Tektronix TDS754D - acquisition board test fail - Page 1
Forum section: electronics / repair

author: Rax
author_rank: Super Contributor
timestamp: March 27, 2024, 12:32:10 am
content: Hi all,I recently got this TDS754D with issues and am now trying to assess and hopefully repair it.It'd fail, initially, the startup tests with an acquisition board failure (see first image below), but now it'd cursorily pass the startup tests, but fail its acquisition board checks if I run the full tests. So it seems very clear where the issues lie.I enclose the log from the recent tests. March 17th is when I got this, so that's the area of interest for my own observations. Per these logs, it seems U207, U208, U214, and U216 seems to be repeat culprits, though there's some odd variability from test to test in what it returns in the failure log.In rather general observations, CH1 seems the most able to display a waveform (in DPO mode), while the other three seem more cranky. In non-DPO mode, though, things seem pretty stable. DPO mode brings a ton of "snow" and garbage on the screen (see image included).Here's some thoughts I put together:This unit's not impacted by the notorious caps failures in some of the related units. Additionally, I see no leaky caps (at least on the top side)I'm considering replacing the ICs mentioned above with 71024S12TYG. Has anyone gone through this? I have a Pace TT-65 which I think fits the bill (though I need to identify and purchase an appropriate tip for the job...)I welcome input on where I should head next.

author: denimdragon
author_rank: Regular Contributor
timestamp: March 27, 2024, 03:25:05 pm
content: Check your power rails. That power supply is due for some service. I useto repair a lot of these old scopes. The 754D is one the best in that TDS5,6,7XX series.

author: Rax
author_rank: Super Contributor
timestamp: March 28, 2024, 12:07:03 am
content: Quote from: denimdragon on March 27, 2024, 03:25:05 pmI also always start with the power supplies in a repair. "Unfortunately," I think in this case they check out. Per Table 6-5 at p.6-66, all rails under load seem to be in spec.I wonder if others have found the errors logging system to reliably report faulty ICs. I'm trying to assess whether I should trust my error log - indicating likely culprits are U207, U208, U214, and U216 - enough to go ahead and replace them. From what I'm learning, others have needed to dig deeper than just that, in order to nail down the faulty parts. But maybe I'm reading too much (or not keenly enough) in prior casework I was able to find.

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 28, 2024, 03:03:59 am
content: I have a TDS794D that had bad RAM chips, it presented as the waveform on the display dropping to zero periodically as the faulty memory cells were stuck at 0.I have a bunch of spare good chips that I pulled from salvage boards and have tested (I used a Retro Chip Tester Pro), so I can re-test a handful of them and send them your way if you need some.

author: Rax
author_rank: Super Contributor
timestamp: March 28, 2024, 04:12:23 am
content: Quote from: TERRA Operative on March 28, 2024, 03:03:59 amI'm not convinced that's what I'm seeing here, as to me it seems my symptoms are pretty random; I don't see much of a pattern - though I should probably take a closer look. They're still my main culprits either way. How did you determine which ones were bad?I appreciate your offer, and will keep it in mind. I think the 71024S12TYG are available in the US at the "big box part stores" - are those reasonable replacements? Not sure what others have tried with good success. Or why you felt you needed to reuse those ICs - other than already having them at hand.But if you removed and maybe replaced some of these, I could use some direction on how to go about that. What worked, what didn't work so well? I think these are on an SOJ package, which I'm not so familiar with.

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 28, 2024, 06:33:53 am
content: I knew which banks were bad as only inputs 1 and 4 were affected (Note, the 4 banks of RAM and Acquisition chips DON'T line up with the physical order of input jacks!)Then from what errors I had, I took some educated guesses as to what actual RAM chips needed to be replaced.A bit of hot air and a fine soldering tip and I swapped out the chips one by one until I had no more errors or distortion of the displayed trace.I had some salvaged boards from scrapped scopes so the chips were free, so I tested them and kept all the good ones to use which allowed me to just swap out chips freely until the scope worked.I can also pull other chips if you need them too. I still have the scrap boards here.It looks like the RAM chips are about $3.50 each from Digikey/Mouser, but if you want a bunch I won't ask nearly that! Postage from me to you will be pretty ok for a small parcel I think, so PM me and we can compare which way is cheaper if you want.I found my notes. The error I had was "ERROR: diagnostic test failure, dsyRastModeV0Walk, raster fail @ 1,1 read: 8, expect: f" and I've attached the display I was getting with no input.You can see the spikes from the bad RAM. (I said things were stuck at 0, the error message has the correct details, but the RAM was bad regardless).

