Finally, after several years of service in my home lab, my 616 is dead.
Or to be more precise, I have killed it.
[Boring explanation starting]
Everything started with a stupid accident involving a broken keithley 220
with killer attitude.
Anyway after the first accident I repaired it (fortunately no sensitive electronics were involved)
and I was curious to track some small leackage problems
of the 616. The first was the 10
-10A range that was way off.
This problem was solved after cleaning the seemingly already clean 10
10Ohm resistor.
Then I started playing with the residual leackage read by the meter, it was measuring
0.3pA with open leads, aiming to understand what was the source of it.
[Boring explanation ending]
Well, I have played too much, I managed to kill Q101 mosfet with a gate perforation.
So I have ordered some 3N163 with the hope to be able to fix the damage
Waiting for them to arrive, I'm playing with some opamps to try to replace all the above circuitry
with something more modern.
This is the schematic I'd like to try, for now I've only tested the principle with bare minimum cimponents,
but I need to test and verify the behavior with the voltage regulators and the zero circuit.
Is there any hope to be able to create a decent electrometer input this way?