Hello Folks,
Just in case a Google searcher has run into the same issues, and some most likely will: I finally found what caused the HP4191A impedance analyzer to become flaky on the calibration values and interpolation.
Found a busted transistor and a couple other things. Still wouldn't work correctly. It wasn't noise, checked it out from A to Z. It was the EPROMs causing the controller to rely on wrong numbers. It seems that the checksum feature (if there is one) didn't work well because it passed all self-tests. The EPROMs are top notch ceramic packages but the sockets they sit in are the cheesy kind, basically consisting of bent metal tongues. So I took all EPROMs out, cleaned the sockets, straightened the pins and inserted them again. Did the same with the uC and two other socketed chips since they reside in the same cheap socket type. All this stuff is inside the fold-out top module. While I didn't like some of the circuitry I found this old HP gear sure is service-friendly.
Bingo, works like a champ again. Sometimes it helps if an analog guy picks up the logic analyzer.
Oh, and while at it check the NiCd batteries on the little board next to the controller. If they are old it might be good to change them.
While waiting for a few cal routines to trundle through I also found out what caused a Minolta film camera not to work anymore. But more on that later when I have the parts and I know that it's running again.
Regards, Joerg