Keithley 7002 switching mainframe with excess noise current

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?

Were those cards that you swapped out working in another mainframe? The current paths are through the cards, not really through the mainframe, so if anything is up, I would assume it would be on the cards. You can check for offset current – close the switch and leave the connections to the DUT open and see if the current in the system is really high. Repeat without the switching system connected. If the offset current with the cards is much higher than without (supposed to be < 1pA offset current added for the card), then it’s probably something wrong with the card. It could be a lifespan issue. The relays have a limited contact life, I believe their resistance increases as they age so that will increase noise. You could try measuring the relay contact potential as well.
For future reference to this thread, the 7002 is a retired product and will be moved to the " Vintage and Retired Models" category.
 

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.

Since you have a number of new changes in behavior, I wonder if one of the power supplies in the 7002 is having some trouble.
The instruction manual on Tek.com for the 7002 has a section on maintenance and running diagnostics.

On the DB9 connector for the digital IO, a 5V supply is available (pin 2).  Look at that on a scope.  Is it steady DC average value?

Our service dept will perform functional check and repair of 7002 still.
https://www.tek.com/en/service-quote