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 29, 2024, 11:20:16 pm
content: RAM seems to be a common failure although I've never encountered it myself.I don't know if the solders to the RAM MUXed fail, try applying pressure to MUXes and RAM while it's working to see if that affects the displayed traces.

author: Rax
author_rank: Super Contributor
timestamp: March 30, 2024, 12:03:58 am
content: Quote from: Rax on March 28, 2024, 12:07:03 amMaybe I was glazed over that too quickly. DCV levels seem fine, but the ripple on the 25V rail doesn't seem so OK to me - see enclosed scope screenshot. This is done with my scope (HP 54522A) over a 10x probe, but my Fluke 189 agrees with this about 160mV level of ripple. If this is not such a smoking gun as it seems to me, I welcome input.There's some healthy level of disassembly, so I won't go ahead with this tonight after a "week-tail-end" day at work, but I plan taking apart the LV PS board and taking a closer look at what may be off on it. Before doing that, I'll probably also look at the "no load" ripple levels on the various rails (in case some module is spilling this crap on the 25V rail). I am using the "component level" SM from the 520B, which is supposed to have some good resemblance with this scope.The ripple on the other rails is in the mV ACV range (for instance, about 8mV AC on the 5V rails).

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 30, 2024, 12:57:06 am
content: These PSU's are known for bad caps occasionally, so you may be making a shopping list for new ones soon...There's also the possibility you have a version of the PSU that absolutely needs at least one cap replaced regardless.

author: Rax
author_rank: Super Contributor
timestamp: March 31, 2024, 01:46:52 am
content: Quote from: denimdragon on March 27, 2024, 03:25:05 pmQuote from: TERRA Operative on March 30, 2024, 12:57:06 amThank you both for this direction. Is the PS quite as dead on at the core of the whole unit as I'm finding it? I think there's a minimum of two maybe three major other modules that have to come out whichever way I go about this (and I am reading the manual, which has about four different sections on pre-pre and prerequisite to disassembly, and will follow its steps).Any good tips?(aside from "stop complaining and get to it!!!")

author: Rax
author_rank: Super Contributor
timestamp: March 31, 2024, 03:36:59 am
content: I did exactly what my last statement above indicates and here's a pic of my actual PS board.

author: Rax
author_rank: Super Contributor
timestamp: March 31, 2024, 04:19:31 am
content: On further work on this, I seem to have a PS board (nearly?-)identical with the TDS520B. Yes, that one - the closest scope to this still supported with component-level SM.So, if I were to believe that, the 25V may - possibly - be a 1V lifted version of the 24V rail on the 520B? The current rating seems to match. I'll examine the corresponding parts.

author: Rax
author_rank: Super Contributor
timestamp: March 31, 2024, 06:04:38 am
content: Further report - replacing C21 (for which I used a UC KYB 1000uF/50V) and C102 (Nichi HE) - the two electrolytics on that rail - hasn't improved anything. The ripple on the +25V is still about 160mV, and the acquisition tests still fail.I think I'll call it a day and think further where I am with this in the morning.

author: Rax
author_rank: Super Contributor
timestamp: May 19, 2024, 04:04:28 am
content: I finally got to get back to this. Some pieces needed to fall in place:Get the right Pace tips for SOJ packagesGet chips from Terra Operative (Thanks a bunch!!)Read a lot and watch a ton of Pace vintage videos! A feast in their own right, to be fair.So I'm half way through the job (though something tells me I may have at best done a third?...) and have removed the suspect chips (207, 208, 214, 216).I welcome any good tips on soldering the replacement SOJ chips. I do have some low-temp Chipquick paste, plenty of syringe-disposing flux, thin regular solder, etc. I do have a hot air tool (cheapo thingie I am not in love with but works OK). I am thinking of cleaning the pads very well (Pace SX-90), be generous with flux on the clean pads, then try to use a chisel tip with careful application of thin solder and run through the pins at deliberate speed. The alternative is to use the Chipquick and the hot air tool, but I have far less confidence I'm not going to end up with bridges and maybe even loose stuff going stray in there. There's some chips in there that get very hot and low-temp solder may get a bit too excited with that in time.

