I have a Tek 7704A with a -15V issue, fortunately it is on the low voltage regulator and not in the switcher part (no 'ticking'). I found two shorted tantalums on the interface board (the most inaccessible location) and replaced them. The impedance went from ~4 mOhm to 40 ohms, so I think
that is OK now.
However, it still is not working. I took a bunch of measurements and nearly everything is off, making it hard to pinpoint the problem.
If anyone sees anything jump out from the picture I'd appreciate it.
But also, I was wondering about this debug strategy:
- disconnect the 470 ohm between -15V and -15V_sense (a safety, I suppose)
- break the -15V sense connection from the interface board
- inject -15V from a bench supply into the feedback node
This should put error amplifier Q3275 in the normal spot, allowing other measurements to
taken and hopefully pinpoint something. Any reason not to do that?