TDS684B fails acquisition self test

[Jwalling]

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

[tec5c]

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?

[Jwalling]

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.

There's only a handful of aluminum electrolytics on the CPU board. None on the ACQ board.

Are you saying that the failed bit test was due to the two pins?

That'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

[nctnico]

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.

[John_ITIC]

It turns out that it's the ceramic attenuator on CH2.

May 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...

[nctnico]

The TDS520B component level service manual can be used to track most problems but you have to be bit creative.

[John_ITIC]

The TDS520B component level service manual can be used to track most problems but you have to be bit creative.

Like this one? http://exodus.poly.edu/~kurt/manuals/manuals/Tektronix/TEK%20TDS%20520B,%20540B,%20620B,%20644B,%20680B,%20684B,%20724A,%20744A,%20784A%20Service.pdf

There'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...

[nctnico]

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.pdf

The TDS500/600/700 series are very similar so the diagrams will help to find most of the problems.

[John_ITIC]

Great; thanks.

[Jwalling]

It turns out that it's the ceramic attenuator on CH2.

May 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...

No problem there - I've replaced hundreds of them. I cut out the bad one, clear the pads and solder on new ones.
Jay

[Jwalling]

It turns out that it's the ceramic attenuator on CH2.

May 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...

As 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

[Ice-Tea]

It turns out that it's the ceramic attenuator on CH2.

May 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...

As 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

Hey 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...

[Jwalling]

Hey 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...

I 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.

[Ice-Tea]

U1252B.

Double checked with a Rigol DM3058E in Kelvin mode.  There I measure 40-50m for a good one.

I think I will use 0.75 on the Agilent as a threshold, will measure a bit esier.

Thanks for the input!

[Ice-Tea]

6 relays have been nominated for the bin.

Also: WTF?

[Jwalling]

6 relays have been nominated for the bin.

Also: WTF?

Oddly enough, I've seen that many times on that relay!

[Ice-Tea]

That's...

Seriously, how could QA have missed that???

[andy2000]

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.