to plug the new fan on it...Spent some time checking each and every semi-conductor on the board, quite a few of them (when you add up all the signal diodes and small trannies)... all were fine. The big 5W 3,9 resistor s fine too.BTW the PSU is well designed, a joy to work on : the two heat sinks are not soldered to the board but rest on stands, they just click into place. The components are not screwed on the H/S but simply held in place by a metallic "spring"/clip, neat. And there is no dirty/greasty white cthermal compound either, but rather a rubbery pad. So it was very easy and quick and "clean" to removed all these power semi-conductors and their H/S.Also, the PSU is really safe : the big mains filter cap, which is charged at 320V DC then (in my neck of the woods anyway), must be fitted with a bleed circuitry because it takes literally only a mere 5 seconds for the voltage to drop to a single digit voltage !So I just didn't know what could be wrong.. decided to power it up to try and probe around... go figure, the thing worked just fine !So... I plugged the fan in and... again it failed to start, though you can see it's trying to. It's jerky. At the same time... I notice that all the other rails are WAY down, like 5V instead of 8.5V for example ! 40% off or so, quite a hit.So I thought, maybe the 15V output supplying the fan (and CRT board), is faulty and fails to supply enough current. So plugged the fan back in, but also measured the current it was drawing, or trying to at least ! Didn't bother hooking up the scope, just did a quick check using my DMM and its "PEAK" detect mode... first time I use this feature, quite handy in fact ! So, the short-circuit current the fan draws every time it tries to start is about 250mA tops., but the fan requires about twice that.So, looks like we are on the right track here ! Might explain why I tossed the previous fan 4 years ago then : I probably noticed it failed to start, assumed it was faulty so just figured I would have to replace it.... which I am doing 4 years later, and the fan still doesn't start...Looked at the parts list in the service manual... I was obsessed with the CFM/air flow rating, not wanting to overheat the scope, so bought a fan with a similar flow... and in doing so I overlooked this important piece of info : OEM fan was rated at 2 Watts, the new one I bought is 5W ! It all adds up : the board peaks at 250mA or so, enough to drive a 2 or 3 Watt fan, but definitely not my 5W fan.Still.... the previous fan, if it were OEM... it should have started ! So maybe it was indeed faulty, or it was not the OEM fan, it got replaced in the past before me, and the guy ran across the same problem I am having right now.So I guess it will all end up nicely, though : as said before, this new fan is way too noisy and produces way too much air-flow at its nominal 12V, never mind the 14V that the board is outputting. So adding a series resistor to halve the current should be able to make the PSU happy as well as make the fan spin more slowly. I just tested it using the bench power supply : adjusted to 14V and current limited to 250mA so as to simulate the PSU. Under these circumstance, the fan is a bit lazy to start obviously, but it does start reliably, and once the speed is stabilized, noise and airflow are at adequate levels.Phew ! That's one problem sorted. Lesson learned : next time I replace a fan... don't assume that the PSU has ANY headroom current wise, assume it can not deliver more than what was strictly necessary for the OEM fan to work ! In this case my 5W fan 2.5 times more powerful than the OEM 2W fan, my bad.I do have a bucket load of power resistors salvaged over the years, I am glad they an be put to use at long last !Oh, and as far as topology goes, I was confused about the apparent lack of transformer/coil for the FET in the pre-regulator section... in the end there is no pre-regulator ! The FET drives directly the inverter transformer apparently ! I guess it works...Also, the TIP 50 / power BJT mounted on the FET heat-sink... has nothing to do with driving the gate of the FET (like it does in the 2232). The PWM chip has a totem-pole output capable/designed to drive the FET directly, so no need for an extra tranny to drive the gate. So, the TIP 50 what does it do ? It is connected directly to the 5W ceramic resistor of the start-up circuitry, so must be involved with this part of the PSU...So I will add this series resistor and call it done. A bit of a hack sure, but that will do. Next time of course I will make sure I get an appropriately rated fan (if at all possible) to avoid this kind of hack !

