TDS684B fails acquisition self test - Page 1
Forum section: electronics / repair

author: Jwalling
author_rank: Unknown
timestamp: October 30, 2015, 12:22:52 pm
content: Just had this TDS684B scope come across my bench for repair. Purchased on Ebay as working:http://www.ebay.com/itm/141797300530?(Yeah, right. If I had a dollar for every time I've bought something off Ebay as used and fully functional that really was...)Would pass the power on self tests and SPC, but when the full self test was run from the utility menu, it would fail with the following errors:"Thu 10-29-;5 13:58:28 ERROR: diagnostic test failure, digFisoShortPipeDiag, ffffffc4 <= exp <= 3b was = -61, CH2: addr = 7340004, BWL=20MHz";"Thu 10-29-;5 14:08:08 ERROR: diagnostic test failure, digFisoShortPipeDiag, ffffffc4 <= exp <= 3b was = -61, CH2: addr = 7340008, BWL=20MHz""Fri 10-30-;5 06:37:03 ERROR: diagnostic test failure, digFisoShortPipeDiag, ffffffc4 <= exp <= 3b was = -61, CH2: addr = 734000c, BWL=20MHz"Found two unsoldered pins (51 and 52) on the A/D converter for CH2. This fault was there since it was originally manufactured in the 90's!Looking back through the logs, the first occurrence was in 1999. One would think that it was probably worth having Tektronix repair it. I wonder if it was under warranty then?Took a picture - wish I had a better camera.Jay

author: tec5c
author_rank: Frequent Contributor
timestamp: October 30, 2015, 12:36:51 pm
content: Quite certain that the 684A's were the model when Tek started to phase out the smd electro. caps and switched to tantalum.The smd electro's have been known to fail too.Are you saying that the failed bit test was due to the two pins?

author: Jwalling
author_rank: Unknown
timestamp: October 30, 2015, 01:24:17 pm
content: Quote from: tec5c on October 30, 2015, 12:36:51 pmThere's only a handful of aluminum electrolytics on the CPU board. None on the ACQ board.Quote from: tec5c on October 30, 2015, 12:36:51 pmThat's what I thought initially in my hurry to post this, however, the failure persisted after soldering the two pins (one of which was a GND.It turns out that it's the ceramic attenuator on CH2. Putting a known-good one corrected the failure. Will now check the relays as well as those little copper interconnects between them and the ACQ board and see what I find.Jay

author: nctnico
author_rank: Super Contributor
timestamp: October 30, 2015, 06:10:54 pm
content: AFAIK the relays in the attenuators can fail but are easy to repair. Hot-air is probably the best method since it is easy to rub a trace from a hybrid circuit but better ask for advice from others on how to repair the hybrid attenuator.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: October 30, 2015, 09:33:05 pm
content: Quote from: Jwalling on October 30, 2015, 01:24:17 pmMay I ask how the heck you found this? I previously repaired my old TDS544A scope, which failed signal path compensation, but that was the obvious fix by replacing all SMT electrolytic capacitors. Anything else bad, without board-level schematics, I've found it to be hopefully impossible to trace to the error...

author: nctnico
author_rank: Super Contributor
timestamp: October 30, 2015, 09:41:35 pm
content: The TDS520B component level service manual can be used to track most problems but you have to be bit creative.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: October 31, 2015, 12:35:51 am
content: Quote from: nctnico on October 30, 2015, 09:41:35 pmLike this one?http://exodus.poly.edu/~kurt/manuals/manuals/Tektronix/TEK%20TDS%20520B,%20540B,%20620B,%20644B,%20680B,%20684B,%20724A,%20744A,%20784A%20Service.pdfThere's not a single schematics so there's no way to pursue an engineering approach to searching for errors. Anything board-level is blaringly missing. Of course, these days, we are not suppose to repair but rather throw away and buy new. Also, I suppose the manufactures don't want to give away any engineering information to the competition. Gone are the days where full technical information was published for complex equipment...

author: nctnico
author_rank: Super Contributor
timestamp: November 01, 2015, 01:51:39 am
content: Duck-duck-go before ranting... One of the first results:http://www.austexsoftware.com/documents/tektronix/service/070971003%20TDS520B%20Mod%20CM%20Digitizing%20Oscilloscope%20Component%20Service%20Manual.pdfThe TDS500/600/700 series are very similar so the diagrams will help to find most of the problems.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: November 01, 2015, 05:39:44 am
content: Great; thanks.

author: Jwalling
author_rank: Unknown
timestamp: November 01, 2015, 10:23:59 am
content: Quote from: John_ITIC on October 30, 2015, 09:33:05 pmNo problem there - I've replaced hundreds of them. I cut out the bad one, clear the pads and solder on new ones.Jay

author: Jwalling
author_rank: Unknown
timestamp: November 01, 2015, 10:38:52 am
content: Quote from: John_ITIC on October 30, 2015, 09:33:05 pmAs I said, I found it by replacing it with a known-good one.It turns out that one of the relays was bad in the N.O. position. When energized, the N.C. pins would remain shorted, and the N.O pins would remain open, even though I could hear the relay click.I wrote an X on the bad one. Oddly enough, of the hundreds of relays I've replaced on these, this particular one fails very rarely.The most common one is to the left and slightly lower. The next one to fail often in terms of frequency is the one above that.It's almost like there's patterns to these failures.Jay

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 08, 2018, 09:04:36 pm
content: Quote from: Jwalling on November 01, 2015, 10:38:52 amHey Jay,Sorry for the necroposting, but what do you consider a bad relay? I'm measuring relays anywhere between 0.5 and 3.5 Ohm between com and NC...

author: Jwalling
author_rank: Unknown
timestamp: May 08, 2018, 09:49:34 pm
content: Quote from: Ice-Tea on May 08, 2018, 09:04:36 pmI use a 4 wire (kelvin) measurement with a Keithley 2015 in REL mode when doing this. What are you using?Anything over 60mOhms gets replaced.

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 09, 2018, 07:17:31 am
content: U1252B.Double checked with a Rigol DM3058E in Kelvin mode. There I measure 40-50mfor a good one.I think I will use 0.75on the Agilent as a threshold, will measure a bit esier.Thanks for the input!

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 09, 2018, 07:47:06 am
content: 6 relays have been nominated for the bin.Also: WTF?

author: Jwalling
author_rank: Unknown
timestamp: May 09, 2018, 09:37:41 am
content: Quote from: Ice-Tea on May 09, 2018, 07:47:06 amOddly enough, I've seen that many times on that relay!

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 09, 2018, 05:09:15 pm
content: That's...Seriously, how could QA have missed that???

author: andy2000
author_rank: Frequent Contributor
timestamp: May 09, 2018, 06:05:44 pm
content: That solder splash could have happened when someone removed it to try to repair the bad channel.Here's a picture from the power supply of a 744A I used to own.