Update:
I double-checked every connection and found merely by accident, that the 3.5V-rail of my PSU-assembly wasn't stable at all times. Turned out that the ribbon-cable socket had loosened up just enough so that the 3.5V-line was slightly wiggling.
After properly reseating the cable everything looks much better
.
So despite not being mentioned in the service manual at all, on a 54502A all these symptoms are directly related to the 3,5V-rail:
-> failed interpolator cal
-> static faulty offset of scope-trace
-> AD/DA-sections fail in self-test
-> Analog Trigger fail in self-test
-> Acquisition-RAM fail in self-test
-> Time-Base fail in self-test
Just ran calibration and self-test once again and all turned out well!
So for the time being I consider this case closed...
Thanks a lot wn1fju & Smith for your support and sticking around!
Best regards,
vanhouten