Hi AArnaud,
?
Congratulations for the successful repair, and for having saved another valuable HP instrument from the scrap heap !
My conclusion is somewhat identical to my initial thoughts : start by checking the "usual suspects", supply voltages and clocks.
The obvious next step for your 3403C is a complete performance check and recalibration, if you have the appropriate equipment. If you're lucky, first adjusting the calibration in DC-only voltage mode may bring the instrument back into full compliance, which of course would be great news.
After that it'll be time for another instrument repair project, and for more fun !
Cheers,
Joe;