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 19, 2024, 04:25:47 am
content: Looks like you got the old chips extracted pretty well.I used my Hakko hot air gun to remove the chips, then solder sucker to clean the pads. To install the replacement chips, I used 0.3mm solder and a conical tip on my iron to solder the legs one by one.I position the chip, solder one corner, adjust then solder the opposite corner, then finish the rest once the chip is perfectly positioned.Lots of flux to make soldering easy, followed by lots of isopropyl alcohol to clean it all up at the end.

author: Rax
author_rank: Super Contributor
timestamp: May 19, 2024, 04:30:30 pm
content: Quote from: TERRA Operative on May 19, 2024, 04:25:47 amDo you use lead-free solder for this? I'm thinking, if lead-free was used in manufacturing it, it may be a good idea to stick with that so one wouldn't mix them.

author: Rax
author_rank: Super Contributor
timestamp: May 19, 2024, 06:29:42 pm
content: I'm having a hard time soldering these on. I have tried both a very fine chisel tip, a long and fine conical, plenty of flux on the board. The solder just doesn't seen to want to go on there.Maybe the .3mm solder I'm using is not very good. An AMZ purchase, terrible product design by using a sticky label which left glue residue on the solder. I've unwinded about 2/3rds of it and I think it's clean, but maybe the product is even worse than I thought? Meaning the rosin core missing in spots, etc.Maybe I should just switch to new chips as maybe the pins of these used ones are no so friendly to rework? To be clear, Terra Operative sent them pristine clean and NOS looking (so this is as far from a complaint as can be). But maybe there's stubborn oxide on those legs?Maybe I should use lead-free solder, in case the original manufacturing of the board used that (which I didn't think would be the case).Oh, I wish this would take a turn to the smooth path!... Pretty exhausting and frustrating.

author: Rax
author_rank: Super Contributor
timestamp: May 19, 2024, 08:29:10 pm
content: Well, no luck with the overall repair... I replaced those 4 SRAMs - I think work is good, see pics (I've further cleaned up the flux from there, BTW) - but I still have acquisition board errors (see log). Just maybe different errors - U200 demux now?... Or maybe just errors in its module.

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 19, 2024, 10:39:44 pm
content: I just used regular leaded solder with additional standard Hakko rosin flux. Nothing special at all and the solder flowed nicely when I replaced the chips in my TDS794DIf your board is clean then you might have bad solder..... I'd get new stuff and reflow the joints to make sure you don't have a dry joint there somewhere.Once you have that sorted, then we can dive deeper.

author: Rax
author_rank: Super Contributor
timestamp: May 19, 2024, 10:44:18 pm
content: Quote from: TERRA Operative on May 19, 2024, 10:39:44 pmSoldering completed successfully, as per my prior post. I've had to look close with a magnifier and eliminated a few bridges. I'm very thorough, so I'd never turn it on before being as certain as I can the work is good. My back hurts more from examining than the actual soldering.Though probably my "finished" pic doesn't quite depict it well, the solder joints I've done here as good as anyone's, I wager.

author: Rax
author_rank: Super Contributor
timestamp: May 19, 2024, 10:57:42 pm
content: Anyway, so I'm back at the drawing board and there's just a lot of scratching my head is all.

author: Rax
author_rank: Super Contributor
timestamp: May 19, 2024, 11:48:19 pm
content: A couple of notes on the rework:I think the .3mm solder I have is indeed crap. I switched to my default .8mm good quality solder, and I had to judiciously distribute each load across a few pins. Abundant rosin helped do that relatively easily. So, my solid belief quality tools and materials is everything is reinforced.The TT-65 and the SOJ tip made the desolder job very enjoyable. It's not a cheap solution, but I'm certain it cut down my time on that part of the job by a sizable percentage.

author: Rax
author_rank: Super Contributor
timestamp: May 19, 2024, 11:50:58 pm
content: As I evaluate the next steps here, I am trying to read the entirety of the error message. I am positive I read a while back a thread where this issue was discussed - can't recall if clear directions were presented - but can't find it now for the world.I'd appreciate pointers on how to read the entire body of the individual errors.

