Tektronix TDS644A repair - Page 1
Forum section: electronics / repair

author: jkprg
author_rank: Contributor
timestamp: December 08, 2017, 01:53:36 pm
content: Hi,I've got TDS644A from ebay with failed Cal and Acq part. I replaced all el.caps on acq. board, cpu board and front panel board. The acq. failure disappeared but Calibration error stays. So I run SPC procedure but without success. Error log says:ERROR: diagnosis test failure checkIfCalFailed, Calibration Failed.nothing else.I can see from the test signal waveform the input is very noisy. Do you have any idea how to fix it or what to test more? Thx

author: jkprg
author_rank: Contributor
timestamp: December 08, 2017, 01:56:06 pm
content: It looks only ch1 is so noisy... ch2,3 and 4 looks ok...

author: ArcticGeek
author_rank: Regular Contributor
timestamp: December 08, 2017, 02:32:51 pm
content: I've repaired quite a few of these TDS5XX/6xx/7XX scopes so I can give you a few pointers.First off, even though you have replaced all of the electrolytic caps make SURE that the acquisition board has been thoroughly cleaned. This means scrubbing the board with a toothbrush with a solution of degreaser such as Simple Green or Krud Kutter followed by a scrubbing with isopropyl alcohol. The TDS 500 and 600A series scopes were the absolute worst with the leaky caps, and once that electrolyte gets on the board it must be completely removed before it works again properly. After cleaning with Simple Green and IPA, I usually rinse the board with plain water and then dry the board thoroughly overnight with a fan running on it.Once clean, you need to run SPC to re-iniitialize the calibration values in the scope - this will often clear up the "noise" you see in the plots you posted.Good luck.

author: jkprg
author_rank: Contributor
timestamp: December 08, 2017, 04:29:46 pm
content: I've already tried to clean it but only with isopropyl alcohol. Ok. I'll do it again and properly :-)Thx a lot

author: cncjerry
author_rank: Unknown
timestamp: December 08, 2017, 04:34:52 pm
content: Don't know if it applies here, but the cal signal on my tds744 looks very noisy due in part to the scope bandwidth.

author: Armadillo
author_rank: Super Contributor
timestamp: December 08, 2017, 04:52:13 pm
content: Quote from: ArcticGeek on December 08, 2017, 02:32:51 pmOn Top of that, you have to check for broken vias and traces. You would need continuity testers and best with microscope. Those broken traces are very elusive, at times hidden below other components.All the best.!

author: jkprg
author_rank: Contributor
timestamp: December 08, 2017, 07:03:43 pm
content: Btw I’ve found all caps failed and melted traces around them on RS-232 communication board. Unfortunately I haven’t found schema of that board. It should be same board for other models. Does anyone have it? Thx

author: james_s
author_rank: Super Contributor
timestamp: December 09, 2017, 12:50:07 am
content: SPC was failing on my TDS784 and replacing all of the relays on the attenuator modules fixed it. That was a bit of a scary job but it was successful and the unit now works perfectly. I would definitely go over the boards again with a fine toothed comb looking for rotted traces and vias before digging into the attenuators though. You could also try swapping the attenuator module between two channels to see if the noise follows, just be sure to mark them first and be gentle, the ceramic substrate is brittle.

author: oventech
author_rank: Contributor
timestamp: December 09, 2017, 01:55:27 am
content: Check out this page. Tek WIKI page. Manualshttp://w140.com/tekwiki/wiki/TDS644.Hope this helps. Good luck with the scope.

author: JoeO
author_rank: Frequent Contributor
timestamp: December 09, 2017, 11:50:47 am
content: Quote from: oventech on December 09, 2017, 01:55:27 amWhat good is a webpage with no pertinent information? In fact no information on it at all.

author: Jwalling
author_rank: Unknown
timestamp: December 09, 2017, 12:11:14 pm
content: Quote from: JoeO on December 09, 2017, 11:50:47 amThe URL is wrong, it has a "." at the end.http://w140.com/tekwiki/wiki/TDS644There is a link there that points to the schematics.

author: andy2000
author_rank: Frequent Contributor
timestamp: December 10, 2017, 01:58:42 am
content: I usually find a number of open traces hidden under components. They tend to break right at the solder pads. You'll need to check everything in the corroded areas.

author: jkprg
author_rank: Contributor
timestamp: December 15, 2017, 04:54:10 pm
content: OK. I did the proper cleaning of acq. board (I hope). I even checked contact resistance of all the relays in attn. modules (everything was ok). The result is all the inputs behaves the same way now. I'm still waiting for new microscope to check vias but meanwhile everything looks ok apart I get Voltage Reference and Frequency Response fail. Signal Path test passed and Pulse Trigger passed too. So I guess I still have problem with Voltage Reference circuitry or calibration values stored in memory. I think it's the Voltage Reference but I can't find it in schemas of acq. board. Before I'll do the adjustment procedures (I don't have the necessary tools and I don't know where to borrow them in Prague so far) I'd like to check the circuitry. I can't find it in schemas from zenith. Any hint where it's located please? Thank you.Anyway the scope behaves normal although it get stuck sometimes (buttons don't response at all apart power) when I played with horizontal position knob. But it has happened only 3 times so far. Noise Vp-p is about 5mV. I don't know if it's within tolerance.

author: james_s
author_rank: Super Contributor
timestamp: December 15, 2017, 07:21:22 pm
content: The buttons and knobs are multiplexed, so if you have a stuck button it can cause others to have problems.

