Tektronix TDS544A Repair - Page 1
Forum section: electronics / repair

author: skennedy
author_rank: Contributor
timestamp: April 13, 2013, 09:00:51 am
content: Was browsing ebay last week and noticed a scope that took my interest, a semi-functional Tektronix TDS544A, 500MHz, 4 Channel, Colour CRT Oscilloscope. Won the auction and now await delivery. The seller listed the faults as "Very dim Screen - see photo", and "Scope not recognizing when probe is attached to any input". The first fault doesn't concern me too much as it has a VGA out so I can always connect the screen to an external monitor. The second is a bit more concerning. It would appear that channel one on the screen capture is sitting with a ~20V dc offset. I'm not very familiar with this scope and wonder if any sort of fancy probe detection is carried out. This fault would seem not to be related to recognition but to be something failed in the analog input/capture stage. Any one have any thoughts or even schematics? I will post lots of nice pictures when the scope arrives. Looking forward to an interesting repair project. Hopefully this will be a nice upgrade in performance from the Rigol DS1052E (with the 100MHz upgrade)Ebay Link:http://www.ebay.com.au/itm/370790593983?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649Simon

author: AndyC_772
author_rank: Super Contributor
timestamp: April 13, 2013, 09:16:58 am
content: The dim screen could be either a failing tube, a fault on the driver board, or a problem with the LCD shutter which sits in front of the CRT. There is a fairly easily accessible brightness pot inside the scope, accessible through a hole in the metalwork on the CRT side near the front. You just need to slide the outer cover back by a few inches to expose it.As for the offset, this is more than likely a result of the leaky capacitor problem that this range suffers from. There are surface mount electrolytic caps on all the boards (acquisition, logic and front panel) which leak corrosive electrolyte onto the PCB. You'll need to remove them all, carefully to avoid lifting pads, then clean the board very thoroughly with a suitable solvent. If you're lucky, simply replacing all the caps with new ones will fix the problem. If not, you'll need to look around for components or PCB tracks & vias which have corroded, and perform whatever repairs are necessary.The acquisition board is usually worst affected, since there are are more caps on this board than on the logic board, and they're closer to other components which can be damaged. Go round the whole of this board with a camera and photograph everything close-up before doing anything, you may well want to refer back to see which caps went where and what condition various parts are in.The good news is that many of the components on this board are actually fairly generic op-amps and 74 series logic, which can be replaced as needed at minimal cost. Although there are some much bigger, unobtainable components too, they're generally located far enough away from the caps to avoid damage.If you're lucky, you've got yourself a very good scope indeed at a bargain price. If not, I'm afraid it's a rather bulky paperweight - but still one which you might be able to break for parts and sell for spares once you've identified which bits are good and which are dead.

author: ivan747
author_rank: Super Contributor
timestamp: April 13, 2013, 12:16:25 pm
content: If that is really the fault, then I'm really looking forward to this repair. I skip these kind of early DSO when browsing though eBay. There's too much that can go wrong. Failed EPROMs, dying CRT, damaged ASICs. Just look at Dave trying to fix that LeCroy oscilloscope.Or sometimes you get lucky and just have to replace caps.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 13, 2013, 12:23:58 pm
content: Quote from: ivan747 on April 13, 2013, 12:16:25 pmBut the flipside is fixing it, having a 4ch 500MHz scope for peanuts & learning a lot on the way!