author: tautech
author_rank: Super Contributor
timestamp: September 24, 2017, 08:36:03 pm
content: Nice one Vince.Did you check the LV caps...value, ESR ?

author: Vince
author_rank: Super Contributor
timestamp: September 24, 2017, 08:42:52 pm
content: Nope, I don't have an ESR meter jsut yet... so I have been making progress recently on what gear brand/model might find mty fancy and fit my budget. So as soon as I can afford it, will do. By the end of the year I think.As far as measuring values, measuring in circuit is not reliable in circuit since I don't have the schematics to see the surrounding circuitry...overall I didn't bother because they all "looked" fine and the PSU was working just fine so it seemed odd to me that it would fail suddenly due to caps. I was right of course, retrospectively...But yeah I can't wait to get the gear to check ESR (will get I think an LCR meter altogether actually, and avoid paying twice for this feature...)

author: Vince
author_rank: Super Contributor
timestamp: September 30, 2017, 07:20:23 pm
content: OK, just worked a bit on the thing. Tried to add a series resistor to the fan , as described earlier... doing so was a total failure and turns out the repair might be more interesting than envisioned at first : might well be that the PSU is indeed faulty, and my fan replacement far being a more powerful than the OEM one may not be the actual issue ! Might still be an issue will see, but it's definitely not the only problem here.... here goes :At first, I used the bench supply to figure out the appropriate resistor, again set to simulate the PSU open circuit voltage (14V)) and short-circuit current (around 250mA), as it tries but fails to get the fan spinning. As a safe side approximation, I simplified and took the fan winding resistance out of the equation, assuming it had to be small, and the back EMF being zero of course since the fan fails to spin. So 14V divided by around 250mA, 56ohms. Had a 47ohms, tried that, yielded to a 250mA current so perfect ! Fan would start reliably but air flow was not good enough to me. Tried a 33 ohms one, air flow was now adequate.So... I then tried that on the actual PSU.... failed to start. Reverted to the 47ohms one... still fails to start (and again all power rails pulled low consequently). Now I am starting to worry... so I tried higher value resistors. Fan obviously would not start but I wanted to see at what point the power rails would start NOT going down anymore. Tried 150ohms... rails still down (about 40% lower than normal, 8,5V rail would read 5V, and 5V would read 2+ V) !Tried 150ohm, rails still down. 330 ? Still down. 470 ohms ? YEAH they are up at last !Measured voltage drop across the resistor, it implied a current of around 20mA !Soooo.... this fan connector is supposed to spit out at least 165 mA in order to be able to drive the OEM fan (parts lists says 2 Watts 12 Volts), but can managed only 10+ of that ?!Surely my fan can't be responsible for that can it...So looks like the PSU/ fan output is too weak... and the fan I tossed 4 years back when I first got the scope... probably was the OEM one and was probably just fine... oops....So... PSU troubleshooting ! Without schematics... a first for me. I meant this TDS 310 repair to be a learning exercise above alll, and looks like this scope heard me ! LOLHad a quick first go at it. There are 4 main rails : +/- 5V and +/- 8.5V. We know that +5V is regulated from the primary side/PWM chip, so we are left with 3 rails that need regulating.In the bottom right corner of the board, just near the main transformer and main output connector, we see three x3 terminal regulators, good start. A positive one (LT1086) and two negative ones (LM 337), it all adds up.Now for the two auxiliary connectors, teh fan and the CRT supply... The fan is supposed to be 12V and the CRT is supposed to output 15V. but in practice, open circuit, they both deliver a similar 14V. OK, so maybe Tek decided to use a single 14V, half-way between 12V and 15V, to cater for both needs with one supply only, why not. Still, problem remains : 14V from... from what ? Higher voltage main rail is 8.5, so it can be used to derive 14V from it without a DC/DC converter. We need at least 15V to produce 14V... and I using 15V to feed 5V and 8.5 Volts linear regulators would produce way too much heat and waste way too much energy... so...... the 14V must have it's own tap on the transformer, and it's own voltage regulator.... but I can't see any. We have run out of 3 terminal regulators already, and no fancy DC/DC boost up converters of course, and no DIP package regulator either...Hmm. Wait, in the top right part of the board (highlighted in yellow in the picture), where the fan and CRT connectors are located (in light blue), I see a tight group of 4 little T092 packages (in red) ... hmm maybe some discrete/hand made regulator ? Let's see what the part numbers say : we have two TL431, which are adjustable Zener diodes, and two 2N4403 which are general purpose PNP transistors, capable of a continuous 600mA collector current, plenty enough for the fan... So looks like the CRT and fan use independent regulators after all, and both use an identical discrete circuit made up of a Zener and PNP pass transistor.The CRT on this scope works just fine so let's not worry about its 15V supply, it's just fine even though it output only 14V not 15V.So I guess I must figure out this fan supply then... maybe the pass transistor is faulty, or maybe the Zener is faulty, or maybe some of the passive around either of these two guys...The good thing is that the two regulators being identical in design and open-circuit voltage, I guess I can use the CRT regulator as a reference, and why not, swap the Zener and transistors to help figure things out.I think I most likely have a decent substitute for this 2N4403. The 2N222 counterpart, 2903 is it ? The other day I spotted these in my drawers.... must be good enough for the task, could try that.The Zener I can always test on breadboard.Passive/resistors around the regulators, can't see any burnt or damaged ones.There is a "big" electrolytic cap next to both connectors, I guess they must be in parallel right across the connectors. Maybe the one for the fan is leaking badly and sucking up most of the output current, leaving only a spare 20mA for the fan...Who knows, but anyway I have a clear path of investigation now, so let's gooooo...I guess the very first thing to do is probe around to figure out which tranny and zener correspond to the fan output.

