normanator
Experienced Member
Need help fixing a couple of dead Olivetti/AT&T PC6300 power supplies. One is completely dead, with no 5v or 12v output. The other has +5v and +12v, but no -12V.
Focusing on the completely dead one for a moment:
I haven't yet started on the second PSU that's just missing -12v, thinking the completely dead one would reveal a much more obvious issue. But that's not been the case so far. Diagnosing using a schematic is not by strong suit. I'm a trace and replace bad components guy. Any assistance or guidance is greatly appreciated.
Focusing on the completely dead one for a moment:
- I have the (poor resolution) schematic for the PSU. Unfortunately, the portions directly near the 5v/12v outputs are illegible.
- Testing the +5v rail and +12v rail at the output connector shows both shorted to ground both with and without a load attached.
- The PSU is comprised of 2 boards (stage 1 & 2?). I've confirmed on the primary board (the one with AC input) that the big caps after the rectifier bridge are generating 160vDC, which seems appropriate given 120vAC input. Beyond that I'm not seeing anything on the second board.
- I've been probing with a multimeter on the second board for a handful of days now, even removing and testing many components out of circuit (diodes, caps, transistors), but everything tests good so far.
- Just about every component that I know how to test has tested fine.
- Testing the diode leads on the 3-pin power regulators (the ones that look like diodes) shows the diodes are not blown, but I'm not sure how to test the third power pin. Is this only done with the PSU powered up?
- There are 3 transformers: 2 Gammatron and one ordinary transformer. I'm not sure how to test these, but what's the likelihood of a transformers shorting out?
I haven't yet started on the second PSU that's just missing -12v, thinking the completely dead one would reveal a much more obvious issue. But that's not been the case so far. Diagnosing using a schematic is not by strong suit. I'm a trace and replace bad components guy. Any assistance or guidance is greatly appreciated.