Hi I'm trying to repair an old Keithley 619 multimeter, just wondered if anybody has any idea's
I'm following the cal procedure as per the manual, I can set the 0.00v ok, and I can set the 190mv ref ok and it will sit there around 190mv fluctuates a bit but within spec.
What I'm having problems with is the 1.9v ref, I've reached the end of the trimmer pot adjustment and still can't get the voltage down below 1.9037v (max spec 1.9003) bizarrely it will occasionally, and suddenly, drop to 1.895v (min spec 1.897v) sit there for a little bit and then go back up again.
I am using a DC calibrator to feed stable 190mv and 1.9v into it, and I've verified the voltage with a modern 6 1/2 digit multimeter. I have tried both input channels and get the same problem on both channels, so that rules out a problem with the input cards. The CPU part is common to both 190mv and 1.9v ref so I can only think there's an issue with the Filter/Mux board.
I've checked the obvious resistors and things, I pulled and tested the JFET attached to the 1.9v ref part and it tests ok with an Atlas tester.