author: Old Don
author_rank: Regular Contributor
timestamp: October 01, 2017, 02:35:29 am
content: Vince, here in the States Harbor Freight has a Chinese made head mounted magnifier levels of magnification plus a flip into position "loupe" for extra magnification and it sells for about $5.00. Or well under 5 Euro's. Don't know if you have a company like Harbor Freight, but even if you have to pay 10 Euro's that should be all you need for most work.Here's a link so you can see it:https://www.harborfreight.com/catalogsearch/result/index/?dir=asc&order=EAScore%2Cf%2CEAFeatured+Weight%2Cf%2CSale+Rank%2Cf&q=magnifer+strap

author: Vince
author_rank: Super Contributor
timestamp: October 01, 2017, 12:08:13 pm
content: Hi Don,Yep as discussed farther up this page, I will definitely give this type of thing a try, cheap and can only better than nothing, so not much to loose....Made some progress on the troubleshooting last night. Another lesson learned : just because something seems obvious and all adds up... does not mean it's true !I mean, these two Zener and tranny pairs around the fan and CRT outputs looked all too tempting and plausible for sure, but after spending some time probing around both power off then on... I just could not make any sense from it !I Had no choice but declare that these Zeners/tranny combos had nothing to do with the fan and CRT ! No idea what they are used for, would love to have the schematics one day, to find out, but this is never going to happen of course, sadly...So I stopped worrying about this section of the board, I stopped making assumptions, and just probed and followed traces.From that, it appears that the fan and CRT are running from the same supply as I originally thought, with the only subtlety being that the CRT runs straight from the supply whereas the fan is fed through a series 16ohm 1W resistor, presumably to protect from shorts, or at least limit inrush current at start-up (to under an amp then), something along those lines...Power comes from a two taps from the first (not the main one strangely) transformer, each tap going to one of the legs of one of the beefy/T0220 dual diode packages which are mounted on the secondary side big heat sink. So I guess it's the classical arrangement where middle tap is used as ground and then each tap of opposite phase has a single diode on it, this way with only two diodes you get a full wave rectifier. Then the output / common cathode of the diode package goes to big 1000µF filter cap, then straight to the CRT output then to the fan output via that series 16ohm resistor.Well I still need to investigates further, with the scope this time, to make sure I am not talking BS again, but while probing with DMM and looking at the actual traces on the PCB, that's what it looks like to me.I guess it makes some sense : the digital 5V rail is regulated direct from the primary side PWM chip, so if that regulation is good enough for digital chips, sure enough it's plenty good enough to supply a bloody fan...Now having that all this, I am even more at a loss as to why the fan output can't supply more than 20mA !1) the 16ohms resistor... looks like new, and measured spot on at 16ohms, so it's not this guy limiting the current2) CRT output is upwards of the fan, and has no problem supplying current, judging by the fact the CRT/scope at large, works just fine !So on the one hand I am happy because I now understand better how it's put together... but on the other hand I am very unhappy because I just don't see how something this simple could go wrong, especially given the afore mentioned preliminary test result !? Grrr... not sure what to think of this... I witness a problem... can't explain it, frustrating... any suggestion welcome at this point !

