On Fri, 19 Feb 2016 22:44:08 +0100, Dimitrij Klingbeil wrote:

But once you have it out and disconnected, please make another test:
apply ca. 15V RMS to the 12.7V winding (to the one where you measured
3V) instead of to the primary. And check if any isolation looks like
breaking down. Note that the 15V value contains some compensation for
the fact that the power supply uses inductors after the rectifiers (and
therefore the normal winding voltage is higher than the normal output
voltage). That would load the transformer close to its normal condition
and any breakdown should become apparent.

I did just try this a moment ago, Dimitrij, but doing this just flattens
the output from the sig gen, I'm sorry to say. Hardly surprising since
it's a 600ohm unit and the 12.7V tappings are 0.52ohms 'apart'! To
perform this test properly I'd have to adopt the work-around suggested by
another chap here who said use an audio amp to get the current up. I may
well have to do this if it comes to it. The other problem is, my
oscilloscope current probe is lacking a termination unit so it's readings
will be meaningless and I can't use the true RMS current range on my DVM
because it's probably going to be out of its bandwidth at this frequency
range.