author: NiaDebesis
author_rank: Frequent Contributor
timestamp: December 15, 2017, 09:54:08 pm
content: Try to check the voltage on the caps, on another tread a guy had your same problem, and he discovered some broken traces/viaInviato dal mio SM-G920F utilizzando Tapatalk

author: jkprg
author_rank: Contributor
timestamp: December 20, 2017, 04:38:09 pm
content: Another episode: I checked voltage on all el. caps. Everything looks OK. Including two 10V caps overloaded by 13.4V which is by design :-( (consulted with documentation). I traced Voltage Reference generated by U900 all the way to the extended trigger U1703. Everything seems to be ok. So the Voltage Reference test must fail either due to corrosion which I have no chance to check without microscope (I'm waiting for delivery) because board looks really good. Or maybe it only needs Service Adjustment procedure. Unfortunately I need to find someone with GPIB adapter in Prague and find software for TDS644A adjustment. Do you have any tip where I can download it?

author: denimdragon
author_rank: Regular Contributor
timestamp: December 21, 2017, 05:10:23 pm
content: Quote from: jkprg on December 20, 2017, 04:38:09 pmThe field adjustment software:

author: Shiv
author_rank: Regular Contributor
timestamp: October 08, 2020, 09:55:04 am
content: Quote from: james_s on December 09, 2017, 12:50:07 amHi James, I´ve been having trouble with my 784D, SPC fails and error in selftest (ACQ).Did yours the same?Thank you and sorry for the late answer to the topic.

author: james_s
author_rank: Super Contributor
timestamp: October 08, 2020, 07:23:08 pm
content: I don't think mine had an error in the self test, only SPC was failing, and not always, sometimes if I ran it 5 or 6 times it would eventually pass. Replacing all of the relays was a tedious job but it resolved the problems.

author: Shiv
author_rank: Regular Contributor
timestamp: October 09, 2020, 07:36:59 am
content: Quote from: james_s on October 08, 2020, 07:23:08 pmThank you!In this, selftest passes when SPC passes, when fails, it works badly and fails selftest (sometimes OFFSET on CH1 or spikes in trace), but allways related to 1-2-5 position, I mean: the fails are the same for 1/10/100 scales and others for 2/20/200 scales.yesterday ran it 9-10 times, and 2 or 3 it worked well after SPC passes.I´ll get the relays and I´ll tell you.Thank you

author: jkprg
author_rank: Contributor
timestamp: October 09, 2020, 09:04:19 am
content: Quote from: james_s on October 08, 2020, 07:23:08 pmWhere did you get new relays? Thx

author: Smoky
author_rank: Frequent Contributor
timestamp: October 09, 2020, 03:40:58 pm
content: The Tech Genie on Youtube has a good video on how he went about replacing the relays on the ceramic hybrids inside of a TDS784A.I can see the part number on the relays in the video. He used a Panasonic DS1E-M-DC12V:https://www.mouser.com/productdetail/panasonic-industrial-devices/ds1e-m-dc12v?qs=HLLy2pIPwusODMqCzaiVHQ%3D%3DWatching the video, it looks like a tedious job requiring a very thin soldering iron. He also found that some of the relays had cold solder joints which may have been the problem all along. It looks like fun though

author: james_s
author_rank: Super Contributor
timestamp: October 09, 2020, 06:24:05 pm
content: I don't actually remember, I think I bought the relays from Digikey or Mouser.It was a very tedious job, nerve wracking too. I was sweating while handling the precious ceramic hybrids, they are very delicate and the only way to get replacements is to rob them out of a parts scope. You have to plan very carefully when installing the new relays so you don't "paint yourself into a corner". I ended up fashioning a custom tip for my soldering iron out of a piece of solid copper wire, I needed something long enough to reach down under the relays and get to pins in the far corner. To remove the old ones I snipped the leads I could reach and used the soldering iron to heat a few until they let go. It is not something I'd recommend someone inexperienced try.

author: Shiv
author_rank: Regular Contributor
timestamp: October 13, 2020, 07:55:29 am
content: Quote from: jkprg on October 09, 2020, 09:04:19 amI get the part numbers in the video from Tech Genie just uploaded Smokyds1e-m-12vdcMouser & Farnell got them from 5 USDIn ebay you will find them cheaper (10x 12USD) but surely fake (not silver joints)

author: Shiv
author_rank: Regular Contributor
timestamp: February 18, 2021, 08:49:02 am
content: I´m here only to say thanks.Finally I didn´t change the relays, my soldering Iron was too big to do it and I realized that when I get out the att board.Instead, I gave them a run of the old trick:12V Square signal from 10 to 1k HzNow it never fails SPC, neither selftest.

author: CaptDon
author_rank: Super Contributor
timestamp: February 18, 2021, 02:35:51 pm
content: The TDS644A and B seldom have problems inside the sealed attenuator unit. The leaking electrolyte on the acquisition board migrates and in particular effects the 4 high-speed samplers by creeping under them. It is a must to lightly scrub the board with a grease cutting dishwashing liquid and then rinse with clear water. I know most will disagree with my next comment. Blow dry the board with a hair dryer to evaporate the water out from under the surface mount I.C.'s!!!!! People will ward you of 'static charge damage'. I always held the board with CLEAN hands and spent as much as 20 minutes with the hair dryer until no more traces of water creeping out from under things could be found!!! It will take longer than you think!! I have repaired several TDS 5XX and 6XX scopes and all work great now. It seems you may have fixed your already but I posted this to share my experience. Cheers mate!!