Alright then here is some more TEA stuff.
I picked up a Keithley 617 Electrometer on ebay. Listing said that the voltage output is broken but it otherwise works.
Few weeks later it arrives and i hear a noise inside of it if i move it around. Also it was 110V and no external switch to take care of it so it was a case of "Don't turn it on, take it apaaaaart"
Getting to see the nice front end magic there, but i start finding screws laying about in places, the top analog board was barely held in as half of its screws came out. So i kept taking it apart to find all the remaining screws.
Got a nice look at the front panel too, no runaway screws there.
Got the whole thing out and indeed found some more of the screws under the main board
Once i was sure there are no screws left in there i assembled it back together and turned in on for the first time! Aaaand its reading OL on all ranges.
Well then... Out comes the multimeter and the manual. Checking all the voltages is a bit annoying, as none are marked where they are, so you need to look at the schematic for that. Turns out they are all there except for the +/- 110V rail. This rail runs the 0-100V output on the back so that would explain why that does not work, but it has nothing to do with the ADC or the front end so it should not make it read OL all the time.
I cobbled together a ribbon cable extender so i could power it on and probe things with the upper analog board out of the way. Everything seamed fine until i stuck my probe on to the ADC clock line. It was moving but rather chaotically with weird sloping rise times. Tracing it back shown the same weird chaotic clock at the crystal oscillator circuit. To quickly test this i soldered a wire to the oscillator circuit and injected my own square wave of the right frequency and suddenly the display starts showing numbers! The only problem is that the crystal is a rather oddball frequency of 1.2288 MHz. Eventually i found one on ebay that was salvaged out of gear, it was even in this gigantic can package that fit the footprint!
After finally getting it i put it in and there we go it works! Next is the 110V rail for the voltage output feature. Probing around has shown that the winding on the transformer for that rail is completely dead. Fixing a transformer is not easy and its not easy to find a replacement so instead i went for adding a new transformer just for that rail.
This instrument has a bit of a weird arrangement inside where the big main transformer is run off mains and has electrostatic shielding and produces a 9V AC supply that goes to the 2nd smaller transformer on the analog board where all the analog voltages are generated. These voltages then feed back down to the main board to power the analog parts of the ADC. Very convoluted power arrangement. Because of this what i did instead is get a 24V transformer brick and stick it reverse on this 9V AC output to boost it up to roughly 110V after messing with the taps a bit. This was then given a filled out notch to snugly fit on the top of the existing transformer before getting glued and zip tied down.
And there we go. The entire unit put back together and she works like a dream now!
Here it is showing a zero offset current of 0.0026pA . That is 2.6 femtoamps!