author: tautech
author_rank: Super Contributor
timestamp: October 01, 2017, 06:44:45 pm
content: Ask yourself, why use a TO220 dual diode capable of supplyingAMPSfor a measly load ?What's the specs for it ?Let's say one diode is OC, the CRT inverter circuit won't mind but a DC fan most likely will.Put a scope on the 14V rail to check for full rectification.Is the 1k uF sick and leaking badly and/or not to nominal value ?

author: Vince
author_rank: Super Contributor
timestamp: October 01, 2017, 07:37:35 pm
content: Hi Tautech,Quote from: tautech on October 01, 2017, 06:44:45 pmYes sure.... I tried to make sense of that by the fact that the CRT uses this supply too. I don't know how much juice a small CRT display like this requires, but it's not minuscule I would think.Other thing is that, though I didn't mention it in my post, these two taps from the first transformer don't just stop at the diodes... they keep going and eventually go to pins on the second transformer... so I thought maybe this is the primary winding of that transformer wand they use this 14V to produce lower voltages for the 5V and 8.5 rails, so some more juice needed there as well.QuotePlenty enough of courseenough to power half a dozen of these scopes I would think ?! It's an FEP16DT, 8 amps per diode no less ...QuoteSure, I checked all power semi-conductors a few messages ago, they all checked OK. I guess I could have gone unlucky and one of these diodes went bad during this repair, you never know your (lack thereof) luck !QuoteYup, will do shortly.QuoteVisual inspection revealed nothing, everything on the board looks sparking new.Just checked the cap and in-circuit, the DMM test is inconclusive, can't get a reading. At first the DMM acts as if external circuitry was drawing current, so I thought OK maybe it's shorted, but then the display briefly indicated "open circuit", then back to trying to measure the cap. Maybe I was dreaming or maybe I just moved the probed inadvertently slightly... or maybe the cap is indeed dodgy, no idea, but sure will find out !OK, so first : 1) check (again) the dual diode to be sure, then 2) power it up and check the waveform on the scope to make sure it's full wave rectified then 3) remove the cap so I can test it properly/reliably.Stay tuned...

