Keithley 614 Electrometer repair - overvoltage on current measurement

[balage]

Hello,

Shit happened today. I was measuring microamps in 20uA range when I disconnected the loop and connected it back again. The source was a current generator with a compliance voltage of 600 volts (max voltage for the 20uA range is 75V for the 614). So the input was fed with 600 volts. 

The 614 now measures exactly the half of the input current with opposite polarity. So when I fed 20uA in it measures -10uA. Resistance and voltage measurement is still fine.

I was checking the schematic (full manual with schematic is attached) and trying to find what was damaged. So the input stage is almost the same for R, V & I measurements that includes the mighty Q104 and U102. So I think (hope) that if R and V measurements are okay than this stage is not damaged. The I measurement configuration is here:


I don't really understand what does "Provides the inversion necessary for feedback current".

Do you think it is possible that only this opamp is gone? It is still unobtainable and some replacement part must be found.

[Kleinstein]

It is possible that only the OP U103 is bad. though there may be more things wrong, like some diodes. The input path to the JFETs is reasonably well protected with high resistors. The FB path has mainly the 10 K for surrent sensing to limit a current surge.

It probably needs a few more readings to really tell that the OP is bad.
There are still a few high voltage OPs available. At least the actual performance / parameters of this OP should not be that critical. So a substitute should be posssible.

[balage]

Thanks for your ratification. The opamps are on their way to me. I will replace and will see.