I swapped the small PCB with 4 populated EPROMS (it is locked by a small aluminum angle plate that is bolted on from the instrument's back side).
I did not swap one of the 3 RF modules (just released them, blow the dust off and reseated them).
I have no history of the donor/reference but at least it seem to work (output signal comes out) without error messages.
I checked all PS control LEDs (+5V, +5V(F1), +38V, 12V, +12V): all LEDs are on.
I checked PS output voltages at connector J6 (against metal chassis):
Pin1: +5,22V (connected to pin 2)
Pin2: +5,22V
Pin3: 0V (chassis GND)
Pin4: 0V (chassis GND)
Pin5: +14,97V
Pin6: 0V (chassis GND)
Pin7: -14,95V
Pin8: +37,66V
(doesn't look too bad)
I found another point the defective unit behaves different to my reference unit:
After turning the instruments on while pressing "memory DEL" key to perform a clear memory, the GPIB adress is not reset to 19 an no "ERROR 627: battery RAM failure, memory lost" (as the manual states to be normal, and what is confirmed by my reference unit).
What the hell is going on here?
(Edit: I moved part of text of my last post to this post because I added after you wrote your reply)