Always good to do a proper test of equipment after repair – especially, after a power supply repair – see recent post 8663A pwr supply repair. In most cases, all will be fine, but this time, another failure followed the initial repair: the -10 V rail showed an error, only providing about -8 V to the instrument, not enough, to keep it going. An issue with the A7A1 assembly, linear regulator board, hp part 08662-60157 (the HP 8662A uses an identical assembly).
After some checks it was clear that the final pass transistor Q2 was OK, and that there was no current limit issue (by checking the voltage drop over the sense resistor, R36).
So, I guessed, something must be wrong with U2, the actual regulator. This is a 1826-0016, alias LM104H. Not quite a common part, at least, I didn’t have a spare on hand. Found some new old stock online, about USD 3 per piece, well, not too bad. It arrived a few days later, but, the exchange of U2 was to no avail – still no regulation.
Well, I should have done a proper check earlier – turns out, the transistor Q7 (2N2904A) didn’t provide enough current for the final stage, despite being fully driven by U2. This time, I had more luck and found a 2N2904A in my parts collection (datecode: 7050 – 46 years old – but still working, hfe=170).
Some final test with a 25 Ohms power resistor to test under load, before risking any damage to the venerable 8663A. And, long story short, all is good now.