Electronics > Repair
Keithley 2002 repair help
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saturnin:
It is much better now! Good job :) Although there are still some missing connections to important nets/pins (U231-3, U231-5, U228-3...), it is not necessary to track them completely - it is enough to measure voltages at these points. It will tell you a lot about circuit function...

As concerns the leakage, check U228 works properly, i.e. there are same voltages at its pins 3 and 6. If so, CR208 or the switch connected to R272 may leak some current. (When 2M range selected, there should not be any significant current flow though R272, definitely not 0.5 uA...)
Kleinstein:
The circuit plan so far looks reasonable - At least I don't see an obvious mistake. There is some connection missing around R276 / U232 pin 2. probably a connection to a fixed reference voltage (around 7 V).
The still open connection of  3 of U231 mit be towards pin 5 or 7 of U231. At least this would be a logical target.

Leakage through R272 has only 3 ways to go: dirt on the board (e.g. below U223), the diode or the CMOS switch (U223). Checking the diode path it likely easier, up to the point of de-soldering the diode. U228 is likely OK since otherwise there should have been even more leakage trough the other resistors. It would be more like the 100 K resistor open or U221 Q6.
nikonoid:
Kleinstein,

You are right on the money again. I found Pin 3 to Pin 5 connection for U231.

I did not have much time today to troubleshoot the circuit. I did check all 4 100k resistors in the vicinity of U220. I got readings of about 98k for all of them. For 100k resistor connected to Q251 I got 83k. I guess there is some parallel circuit somewhere.

I also checked how much current is flowing through R270,R271,R272 and R277 on working meter and it is quite a different picture. I only got between 0 and 0.003uA there.

I measured all four diodes (in circuit) and got pretty analogous readings between meters: 0.66V in one direction and 2.5V or 2.6V in the opposite direction.

Not sure what to check next...
Kleinstein:
The 100 K resistors don't need to be accurate, they are just there to cope with any leakage from CMOS switches above. One should measure the voltage on both sides of CR208 - ideally it should be very close to zero voltage over that diode, unless the resistor is chosen as the active one. The voltage should also very close to the point where the 4 precision resistors join together. As there is some extra current through the resistor, the polarity of the voltage over CR208 should tell it the current flowing this way (e.g. leakage in the diode) or the other way (through the other CMOS switches). In the 2nd case is would be de-soldering the switches U223: as it would be either that chip, or maybe residue - likely under it. Likely a new chip for U223 is than needed - as there is quite some risk it is damage (either from the beginning or when removing it).

If it is current through the diode, one could for a test remove CR208 - trouble is than more like with the other CMOS switches (U221-Q6).
TiN:
Prettified nikonoid's handiworks just a bit. Time to get back with pensil again ;)

PDF-version.

EDIT: Bugfix ver3.1

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