Electronics > Repair
Keithley 2002 repair help
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nikonoid:
I did a number of measurements comparing good 2002 that I have and a Bad 2002.

I used 100k resistor on a 2MOhm range. Most measurements were done relative to Input LO.
On Bad instrument ohm reading was at least 10% low, sometimes more depending on how warm the instrument was.
Please keep in mind that all values from good 2002 were stable, while many values on bad 2002 were drifting with temperature and since readings were taken at different time they may not correspond to each other precisely.

I compared values and highlighted any place where I found significant difference. Please tale a look at scans attached and let me know what you think. Pardon my horrible handwriting :(.

Some most notable differences include Q251, Q226, VR204, Q210 - Q214. Thanks a lot.

PS. I have not had a chance to get to measurements with oscilloscope. It will have to be sometime this week.
Zucca:
 :popcorn:

PS: Excel next time?
Kleinstein:
It would be much easier to have to voltages from both meters on the same sheet.

The voltages around Q251 look a little odd. One thing is the rather high supply (collector or Q251) with the bad meter. So there might be a slight problem with VR216. However this should not cause this problem it would be more a problem causing higher temperature at Q251 in the low resistance ranges. Still worth checking VR216.

Ar the pins in the same sequence ? It is a little odd to have the base slightly below the emitter. This might an indication that there is some AC super-imposed. If swapped the two values would make more sense. Similar a base voltage around 15.3 V instead of 14.3 V (this would also match U231 pin 1)

The difference in the voltage around Q213 and so one could be a minor difference in CR217 ( different type of diode, with higher voltage drop on the bad unit. This normally should be of no consequence - just in case one could temporarily have a second diode (like 1N4007) in parallel.

So far the only part really indicating a fault is the voltage over CR208 and thus R272. The polarity of the voltage indicated the false current can not flow through the diode. If at all the diode would add a few pA to the output current. So far I only see U223 (pin 10) for the current to flow through - somehow leaking toward ground.

The DC voltages around U223 (max326 - the one that got replaced ?) look good. Still a little odd they supply +-15 V to the chip, as there is no need for a high negative supply and this would only add to leakage. But it seems to be the same way on the good one.

I would really look for AC trouble (like a control loop that oscillates or maybe spikes on the supply).
One might learn a little more about the missing current, with testing in the 200 K range. This would change the voltage at the critical node.
nikonoid:
Thank you. I will remeasure everything on transistors and make note of pins in case I made an error before. I also prepared an oscilloscope to do AC measurements.
I did notice before when working on power supply that bootstrap voltage was a bit high. As per manual it was supposed to be +34V to +38V and I was getting something like +38.6.

This comes to be the most difficult repair I ever attempted.
TiN:

--- Quote ---This comes to be the most difficult repair I ever attempted.
--- End quote ---
I bet you learned a lot already as result.  :-+

If you have more schematics works, I'd be happy to digitize as well.,  :-DD

+38.6 sounds fine to me, I saw up to +42/-42 few times. It depends on line frequency/voltage a little.
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