author: nctnico
author_rank: Super Contributor
timestamp: April 13, 2013, 12:51:56 pm
content: Quote from: ivan747 on April 13, 2013, 12:16:25 pmOn the Tektronix TDS500, 600 and 700 series you can't go wrong unless the display is bad. I have restored several. The problems with the leaking capacitors are well known and fixable. This scope doesn't seem to have a startup failure so chances are the capacitors didn't do any real damage yet. If you want to be safe its better to buy a scope from these series made after 1994.There are 10uf and 33uf caps on the CPU, acquisition, front panel and I/O boards. What I do first is wash the board with a strong cleaner (like 'simple green' or 'st marc' paint cleaner. Look 'm up with Google and find something with the same ingredients) and a tooth-brush. Rinse with water and then clean with alcohol or ethanol. Then rinse with water and let it dry on a heater.When removing the caps I mark the similar values first, remove the capacitors, place new ones and then do the same for the other value. Most capacitors are mounted in the same direction but some are reversed so look at the '+' signs on the boards.While the boards are drying its a good time to clean the front panel, knobs, rubber key mats and housing. It will make you scope look like new. This is TDS544A I restored a couple of months ago (it had a bad CRT driver board and some problems caused by leaking capacitors but nothing that couldn't be fixed):

author: AndyC_772
author_rank: Super Contributor
timestamp: April 13, 2013, 01:18:33 pm
content: Even if the display is terminal, there are LCD conversion kits available for these scopes. Generally I regard them as somewhat overpriced, but you might be able to develop your own for less if you're comfortable working with CPLDs.

author: nctnico
author_rank: Super Contributor
timestamp: April 13, 2013, 01:44:06 pm
content: I already did that:https://forum.sparkfun.com/viewtopic.php?f=14&t=23147Color is possible with about the same circuit:

author: vk6zgo
author_rank: Super Contributor
timestamp: April 14, 2013, 05:00:03 am
content: An alternative to the LCD may be to to what Jose gavila did with a TDS524A.The goo that holds the LCD "shutter" on the face of the tube had gone funny,& he successfully removed it & refitted the shutter.http://jvgavila.com/tds524a.htm

author: nctnico
author_rank: Super Contributor
timestamp: April 14, 2013, 11:04:12 am
content: Unless you have the 'aquarium with bubbles' effect you better leave the display as it is. If it ain't broken don't fix it.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 14, 2013, 11:10:02 am
content: To quote Jose'"This was not visible without an strong front light, so I had not noticed it..."If you just look at the screen without external illumination,you may miss it.

author: Watermelon
author_rank: Contributor
timestamp: April 14, 2013, 11:26:23 am
content: Don't power the scope up when you've got it, it could damage the scope because of all the bad caps in there. Replace ALL aluminum caps rated 10/33uF with new low-esr rated ones with voltage rating > 15V. The caps are on the aquisition board, logic cpu board and the front panel. None in the power supply. They use 10v rated caps on places were 15v is across the cap.When you've got the unit open, desolder and backup your NVRAM chip, it holds all the calibration data for 10 years minimum. I think it is a ds1250Y. Fit a chip socket in there and replace it with a new programmed NVRAM.Like nctnico said: clean, clean and clean more. Then power it up.

author: skennedy
author_rank: Contributor
timestamp: April 20, 2013, 08:28:28 am
content: The Tek 544a turned up yesterday. I couldn't resist turning it on for a moment, and seeing for myself the error log. The Scope booted fine until the Diagnostic Log. Interestingly the screen seemed to be pretty much spot on, so that was nice surprise. Much nicer than the five year lcd at work. More concerning was that we had some diagnostic errors. In this case, Cal Initialization, Acq/Proc Interface, Acquisition, Attn/Acq Interface. The first I suspect is due to the Dallas DS1650Y failing, this is going to be a bit painful to fix I think. Any suggestions for a programmer/reader for this device would be very helpful.Hopefully the other errors can be fixed with replacement of the electrolytic capacitors and corroded op-amps on the Acquisition board. Thankfully inspection of the PCB has shown that damage appears to be limited to fairly mainstream devices (read op-amps), with the acquisition ASICs looking fine.The Scope appears to have been owned by the Department of Defence here in Australia, much like the equipment that Dave saw at the auction. It appears to be last Calibrated in 2003.Digi-Key order for replacement op-amps, capacitors and Maxim 1250Y is on the way. Desoldering the DIP Dallas DS1650Y was pretty challenging without damaging anything, surface mount technology has made rework a lot easier.Attached are a number of interesting photos.(Edited to fix images)

author: skennedy
author_rank: Contributor
timestamp: April 20, 2013, 08:33:58 am
content: More Images

author: nctnico
author_rank: Super Contributor
timestamp: April 20, 2013, 04:11:06 pm
content: Don't bother with the Dallas NVRAM. First replace the capacitors! I'm afraid you will have to do some work on this scope (probably find broken vias and interrupted traces).

