Tektronix TDS520A DAC range test failure - Page 1
Forum section: electronics / repair

author: MetalHead
author_rank: Contributor
timestamp: December 23, 2022, 06:06:19 pm
content: Hello guys!I'm stuck on repairing my tektronix TDS520A scope that is giving "Acquisition" & "Attn/Acq Interface" diagnostic errors. Error log shows"Dac range test ** DAC system failure".I have recapped processor board and acq boards. There were only two caps leaking - one on processor board and another on VCO board, all other caps just had lowered capacitance. There is no corrosion underneath those caps. All traces are in perfect condition as the boards themselves. Only light dust from fan that I blew out with compressor.. But it still failing.I found this threadhttps://www.eevblog.com/forum/repair/tds-620-still-failing-after-recap/and the last reply from "rf+tech" is the most good explanation how this scope is performing diagnostics and this is where it's failing."Acquisition Processor U600 (schematic A10-11) sends 8-bit control data to DAC U900 (schematic A10-12). Follow the analog output from U900 pin 9 (Vout) to U903A to TP912. Continue on to schematic A10-13, U933 CD4051. Analog voltage from MUX goes through each Probe Control Buffer, to schematic A10-16. ProbeOffesets then go to the Attenuator board. Self-test commands the DAC to run the analog voltage up and down, looking for a response via ADC U750 and U700."I downloaded TDS520-SM-V2 and started checking things.1. Signal at TP911 (U900 DAC VOUT) is like that2. Then signal goes to U908 DG444 IC that has however no output at pin 2,15,10,7. Supply V-,V+,GND everything is ok. I replaced this IC but nothing changed. So this is my first question. If there is input (let's take pin 8,6) but pin 7 does not change is that OK or not.3. Also I checked every ICs in that corner looking for missing, wrong supply - everything is fine (U901, U931, U905, U932, U933.. ).That section looks except that I'm not sure about U908 output...So I switched to the A/D section with U700. U750. And there is following that I'm not quite sure.Does it mean that A/Ds are faulty?====Signal at pin 71 of U700 U750 is like that..pk-pk 116mV, freq 243.3MHz (only have scope that can measure up to 200Mhz) and no output at bus A,B

author: rf+tech
author_rank: Frequent Contributor
timestamp: December 24, 2022, 01:02:04 am
content: Quote1. I cannot say if this waveform is correct, or not.2. Please show each DG444 switch control input and corresponding switch signal input, on channels 1 and 2 respectively. Trigger scope on switch control input.- Repeat for each of the four switches inside DG444.- Next, show each DG444 switch control signal and corresponding switchoutput, as above.- If the switch control inputs are out of sync with the switch signal inputs, no switch output is to be expected.3. Inputs determine outputs and time-dependent inputs must occur at the correct time - apply this premise a you work through the self-test signal path.Also, the scope cannot wrap long lines in the error log. What cannot be seen is the measured result, following the expected value - and that measured result may provide additional clues. To see the entirety of long error log lines requires use of the console interface and an external terminal app. Search the forum for "Using the Tektronix TDS Console Port" for details.RF+Tech

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 24, 2022, 05:11:50 am
content: You can also extract the error messages via GPIB. I use the Keysight IO software.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.(Don't type the quote marks)I would first clear the error log, then reboot the scope so you have the latest and crispy fresh errors to concentrate on.

author: MetalHead
author_rank: Contributor
timestamp: December 24, 2022, 09:53:32 am
content: Thanks for suggestions!!!I would do that but I don't have GPIG interface cable (I made few more researches yesterday. Particularly I was looking at U700/U750 A/D converters and their clock at pin 71. And that seemed to me suspicious. The clock signal somehow weak and 243.3MHz... why? And there is a mention about that the output on bus A, B should be in sync with rising edge of pin 71 clock. But the clock signal is not square! it's a sine or similarSo I went to see what's going with the clocks (VCO & ACQUISITION CLOCKS A10). There is U504 (MC12040FN) has two inputs pin 9 and 13. Signal on pin 9 looks like this - a sawtooth, but accordingly with the clock troubleshooting procedure (Vol 2, 9-18) it should be squareWhen I look on schematic (A10) signal to pin 9 of U504 comes from pin 8 of U501. And pin 8 doesn't have clock signal at all.So looks like that U501 is faulty.

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 24, 2022, 11:59:19 am
content: I have heard of the 80MHz crystal for the video section can go bad, so maybe this could be true for the other crystals in the scope? I wonder how they go with shock damage if the scope is dropped?They are self contained oscillators, so you can remove them and power them up with 5V to test them relatively easily if you choose to do so.

author: MetalHead
author_rank: Contributor
timestamp: December 24, 2022, 05:45:48 pm
content: The part that seems has failed is this chip. Is there any replacement for this IC or nothing can be done?

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 25, 2022, 01:28:06 am
content: These are usually a custom Tek part.Basically you are down to salvaging it from a spare board.Let me check my spare parts, Imighthave something, but no guarantees...

author: MetalHead
author_rank: Contributor
timestamp: December 25, 2022, 12:31:10 pm
content: Please have a look and let me know! I will buy it if you haveAlso I'm looking on taobao but there is no spare parts, only broken TDS520A scope but who knows if that chip is ok there.. and there are also few scopes with the same error as mine (

author: MetalHead
author_rank: Contributor
timestamp: January 01, 2023, 02:17:33 pm
content: So......I was not able to find Tek chip anywhere, so I bought used acquisition board for TDS520A from ebay.Curious whether I was right about U501 chip and the clock issue causing all these diagnostic /DAC failures.Will update this post as soon as it will arrive and I put it into the scope for testing.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 01, 2023, 04:35:19 pm
content: Unfortunately I didn't find anything either, not a single one of my spare boards had that part unfortunately.