22 Apr
2023
22 Apr
'23
8:28 p.m.
This seems to be because I measure a steady 0.6V on pin 6 of the transformer
(p4, PSU Sheet 3). I just can't imagine where it might be coming from as the
chopper won't be running. I had previously removed the transformer and there
are no shorts between the pin 5-pin 6 winding and any of the other pins on
the transformer. I checked all the DC outputs of the PSU when powering the
7812 from the bench, both on a working PSU and the non-working one. They are
all at zero except the -12V output on the non-working PSU, which is +0.6V.
But the voltage can't come out of nowhere.
Is the -12V output connected to it's load while you are testing? If so,
perhaps there is a path through the load to the the +12V or +15V supplys
available elsewhere which would forward bias the two diodes connected to
pin 6 of the transformer giving you a small positive voltage at that point.
(I would have thought it would be a bit higher though, depending on the type
of diode. Neither of the two diodes is shorted, is it?)