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 01, 2013, 03:15:59 pm
content: I've a TDS744D That don't show any waveform at all. It only show garbage wave on channel 2 & 4. What can cause that?

author: techineer
author_rank: Newbie
timestamp: May 23, 2014, 09:53:41 pm
content: Quote from: AndyC_772 on April 13, 2013, 09:16:58 amAndyC, you mention an LCD shutter.... can you provide more information on it?I just received a TDS544A on an eBay auction. The seller didn't indicate any problems with it, but it has an extremely dim display.When it arrived, there was a rattle inside, so I slid off the cover and found two loose screws inside. Matched them up to a shield plate on the bottom. Close inspection doesn't indicate any capacitor leakage, and no noticeable odor of fish or cat pee, so the caps _might_ be OK.Would like to figure out whether this is worth keeping or if I should send it back.Thanks!(Yes, I know this is resurrecting an old thread, but I think it's better to keep to a similar topic.

author: nctnico
author_rank: Super Contributor
timestamp: May 23, 2014, 10:48:15 pm
content: The displays on these aren't bright because a lot of light is lost in the LCD shutter. You could try to increase the brightness like Andy suggests.Here is more information on the LCD shutter technlogy:http://www2.electronicproducts.com/Oscilloscope_makers_ride_the_color_bandwagon-article-OL3-apr1993-html.aspx

author: AndyC_772
author_rank: Super Contributor
timestamp: May 24, 2014, 06:35:31 am
content: If the scope is working OK apart from the dim display, then you're in luck. It doesn't necessarily mean the caps are OK, though.If they've been replaced in the past, then that's great. But if they haven't, then they will have leaked electrolyte onto the board, and it's just a matter of time before it does some damage.If I were you I'd open up the scope and look for evidence that the caps have all been replaced. If they have, then enjoy your new scope - otherwise it's a job you need to do before the PCB gets damaged.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 24, 2014, 10:45:21 am
content: Have a look at posts 7 to 9 inclusive!

author: skennedy
author_rank: Contributor
timestamp: February 18, 2015, 01:42:17 am
content: I thought I should update progress on this repair as work and uni have got in the way for a fair while. After a lot of cleaning and replacing op-amps and shift registers the scope is looking a lot closer to being fixed. I purchased a Beiming USB to GPIB adapter which has helped a lot reading and, most importantly, clearing the error logs. I have identified that one of the P4C164-20JC CMOS 8K x 8 SRAM chips is dead. I can't easily locate these chips and wonder if an IDT-7164S20YG or IDT-7164S20YG8 which is at Mouser and Digi-Key would be a suitable replacement. They look almost identical in almost all of the specifications but have had some issues with RAM in the pastThe remainder of the error log looks to be likely caused by the failed memory chip and default calibration settings in the new Dallas part.:ERRLOG:FIRST "Sun 02-15-;5 16:30:34 ERROR: Cal meas clipped at max dig output, DigD Imbalance cal":ERRLOG:NEXT "Sun 02-15-;5 16:31:57 ERROR: Insufficient differential response, Preamp-Pipe gain characterization":ERRLOG:NEXT "Sun 02-15-;5 16:31:57 ERROR: Insufficient differential response, Preamp-Pipe gain characterization":ERRLOG:NEXT "Sun 02-15-;5 16:32:08 ERROR: Characterized values out of bounds, A/D biasing cal":ERRLOG:NEXT "Sun 02-15-;5 16:32:08 ERROR: Characterized values out of bounds, Vertical":ERRLOG:NEXT "Sun 02-15-;5 16:32:25 ERROR: 2260 Calibration failed, Triggers":ERRLOG:NEXT "Sun 02-15-;5 16:32:34 ERROR: 2260 Calibration failed, Repet Cal Failed"

