keithley 195 option repair

[tjl0023]

I purchased a keithley 195 with the option installed but the amps range only works on the 20uA and 100uA range. All of the other amp ranges show "-oflo". The adc works fine so i have a feeling its just something with the range switching but i cant find any info about it. any suggestions on what to do next?

[lowimpedance]

 The online manual has the schematic of the option and also in the maintenance section there are tables showing which relays / FET's that are used for each range/function etc.
Do the ACV ranges work correctly or do they fail in a similar pattern ?.

[TurboTom]

I can't tell for sure in your case but I recently fixed a 195A that had several of the analog multiplexer JFETs blown. These seem to be quite fragile actually. I ended up replacing all of them (Q102~104, Q107~114, Q121, Q122). Some are selected types (Q121,Q122) and I did my best to find failrly good matches to these  -- one of the original transistors was still okay so I was able to measure the on-resistance.

If the DC ranges on your 195 are working correctly, you may want to have a look at Q502 and Q503 and maybe also Q505 and Q506 on the AC option board.

Good luck,
Thomas