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 20, 2024, 12:09:36 am
content: Ok, all good on the soldering, sounds like you got it all sorted.To get the entire error messages, you'll need to read them out via GPIB. I also have no idea why Tektronix couldn't get the text to wrap properly on the scope display...Here's how to read out the errors:To read the first line of the error log via GPIB, send the command 'errlog:first?'To read out subsequent lines, send the command 'errlog:next?' per line in sequence.To clear the error log, send the command 'ERR CLEAR' via GPIB.I would suggest clearing the error log first, then reboot the scope and manually run a full self test and SPC to generate a fresh set of error logs.

author: Rax
author_rank: Super Contributor
timestamp: May 20, 2024, 02:22:25 am
content: Quote from: TERRA Operative on May 20, 2024, 12:09:36 amThis is great, thank you. I was able to initially read an error from the dawn of time, but as trying out different things to read the last didn't work, I resorted to what you recommended and reset the error log. Then ran diagnostics again, and got this:"SUN MAY 19 19:11:48 2024 ERROR: diagnostic test failure, digDataFormatDiag, ERROR!!BYTE mode, in demux 200 ,i= 7 memBaseAdr 0x7380010= data = 800, expectedData= 5500"Looking forward to interpretations from those with experience on this instrument - as for me, I am headed now to do some reading.

author: Rax
author_rank: Super Contributor
timestamp: May 20, 2024, 02:30:09 am
content: BTW - not to jinx it, but I feel stronger and stronger that replacing those four SRAMs was called for, because there seems to be just one error now and those RAMs don't pop in there at all anymore.

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 20, 2024, 02:56:10 am
content: Yeah, looks like you had a few errors to deal with, at least you are making progress!I'll have a search and think about your current error. My feeling is to either track down that address to a RAM chip and replace it as well (if not already done) or figure out what is controlling that RAM chip and stare intently at that.I'd expect that if it was the chip passing the data to the RAM chips, it would be a periodic error across multiple chips rather than just one RAM chip in that case due to how the RAM is written to with bank switching etc.One way to find the right RAM chip is to ground/apply 5V (have to double check which one) to the WE (Write Enable) pin one chip at a time and see what errors are thrown on self test due to that chip not being enabled to be written to.If you get new errors on the same address range as your latest error, you found the chip.

author: Rax
author_rank: Super Contributor
timestamp: May 20, 2024, 03:24:19 am
content: After running SPC, I got:"SUN MAY 19 20:07:47 2024 ERROR: Internal adjustment range exceeded, ( 3 @ 7464): DIGc gain = -0.400113, bal = 1.113079"Bad news?...

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 20, 2024, 05:43:45 am
content: oooeerrrr.... Either bad cal (unlikely) or something is bad on that channel. Maybe as simple as a bad relay on the hybrid. It's not too hard to swap hybrids between channels, just need to be extra careful not to crack the ceramic PCB.Relays aren't toooo difficult to replace, I have a video on Youtube about it.youtube.com/watch?v=b0P1sJBomDQIs that the only error after clearing via GPIB?Can you run SPC maybe 5 or 6 times to exercise the relays, then clear all errors, self test and SPC once more and list the entire error log here?

author: Rax
author_rank: Super Contributor
timestamp: May 21, 2024, 11:22:05 am
content: Quote from: TERRA Operative on May 20, 2024, 05:43:45 amWould that be CH3? The error body is a bit cryptic to me.Quote from: TERRA Operative on May 20, 2024, 05:43:45 amThose two errors above, yes. The second error occurs if I run SPC.Quote from: TERRA Operative on May 20, 2024, 05:43:45 amBeen doing a few of those, and the error seems pretty persistent.I should also say, I don't feel there's anything very specifically wrong going on with CH3 - if that's the one found faulty by SPC. If anything, CH4 seems a bit choppier and there's a bit more freezing and vertical bars "noise." But they all work pretty well, their measurements are accurate as much as my other meters think. It just fails tests. Oh, and DPO mode doesn't work very well - there's a good amount of "video noise" - color pixelation ("snow") on the screen when that's enabled.