author: Vince
author_rank: Super Contributor
timestamp: October 05, 2017, 10:25:59 pm
content: Dear all,It is with much regret that I have to inform you that the PSU is dead. RIP, poor SMPS...I was (scope) probing on the PWM chip, near the mains filter cap, and I guess the probe must have slipped and shorted the mains in some way shape or form but... I do'nt remember exactly the chain of events as I am still under shock !. All I remember was that all of sudden the board caught fire violently, a fireworks followed, all I could think of was run away in case the huge filter cap exploded and might injure me.Luckily it didn't, and after a few seconds the fireworks ended, phew...See pictures. PWM chip is cracked, and a couple components are burnt to complete destruction. From past pictures I would say they used to be a resistor and a little signal/glass diode. The big filter cap did catch fire but it's perfectly fine. I pulled it off for closer inspection, only the outer plastic foil started to melt, but the can/body of the cap itself is completely undamaged. Cap still measures just fine. There just wasn't enough energy in this fire to damage such a big guy.I do happen to have a spare PWM chip of this reference, as luck would have it, but I have no way of knowing the specs and value and the two components that got destroyed. Plus, even if I could fix this, I would still, at best, get back to my original problem which I failed to diagnose.Last night I did manage to probe around the thing with the scope without blowing anything, but all I could see were waveforms that defied common sense, so I gave up. this supply is just too complex to be tackled without schematics... too many ICs, too many discrete transistors, two many diodes...So I give up for now. This scope has eaten enough of my time and money and is not worth it. sometimes you have to draw a line somewhere and call it a day...But for the record, and in case I do come back to this repair (much) later, when I am more experienced and knowledgeable about SMPSes, or if the schematics somehow surface one day (highly unlikely I know...) here are some screen shot from last night.The CRT/Fan supply is properly rectified but ripple is horrendous, > 500mVppinteresting things is that ripple is pretty much as horrible on ALL the other rails ! Just slightly less horrible... maybe 300mV.. but still waaaay out of spec I would think, whatever the specs might be ! To be honest I find it quite miraculous that the scope ever managed to work at all with so much ripple everywhere.What I find even more troubling is the fact that this ripple has a very odd frequency : 15Hz or so ! Not 50, not 100... but 15Hz. Where the hell would that come from...Also, when I tried to check the basics and look at supply voltages of the PWM chip and and nearby LM393 comparator.... instead of a clean DC voltage, I found some weird waveform ! The comparator Vcc pin had huge 7Vpp triangular waveform, again at 17Hz, sitting on a 10Vdc level. Surely this can't be normal...As for the PWM chip, even more weird. The Vcc pin was seeing a PWM like waveform ! Yes, I triple-check chip orientation and pinout in the datasheet... I did probe the proper pin for all I know.So really, without a schematic, I give up on this thing. Still, I will reassemble the scope, which works fine otherwise. I might find a cheap donor to help me fix this scope, or more likely this scope will be the donor to fix another TDS 300 scope featuring better specs...So this scope served his purpose well : I learned some things and got some more hands on experience on SMPSes , without schematics. Valuable to me.Also learned that there is no shame in giving up/admitting defeat... sometimes you have to be reasonable.Now, let's clear the bench and get going with the next project... I have got an old Schlumberger/Enertec universal counter to fix, hopefully with better luck than this SMPS !Going to open a new thread about this little guy soon, for those who might be interested.

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2017, 10:36:14 pm
content: Quote from: Vince on October 05, 2017, 10:25:59 pmMinor damageLesson= use probe tip sleeves to cover the mains ground referenced ring rather than pop the DUT from a dumb slip.

author: Vince
author_rank: Super Contributor
timestamp: October 05, 2017, 10:58:55 pm
content: Yes, good idea.. just that the probe didn't come with such a sleeve. I can't recall ever seeing one either to be honest. I think I must be misunderstanding you...The only sleeve that came with the probe is the usual huge one with a retractable hook at the end. I don't recall ever seeing a tiny sleeve designed to cover just the ground ring. Maybe I am blind, will check the contents of the probe containing all the accessories, might find something resembling what you describe.Anyway...

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2017, 11:21:41 pm
content: Quote from: Vince on October 05, 2017, 10:58:55 pmThese:2 types, a locator for DIP leads and a plain sleeve with just the tip exposed.

author: Vince
author_rank: Super Contributor
timestamp: October 06, 2017, 12:07:33 am
content: Oh ! yes silly me... I had forgotten about these little things ! My old crappy probes I threw away some time ago, had these ! Actually I just found them, still there ! I even have two of them, both different shapes (had two different probes), and none of them fit my current probes (pun intended) : some months ago I bought " NOS " Tek probes, x4 P6139A (500MHz x10 for my TDS 544A) and x5 P2100 (x1/x10 switchable 100MHz, for my 2232 and all my slower scopes in general). They all came with their individual pouch of accessories but none of them had these little tips so my brain kinda forgot about them even existing ! Sorry my bad again...If these can be had second hand for cheap, for Tek probes, I would surely consider getting some !