Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617

[Alex Nikitin]

when I trim zero offset in voltage mode then the current mode offset will be 1000fA+

Do you have a photo of the modded input? Something is not right, either you didn't do the mod correctly or there is a fault. With "Zero Check" engaged the only adjustment that matters is the voltage offset, which cover the range of +/-8mV on the trimmer pot and +/-12mV added by jumpers, more than enough to correct for the LMC662 offset (which is 6mV maximium according to the data sheet). The input current compensation pot is not working with "Zero Check" on. The main reason I did the mod was to get a much better voltage offset stability (which transfers to the input current stability in the current mode), with the original JFETs it drifts too much in my opinion.

Cheers

Alex

[ZhuraYuk]

I double checked everything and all is correct. Here are the photos.

[Alex Nikitin]

Unfortunately, the original photos are not in the thread now, there is one component missing, a 220nF capacitor (I've used PET film one) between pins 4 and 8 of the opamp. Try to add it!

Cheers

Alex

[ZhuraYuk]

The capacitor was there initially, but it makes no difference with it or without.  I disordered it to make better pictures.

[David Hess]

LMC662  was purchased from Mouser.

So there should be nothing wrong with the LMC662.

I already changed input JFET pair to LMC622 in some other device which has identical input schematic, the result was similar - offset in voltage measurement mode.

I assume this replacement is not suitable for this type of devices, it breaks voltage and ohms modes as well. The offset voltage of 20mV cause resistance never going to zero even with zero check enabled, it shows 200 Ohms. 
Amps works great on other hand, stability and settling time is much faster with OP Amp mod.

The differential JFET pair as shown is not quite a pair of source followers, but it should be close enough for the LMC662 to work.  The schematic in the service manual does not seem to be complete, or I would perform a more detailed analysis.

Inconsistent operation of precision circuits often means oscillation, which will appear as DC offset to a low frequency measurement.  The LT1012 supports overcompensation on pin 5 which may be useful here.

[MiDi]

Ok, that is a different error, nothing to do with offset, it is an intermittent fault.
The switched relays are suspect, preferrably the yellow one.