author: skennedy
author_rank: Contributor
timestamp: March 18, 2015, 06:23:40 am
content: I have made some significant progress. Turns out the GPIB error log is invaluable to reading and clearing errors. A USB to GPIB dongle later I have fixed a lot.I have fixed all of the ACQ/PROC Interface errors by replacing two of the P4C164 sram chips. I was able to find some from a US dealer for not too much.I have also removed almost all of the signal path compensation errors to do with the acquisition system and believe that I have only one error left to do with the trigger system. Otherwise the scope is working as expected with four clean traces with pretty good accuracy even without being calibrated. Triggering is working fine for everything I have tested. I haven't been able to test above 15 MHz, where I am apparently going to see some issues, as I only have a 33120a to generate my signals.This error is proving slightly more difficult to get fixed and seems to be quite cryptic. The error ERROR: diagnostic test failure, ctlConfidencDiag, ** 2.50e-9 <= exp <= 1.20e-8 actual= 0.00e-1 delay TI failed" looks like it is something to do with the Time Interpolators used for the equivalent time sampling mode (I hope this is what TI means). It seems that this has troubled a few people in the past and I have spent a couple of weeks trying to work it out.Looking at the TDS520B service manual schematic I have worked out that components U16XX should be related to this. This area is very dense and entirely made up of LM311, TL072 and passives components. It was quite badly damaged by the electrolyte and I have spent a long time trying to repair it as best I can.There is a thread from quite a long time ago on the TekScopes mailing list which suggests that I should have a 3.125MHz signal on U1050 and U1001 for a fraction of a second during startup. The trouble is that I have a 10Mhz signal and then a ~30kHz and I can't seem to work out what could be causing it or if the pin might be wrong.Any one have any ideas? I really would like to have this scope conquered after all of this time and have enjoyed working out what everything does to work out if it needs reparing.

author: Jwalling
author_rank: Unknown
timestamp: March 20, 2015, 11:00:44 am
content: Quote from: skennedy on March 18, 2015, 06:23:40 amDid you clean all the boards and replace all the electrolytics? It is critical that this be done!Jay

author: skennedy
author_rank: Contributor
timestamp: March 21, 2015, 01:18:19 am
content: I have cleaned everything heaps of times with simply green, IPA, de-ionized water and lot of scrubbing. All of the electrolytic caps have been replaced as have all of the TL074, TL072, 74HCT154 and LM311s as they are cheap enough just to replace.Lifting chips and cleaning under them has yielded quite a lot of improvements particularly around the sample and hold circuitry. The residues picked up by the cleaning seems to have got stuck under chips causing issues. I have found a few damaged pads but so far the damage has been quite limited.My major issue is that I'm not sure where I should be looking anymore for resolving this issue. Looking at the TDS520B service manual schematic I can see a basic overview of a possible Time Interpolation circuit. Equivalent time sampling seems to work ok on my scope so I am having issues working out what exactly I'm trying to fix.

author: skennedy
author_rank: Contributor
timestamp: March 30, 2015, 05:17:46 am
content: I have fixed the ctlConfidencDiag error by fixing a damaged via under U1652 a TL072 opamp. The final error (hopefully) is looking at the output of a gamma comparator. I can't seem to find any information on this error anywhere. I am going to keep working through this error but if anyone has any experience it would be helpful. The location of the STRIGA testpoint would also be helpful. I am getting to the stage where lifting U1001 seems like a good option although this is going to be pretty complicated with so many pins.The error code is:ERROR: diagnostic test failure, logicTrigConfDiag, STRIGA must be HI when triggered, wasn't for gamma comparator"

author: cncjerry
author_rank: Unknown
timestamp: April 04, 2015, 07:05:18 am
content: once you have it working, you might be able to upgrade it all the way to a 784. I was able to take my 744 up to a 784 adding option 1M along the way for free. Remove a couple of zero ohm resistors, capacitors and make a ROM change is all you have to do.

