Electronics > Repair
Keithley 2002 repair help
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nikonoid:
Thanks, TiN.

I actually got somewhat very similar numbers on both working and broken 2002. The only exception is that on the broken one they seem to fluctuate rapidly by 1-2%. It was a very quick test and I might have to redo it.

Overall resistance switching seems to work ok. Then in a pinch I used a different meter in a resistance mode to serve as a current source. When I was pocking around all of the sudden current started to work then stopped.

So far it appears that proper voltage is not reaching Pin 3 of U225 (unity opamp LT1097S8, incorrectly marked in the manual as V425 instead of U225).

My next stop is to check for compromised vias or solder pads on the way to U225, especially considering that it was heavily compromised by electrolyt and replaced by me.

As promised, here are photos of the unit after repairs. I am considering shortening bodge wires later, however I will have to get a shield first before I decide on the final route for them. I am considering hot melt glue or E6000 glue to semi-permanently hold them to the board.

Le_Bassiste:
gents,
i need some clarification on your current (Amps) input findings.  were all those measurements taken from outside, i.e., using AMPS  and LO input jacks?
if so, then it's little wonder that measurements are fluctuating. the internal spring loaded white cable "pigtail" connector on the fuse holder is just f* crap.
some intensive IPA cleaning of the fuse holder interior and fuse itself helps alot.

TiN:
In my case, yes, outside AMP + LO jack. I'm not taking apart 24/7 online 2002's anytime soon, unless they go kaboom  ^-^.
I didn't see much fluctuation (random 3458 in reach was used to measure resistance), but again I don't ever remember using 2002 to measure current, other than during calibration  :-DD.
nikonoid:
Le_Bassiste, for rear testing I was using rear terminals (from outside). For front I was using common (Input Low) from the outside and the hole on the PCB where the white AMPS cable goes to. I do not have it soldered yet. So I was bypassing the fuse and fuse holder.

This "fluctuating reading" behavior comes and goes, so I cannot yet pinpoint what is going on. One possibility is something going wrong with control signals that switch in different resistances. Maybe I get a behavior when specific resistor is not completely turned on.

I found what looks like a bad via where the signal after chain of resistors is traveling to pin 3 of U225. I may have to learn how to copper stitch vias. After a did a quick fix to bridge that via, the situation improved just a bit, but not much.

As I am pocking around the miter it behavior seems to change. It could be a mechanical effect or thermal, as I had with a previous K2002. There I had a bad IC switch that depending on a temperature would sometimes be partially closed.

With me fixing that one via the test 500.1 is now passing most of the time, while others still fail. After the fix, the current function seem to work to a degree, it is couple of %s off on 200uA range, little bit more off on 2mA range, and so on with 1A range being about 40% off.

I was tracing the U234 chip that controls 2mA and 200uA switching and found that while 3 channels would get 0V or 5V on their control inputs, one channel got -3V on it, so now I will have to check where control signals are coming from.   

So we are basically down the rabbit hole....


 
nikonoid:
Today I found another bad via. After fixing it, I am down to only TWO test errors 306.2 and 600.2   :-+

Current passes the self test, but it is working strangely. With shorted inputs I get:
200uA range: -000.0461 uA
2mA range:   -0.013751 mA
20mA range:  -00.03802 mA
200mA range:  -001.7962 mA
2A range:   -0.018315 A

If you take this offset into consideration, then DCI reading works pretty well.

I will trace the problem with Ohms next and get back to current later.
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