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 21, 2024, 12:11:20 pm
content: I think the firmware sees the chanel numbering as 0 indexed, i.e. 0, 1, 2, 3 corresponds to physical channels 1, 2, 3, 4. So I would look for problems on channel 4Try swapping the channel 1 and channel 4 hybrid (don't forget the thermal paste on the chip on the hybrid module!) and see what happens.I keep saying it and will say it again.... Be careful with the hybrids, and make sure they are perfectly seated in the metal holder before screwing it to the PCB!Easy tests are easy, so may as well rule out bad relays or bad hybrid module.

author: Rax
author_rank: Super Contributor
timestamp: January 11, 2025, 04:15:05 pm
content: Update on this, as only now I managed to work my way down the projects backlog and get back to this).Putting this back on the bench, I got the error above, but also a new error originating in U208. Suspecting I may have some soldering issues - I absolutely don't love those SOJ packages one bit - I tested continuity pin to pin and indeed I had a short between two of them.Once I fixed that, it seems now to pass all tests, startup or the full range from the Utilities (before this last fix, it would fail both). Given the original issue was reported to occur after extended running (hours), I'll hold back on popping that cork, but I'm hopeful this is it.I think this failure mode occurs due to the heat from U200 nearby, which cooks those SRAMs in time.

author: Rax
author_rank: Super Contributor
timestamp: January 11, 2025, 04:16:50 pm
content: An effusive thanks to TERRA Operative here for his sending me the SRAM ICs (used, but tested) to repair this. Much appreciated! Members like him make this a great place to hangout.

author: Rax
author_rank: Super Contributor
timestamp: January 11, 2025, 07:31:54 pm
content: SPC also completed successfully. This always failed before.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 12, 2025, 04:30:53 am
content: Nice.I reckon run it overnight and give it a bunch of SPC tests too.

author: Rax
author_rank: Super Contributor
timestamp: January 12, 2025, 05:31:21 am
content: It worked flawlessly for part of the afternoon, on and off. Then tonight I fired it up again, and it gives me the artifacts on screen (see below) and essentially the same error regarding U208 as before (see below), just with a slightly different address.Back to the drawing board. I feel I need to check U208 thoroughly for input/address and the rest of the pins. Maybe replace it with another one I have here, just in case this one's taken a dive.

author: Rax
author_rank: Super Contributor
timestamp: January 19, 2025, 01:47:29 pm
content: Some progress last night.I'm now not sure how I fixed the short or what exactly is going on, I've found a "short" between what I think are another two pins of U208 (but always on the address pins). I think last time it was 3 and 4 or maybe 4 and 5. It's possible that was indeed a solder bridge.But this time around my meter beeped a short between 2 and 3 (A14 and A16). Per 520B component level SM, the address buses are shared between U201 through U208, and then separately U209 through U216. So my "pin 2 and 3" short could be with my recently replaced U208 (where the utility flags it), but could also be originating at U207.I removed U208, and I have the same "short." I call it "short" because in closer examination resistance there is almost 10 ohms, which I can't imagine is caused by a solid short, even at a relatively remote point from where I measure it (so through traces). Besides, it's 10 ohms at the U207 pins as well as on the currently free pads of U208 (it being lifted).Any ideas welcome.

author: Tantratron
author_rank: Frequent Contributor
timestamp: January 20, 2025, 02:50:10 pm
content: Quote from: Rax on January 12, 2025, 05:31:21 amHello, I look at the 2nd picture which you posted but maybe i'm wrong, what makes you think it is a memory issue from the acquesition board ?Could it be so graphic video processor generation or some video plane in failure (i.e. grid and text OK but signal KO) ?

author: Rax
author_rank: Super Contributor
timestamp: January 20, 2025, 04:36:17 pm
content: Quote from: Tantratron on January 20, 2025, 02:50:10 pmThat may be possible, but I'm focusing on the acquisition board because:Running all tests indicates an acquisition module failure. This has always been the case (other modules have been passing the tests).The error code indicates a memory address fault, pointing to U208. See the first screenshot.Not sure if the diagnostic utility can point to a different module/part than what's actually causing the failure, but I assume I need to fix that before tackling anything else. Besides, after replacing the four SRAMs in the original errors (again, pointing specifically to the acquisition board and namely U207, U208, U214, U216), I had a fully functional system able to pass SPC. I'm not sure why it failed again after that, but determining the cause of that would probably mean identifying the smoking gun here.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 20, 2025, 06:08:40 pm
content: I would be inclined to give U208 the flick for a replacement part.Rule it out before potentially chasing your tail.

