Keithley 616 Digital Electrometer: input amplifier heavy leaking issue

[Marco_Mar]

Hi,
after some easy fixings, now I'm clashing against an issue well beyond my knowledge and dexterity.

So HELP, please.

This is the problem: when measuring let`s say 5 V with my Keithley 616, there is a 100 mV drop across a 1 Gohm series resistor.
So the unit is draining around 100 pA. Specs state that input impedance should be > 2x10^14, so the effect of a 1 Gohm resistor should be negligible.
As suggested in other threads, I cleaned the front end thoroughly with IPA (and nitrile gloves...)  but no improvement was accomplished.

What do you think?
Should I replace the input MOSFETs?

In order to do that, I read all this interesting thread
https://www.eevblog.com/forum/testgear/does-this-keithley-616-electrometer-look-operational/

where some substitutes were highlighted but none fit K616 p-channel input MOSFETs, if I`m not mistaken.

Thank you in advance

[coppercone2]

if they are hard to get you can desolder them from the PCB and do a leakage test on a protoboard that is clean

[Kleinstein]

It is not just the MOSFETs that can leak, but also the transistors used for protection (especially if there is too much offset in the amplifier) and the relay and switches.

[Marco_Mar]

It is not just the MOSFETs that can leak, but also the transistors used for protection (especially if there is too much offset in the amplifier) and the relay and switches.

You are right but in K616 there is no relay in the signal path and in Voltmeter mode, the input is directly connected to input amplifier.
Furthermore, lifting a lead from a teflon standoff, I disconnected the chain of the two protection transistors from the input gate, with no success: same leakage.
Thank you

[Marco_Mar]

Firstly, I'd like to thank you all for your time.
Kleinstein's insight was spot on!
The culprit was exactly the only relay of this unit, the one that sets the Zero Check mode.
Being leaky and located before the input Mosfets caused all the issues. Disconnected, my K616 works fine.
Now I have to figure out a substitute.

Thank you again!

[Swainster]

Apologies for a slight thread hijack, but I thought it makes more sense to put this here than start a new thread.

I had a similar issue with my 616 - in my case there was a 0.5mV (i.e. 50 count) discrepancy vs zero check and using an external short. This corresponds to 5pA leakage, which I believe is excessive for this instrument. In my case detaching the zero check relay had no effect. By swapping around the input FETs and thorough cleaning, I managed to reduce the leakage to to 2-3pA. I then had the bright idea of detaching the protection transistor to see how much it was contributing... aaaand it's dead.

Well, the FETs were toast but the op-amp was fine, so inspired by Alexander Nitkin's 617 fix, I replaced the FETs with one side of an LMC662. After this fix the leakage was below 1 count I.e. 100fA. I have some electrolytics to replace before buttoning it back up, but after that I'll see if I can use the charge measurement function to get a better idea of the remaining leakage.

Hope someone finds this useful!