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 28, 2016, 07:39:40 pm
content: Felt like rescuing this thread instead of creating a new one.Just got a TDS744A, works well overall. Haven't seen any leaking caps but will replace them nonetheless with 10/35V and 33/25V caps. A range of settings onCH1 don't work well, 200mV to 1V show ground (but from time to time I hear the click from a relay and the signal goes through so it must be a contact thing)Just ordered a DS1650Y and sockets... but my main question is, what about the other DS1486??? do I need to change/backup it?

author: skennedy
author_rank: Contributor
timestamp: September 28, 2016, 11:10:48 pm
content: There was some discussions on the Tekscopes group on Yahoo regarding the relays wearing out. Might be a good spot to look. I think they diagnosed the issue by measuring the on resistance of the relays and more than a couple of ohms replaced them.The TDS744A is quite a bit different from the TDS544A. Most of the capacitor issues that the earlier scopes had were resolved by the time the TDS744A came out.

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 29, 2016, 06:18:58 pm
content: Quote from: skennedy on September 28, 2016, 11:10:48 pmYes, the caps don't seem to be a problem and the relay seems to be a contact issue.However the Dallas rams worries me. I found a programmed that can read/write the RTC DS1486 and the DS1650Y so I'll desolder them and try that next.

author: cncjerry
author_rank: Unknown
timestamp: October 02, 2016, 02:17:47 am
content: It's unlikely the caps are wholesale bad in a 744 like the 5xx series. I wouldn't get carried away unless you find one through troubleshooting to be defective.

author: nctnico
author_rank: Super Contributor
timestamp: October 02, 2016, 02:33:08 am
content: AFAIK the capacitors are only bad in the scopes from the early 90's (say 90 - 92). I had a TDS510A from 1995 and the capacitors didn't leak at all.

author: PartialDischarge
author_rank: Super Contributor
timestamp: October 09, 2016, 09:39:24 am
content: The caps seem to be fine, so I left them.The attenuator problem is solved too. Seemed to be a contact problem although it took me a couple of tries to fix it. I did clean the contacts withalcohol and most importantly I did pinch with tweezers the spongy gold contacts that interconnect the boards as some seemed to be too compressed.I had another issue, in which the signals from 30Mhz and up did not trigger properly, there was jitter in the signal. After chasing a phantom hw problem, eventually it solved with the SPC self calibration.Both NVR are backed up tooNow I can enjoy the scope!

author: ruairi
author_rank: Frequent Contributor
timestamp: February 08, 2017, 07:13:14 am
content: Hi all,I figured it's best to keep TDS series issues in one thread. I have a very clean TDS620A that I bought from a friend a few years back, cal 'd in 2012 and has worked beautifully until tonight.I was probing around on an opamp stage when I accidentally touched the probe off of the palm of my hand, whether coincidence or not the scope flashed onscreen and shutdown. It attempted to reboot but failed every time.I learned about the infamous electrolytic cap issues after I bought this scope so I opened it expecting the worst. Even under the magnifier I cannot see a single leaky cap or corroded trace on the board, everything is super clean. I spot checked the ESR on maybe 10 caps with my Peak Atlas and every cap was sub 1 ohm.Is there something obvious that I'm missing? Any ideas appreciated.Cheers,Ruairi

author: daveyk
author_rank: Frequent Contributor
timestamp: April 22, 2017, 03:23:48 am
content: Quote from: nctnico on April 13, 2013, 12:51:56 pmI have one that fail acquisition during boot up. A lot of the caps were leaking on the acq board but not all. So far I have replaced about 75% of the caps on the acq board and am finished for the night. I tried booting before bed and I get same error. The ones I didn't change yet aren't leaking. There is some corrosion but not a huge amount. I don't hold out a lot of hope that changing the remain caps on that bottom acq board will fix it.It's a shame the crt is bright and the colors nice.It might end up on the scrap heap.DaveSent from my iPhone using Tapatalk