author: Curve514
author_rank: Newbie
timestamp: February 09, 2025, 02:17:42 am
content: Hi,I was having the memory error on my 754D, U206 and U208 were specified in the log in different test runs. I did initially replace U206 and U208 without success. All the 308pins on the QFP controller had good solder joints. While probing for solder bridges after installing the new chips, I managed to catch two consecutive pins shorted.Turns out this short was NOT a solder bridge, it was a bad SRAM chip with the pins (25-26) internally shorted! Not any one already replaced. I had to remove 7 chips from the gang before finding the bad one.-> Note that the 8 gang is split 4 on top side and 4 on bottom side.It is logical that if two address or data bits are shorted (I did not check de sram chip spec), the diagnostic will fail on the first chip affected.No more error! Yay.Cheers!

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 09, 2025, 07:12:11 am
content: Oh nice catch! I'll keep that in mind for the next repair I do.

author: Rax
author_rank: Super Contributor
timestamp: February 09, 2025, 07:41:06 am
content: Quote from: Curve514 on February 09, 2025, 02:17:42 amWell, that's very interesting, because that confirms my latest observations and the direction I was heading into.Given I was still getting errors pointing to U208, I removed my replacement, and I was still getting what seemed to be a short between two (neighboring pins) addresses. Here are my notes from about a couple of weeks ago (I haven't had time to go back in there since):"I checked all of the address lines (say A0 through the four chips on the top of the board, so I picked one chip's pin 12 and checked the other three pin 12s, and so on for all address buses/nodes) and they seem to check out. Then, looking for shorts - "address to address," so A16 to A14, A14 to A12, A12 to A7, etc, neighboring pin to neighboring pin starting at pin 2 - I didn't find any other measurement that'd "beep," other than pin 2 to pin 3, or A16 to A14.Beeping threw me off, as I expected it to be a solder bridge, but I now think I was dealing with something completely different. It seems to me one chip has some odd condition between its pin 2 and pin 3 (maybe leaky/near shorted ESD protection diodes?)."With this last sentence, I meant that, upon closer inspection (resistance measurement vs. continuity), I wasn't actually seeing a true short between A16 and A14, but about 10 ohms. Which to me ruled out an actual solder bridge (too large a resistance). So I take your observations as a confirmation that these 10ohms are the sign of an internal fault with one of the chips. Unfortunately, as you note, there's eight chips sharing those address buses, so any one of them could have this condition. I just hope it's not going to be the eight!

author: Rax
author_rank: Super Contributor
timestamp: February 09, 2025, 07:45:30 am
content: I should also add that my current understanding of this is, depending on which data bits get addressed to U208, and specifically at A14 and A16, any one of the (other) chips with issues between pins 2 and 3 will likely cause this fault. So even if the SRAM flagged is U208, this is just indicative of memory allocation being assigned to U208, even if the "quasi-short" occurs at any of the U201-U207 chips. Not doing us any favors!

author: Rax
author_rank: Super Contributor
timestamp: February 09, 2025, 07:53:48 am
content: What's been suggested to me, and to me makes a great deal of good sense, is to execute a high resolution resistance measurement between pin 2 and pin 3 of U201 through U208, and to try to determine which one chip exhibits the lowest resistance.Certainly, the difference would be extremely minute, and I never really went ahead with this (other than a couple of tests), as simply contact resistance/oxidization, probe pressure, etc could generate false readings and make this pursuit particularly finicky and unreliable.

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 09, 2025, 08:49:16 am
content: You need a heat gun and a chip tester. Pull them all off in one go, test them, and throw away the busted one.

author: Rax
author_rank: Super Contributor
timestamp: February 09, 2025, 09:01:25 am
content: Quote from: TERRA Operative on February 09, 2025, 08:49:16 amProbably a good idea. I am a huge fan of honing into the culprit and busting it pinpointedly, but as sexy as that path is, definitely not always possible.I do have a Pace unit with SOJ tips, so removing the SRAMs is far easier than resolderingI'd just like to find a path forward not involving a lot of rework (testing in circuit somehow). But maybe that's not feasible.Regardless, I'm encouraged by getting solid confirmation on the path I was turning to.

