Keithley 410A Picoammeter, Teardown, Repair

[HighVoltage]

I bought this Keithley 410A Picoammeter, really cheap because it was listed broken and I was not sure if it could be repaired.

Repair
The problem was found fast, so it can not even be considered a real repair. The mains power cable was so old and the insulation was broken off from the conductors, it made a shortage. New mains cable is installed.

Teardown
Since I had it open anyways, I took some pictures of the inside as well.

Now I need to find a way to test it.
I do not have a real pico amp source.

 

[HighVoltage]

Inside 1

[HighVoltage]

Inside 2

[testmode]

Now I need to find a way to test it.
I do not have a real pico amp source.

Yes, you know you need one! (Sorry can't help it after just coming from the TEA group therapy thread )

Congrats on the good find!

[TimFox]

I have also repaired several old Keithleys where the power cord rubber had failed badly.  Past that, most of the problems were solved by cleaning the rotary switches.  (Be careful with switches in high-impedance locations, such as the main range switch on a picoammeter.)
PS:  the easiest way to get a proper diameter cord was to start with a cheap IEC computer cord and cut off the IEC female connector.

[Assafl]

Strange guarding on the high value resistors. Can't say I understand where the guards are connected to.

[Vtile]

So the smallest resolution is 5 fA and the deltaA you can notice is <1 fA ?? Nice find.

[edpalmer42]

Now I need to find a way to test it.
I do not have a real pico amp source.

All you need is a few gigaohm resistors.  I was surprised how cheap they are on ebay.  I searched for <resistor (gigaohm,gohm)> and got over 100 hits.  One vendor sells a set that claims to go up to 1000 Gohms!

That would be interesting to test, but at those levels, be sure to think positive thoughts or you'll upset the reading!