Keithley 196 repair report and question.

[Henrik_V]

I happened to get my hands on a Keithley 196 DMM ..
First run: Funny numbers in all Ranges...
OK, this device is really service friendly, good service manual / schematics (in bad quality, a better copy for schematic and boad overview is highly wellcome). 

Needed a recap in the powersupply and a resoldering of the transformer connection.

Pictures of the internals are already posted, I add only two:
The powerboard is bended due to the mounting/fixing srews of the transformer (after soldering!)    resulting in broken solder joints....

Powerup, shortcut, 300mV range, zero stability    , apply different voltages in all ranges  , Ohms:     AC has to follow a 5700A is available

Check for input impedance: Apply a stable in range voltage in both polarities and insert a 10M in one lead ....
The input protection diodes (FETS) are leaking... 300mV Range has about 11Meg for negative voltages, should be >1G.
So this unit has seen some unhealthy occurences....

The doc tell me Q5-Q10 are 2N4392  ..  4EUR for the metal can   but .65 in SMD ...  is there a reason not to take the SMDs? Other JFETs prefered? 

Greetings from Germany
Henrik

[saturnin]

11 MOhm is input resistance on 30V range. If you measure 11 MOhm on 300 mV range, Q13 JFET might be shorted (or turned on permanently).

[Kleinstein]

The non SMT version of the fets might better fit the existing layout. Other instruments like the Keithley 2000 use the SMT version - so should be good enough here. Possible higher drift in threashold votlage should be no problem in a switching application (could be different if used as am amplifier).

[Henrik_V]

Thank you for the input. 
I will check the Q13... However since the 11M is only at negative input voltages in the 300mV and 3V range, my first action will be the input protection ones .... with SMD

Henrik

[lowimpedance]

Better scans of the schematics can be found here.
https://doc.xdevs.com/doc/Keithley/196/

I fixed one of these DMMs at work some time back and the problem was the same with the transformer and cracked joints.