Keithley 2100 repair help

[Mav_2014]

Hi All, I'm out with my begging bowl. Keithley 2100, errors 612, 613, 617,618,619, which I believe are related to the current source.

What have I replaced, well, u305, u401, q401, q402, q311.

With q311 out of play the errors are the 500na source only with errors 612, 613 and the others gone. However q311 replaced did nothing, nor q401, q402 which I assume is the current source. The craddock divider is also ok. With q311 replaced ALL of the errors are back.

All the errors above were there before and changing all of the above has made no difference to them.....

Help much appreciated as it's driving me round the bend! Many thanks in advance!