author: Curve514
author_rank: Newbie
timestamp: February 10, 2025, 01:23:12 am
content: @Rax:I may be wrong but I don't think you could reliably locate the shorted chip by R measurement. If you want to go all-in getto with it, you could inject 0,5V@15A and check the heating sram chip. You flood the chips with iso and check the one evaporating the fastest. That's a technique used by the people repairing apple motherboards.I suggest just removing the chips with hot air and a lot of flux (amtech). I also have a pace station but for this I use the cheapest hot-air station Amazon.ca (64$C) sells. I have it 5+ years, thinking it would die on me but no, still works.Be calm, careful and zen, no matter what, it will always be the last chip that you remove that will be the bad one!I re-solder the good ones (and new ones) with no added solder, just a lot of flux. Re-soldering the chips requires less heating but careful positioning (the SOJ don't auto-allign as good as BGA, QFN, etc..). You have to be careful heating but the PCBs on these scope are good quality. I know the limit when the flux goes all-in vapor mode.For me, the key is inspection so I have multiple stereo microscopes. I have one with 20cm viewing distance so I can look at the chip/pads at an angle (not over the bench).Regards,

author: Curve514
author_rank: Newbie
timestamp: February 10, 2025, 02:57:55 am
content: -> Just to be clear, I was joking about injecting voltage thru the shorted signals. This can work for shorted power rails but I would not try it for signals.

author: Tantratron
author_rank: Frequent Contributor
timestamp: February 10, 2025, 05:43:17 am
content: Just my 2 cents here since we know via GPIB to flash, read, write, access many memories in the CPU board (i.e. tekfwtools, NVRAM, flash). Would it be more efficient and less risky to have a dedicated application to write and read the acquisation SRAM chips. The idea would be to have a C compiled application which would provide aneducated guessto identify by remote testing (GPIB) which of the SRAM is failed.

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 10, 2025, 06:27:06 am
content: A floppy tool to do an automatic RAM test would be magic.

author: Curve514
author_rank: Newbie
timestamp: February 11, 2025, 12:31:54 am
content: I'm in the process of repairing/servicing my TDS754D. The power supply works but I wanted to check on the caps. It seems I have a different PSU version than the one you described. I can see some similarities but definitely no C17.

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 11, 2025, 02:21:36 am
content: That's the newer type of PSU, no Rifa capacitors and no C17, so no worries.The rest of the electrolytics arelikelyok, but give them a close inspection to be sure.

author: Rax
author_rank: Super Contributor
timestamp: March 02, 2025, 05:11:45 pm
content: I got back to this last night and I have a quick update. I've replaced at this point seven of the eight SRAM chips on that address bus (U201-U208, only U204 is still the original part, and U207 replaced with one of TERRA Operative's recycled chips). This is because once I replaced U203, the low resistance condition between A14 and A16 (a bit less than 10 ohms) ceased occurring. I am pretty confident this was an actual issue and it was causing part of the errors (those directly pointing to "U20X" ICs).I still have a failed acquisition module.I currently have two errors:ERROR: diagnostic test failure digHFStepDiagERROR: diagnostic test failure digDataFormatDiag, ERROR!!BYTE mode, in demux 200, i=4... (the rest of it is out of view; can anyone recall how to wrap/unwrap error codes text?).Ideas welcome. Thank you.

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 02, 2025, 06:08:15 pm
content: To get the full error text, you need to extract it via GPIB.The error screen in these scopes is a bit crap like that with no text wrap...To read the first line of the error log via GPIB, send the command 'errlog:first?'To read out subsequent lines, repeatedly send the command 'errlog:next?' to read each subsequent line in sequence.To clear the error log, send the command 'ERR CLEAR' via GPIB.I like to read out the error log for reference, then clear it all (including clearing all memory and performing a secure erase etc) and reboot the scope and perform a full self test to generate a new fresh set of errors.

author: Tantratron
author_rank: Frequent Contributor
timestamp: March 02, 2025, 06:15:43 pm
content: I did not read all therad but have you done the DIP switch troubleshooting testing (steps 1 through e) ?

author: Rax
author_rank: Super Contributor
timestamp: March 02, 2025, 07:18:14 pm
content: Quote from: Tantratron on March 02, 2025, 06:15:43 pmI have not, I went for the issues reported by the internal utility first. This eliminated at least two faults this far (all memory/SRAM-related). The other reason is, although I see the DIP switches on the top board, I can't find the corresponding directions in the SM (071-0627-02), though I'm still looking... If you know offhand where they are located, I'll appreciate the ability to save some time.

author: Curve514
author_rank: Newbie
timestamp: March 02, 2025, 09:59:33 pm
content: Regarding the error you still have, have you seen this post :https://www.eevblog.com/forum/repair/tektronix-tds754c-digdataformatdiag-error-and-option-2m-239912/You should also inspect the big 300+pin QFP associated with the channel (2) for bad solder joints.

