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Not sure whether it has sense to mess with this monster further on.. Best would be to desolder the 500G resistor and the 10pF capacitor and the rest trashed..
I still think that would be a crying shame. That thing is insanely sensitive and the 1.4fA input leakage current is very respectable, it might be even better after cleaning if it has been lying around in a junk box and you've been touching the insulatoror other parts with your fingers. The noise level doesn't sound that bad (for DC measurement use). I'm not clear on the source, but it might be possible to substitute a more recent, lower noise opamp. The physical construction is good and would take some significant effort to reproduce.
Before pulling the resistor, you need to consider what purpose you want to use your picoammeter for. 0.5mV output per fA is going to limit you to a very few applications - it would be useless for measuring the leakage of most components, reverse leakage of semiconductors, where the output would immediately peg to the supply rail.
Having built a 1mV/pA picoammeter (reply #23), I have found this perfectly adequate for all my leakage measurement needs (even evaluating insulating sleeving). I would still find a 1G, or maybe 10G resistor if you're feeling the need for a little more sensitivity (and noise).
As for the capacitor, I hadn't realized that Porcelain capacitors were actually
that low leakage, but it is something that could be easily matched with an axial Polystyrene capacitor with a lower value feedback resistor - you would want a higher value anyway (330pF in my case).
If you have low motivation for this unit at the moment, I would wrap it carefully and store it in dry conditions. Either that or pass it on to another member. I certainly wouldn't trash any of it, the mosfet for one, has value for other instrument repairs as previously mentioned.
Just my tuppence worth anyway.