Jackson Eric Research Physicist

I noticed the problem with the cards we've been using for several years.  We bought some refurbished cards, but that didn't solve the problem.  Now some of the slots no longer seem to work at all with any card.

Photodiode measurerments using a Keithley 7002 switching mainframe equipped with 7158 low current scanner cards have started to become  obscured by excess noise current that is about 10 times greater than the 1 pA noise floor of the 7158 cards. This is a recent development in a system we have been using for the last 10 years.

We suspect the 7002 Mainframe since we have replaced cards and without seeing improvement, and when we measure individual photodiodes directly, by-passing the 7002, we are able to obtain results in the pA range. 

Is the a diagnostic we can do that will allow us to identify the problem?