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 02, 2025, 11:43:28 pm
content: You'll find a bunch of low level info in this document:It's for the TDS520B, but it'll be mostly relevant.https://w140.com/tekwiki/images/0/07/070-9710-03.pdf

author: Rax
author_rank: Super Contributor
timestamp: March 09, 2025, 03:55:21 pm
content: Quote from: Curve514 on March 02, 2025, 09:59:33 pmGreat point - I reflowed the joints on U200, and I'm getting the same errors. Nothing seems to point to any SRAMs anymore, so I regard that as progress. But I'm still not out of the woods with this.

author: Rax
author_rank: Super Contributor
timestamp: March 09, 2025, 04:10:24 pm
content: Quote from: TERRA Operative on March 02, 2025, 06:08:15 pmWould you be able to provide some guidance with this? I have an NI GPIB to USB adapter, and my bench computer has R&S VISA environment installed. When I tried to install the NI stuff, I got a warning a file will need to be overwritten, so I backed off. I'm weary of going forward from that point, as I have a working installation of a xyphro USBGPIB DIY adapter that's able to run some Python scripts I wrote to automate readings from my 8.5 digit Prema meter. I'm concerned I may interfere with that.

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 10, 2025, 12:20:44 am
content: Any GPIB interface can be used to communicate with the scope.You only need the specific National Instruments ISA GPIB card for running the Tektronix Field Adjustment Software.

author: Rax
author_rank: Super Contributor
timestamp: March 23, 2025, 12:29:10 am
content: Quote from: TERRA Operative on March 02, 2025, 06:08:15 pmFinally got to this point. I'm using pyvisa-shell, where I needed to "open" the instrument and "write" commands to the instrument. It's working, as I was able to clear the error log ("write err clear").But if I send a command to "write errlog:first?" it doesn't return anything. Trying to run just the instrument command, without "write" in front of it returns unknown syntax. Any hints?...

author: Rax
author_rank: Super Contributor
timestamp: March 23, 2025, 12:31:32 am
content: Stop press!! Dumb of me, but I needed to "read," not "write," kind of obviously... Working OK now.

author: Rax
author_rank: Super Contributor
timestamp: March 23, 2025, 12:36:19 am
content: Apparently, I only have one line of error to read:"ERROR: diagnostic test failure, digDataFormatDiag, ERROR!!BYTE mode, in demux 200 ,i= 4 memBaseAdr 0x738000a= data = fffffa00, expectedData= aa00"Can anyone make any sense of this? Other than a suspicion over U200 (assuming that's what's meant by "demux 200") I can't tell anything beyond that.

author: Rax
author_rank: Super Contributor
timestamp: April 09, 2025, 09:33:36 pm
content: I wonder if anyone has noted down the order of the channels on this acquisition board (it's a 671-4166-02). I've been making some assumptions regarding PIPE-A through D representing CH1 through CH4, case in which (per 520B component level SM) U200 would serve CH3.I am increasingly suspicious the remaining issues are localized to CH4, but I need to locate it clearly on the acquisition board. If the above assumption is correct, CH4 demux would be U100.

author: madao
author_rank: Frequent Contributor
timestamp: September 12, 2025, 12:41:14 pm
content: I give notice after few successful repair of TDS700D Series with bad SRAM (only with Firmware 6.xx )A TDS794D has two bad SRAM, other TDS754D has bad three SRAM.I have looking bad SRAM with shorting methode. (shorting of data pin), if short-spike has distance 16 sample to spikes by bad memory -> you had it !But, my both case: it looks like, bad SRAM is sit on bottom of PCBs.. i replace all 8 pieces. Fail self check message is away and fine waveform. (Except TDS794D, which it has one bad SRAM on top )https://www.wellenkino.de/557/viewtopic.php?p=7901#p7901(Sorry german)SRAM is not lined to channel input, CH1 & CH2 and CH3/CH4 is crossoverI have a dead TDS754D acqusistion board (mechancial damaged )-> i pull their SRAM...