On Sun, 06 Mar 2016 23:34:55 +0100, Dimitrij Klingbeil wrote:

If I understood the service manual correctly, they seem to suggest to
start from the lowest frequency when performing an adjustment and going
up until output regulation is reached. (They write from "fully
counter-clockwise" actually, referring to the "FREQ" trimmer, and
looking at the schematic that would likely be from the "lowest" position
of the wiper, meaning starting at the highest resistance and going
towards lower resistance.)

OK, well I can easily establish that safely by popping V1812 out of
circuit temporarily so sweeping the frequency adjustment pot won't have
any effect.

It's only my guess, but I think that they intended this supply to run
rather somewhere below resonance than somewhere above. This would mean
that adjusting the pulse frequency down to 17.35 kHz should do no harm
as that value would be lower than the setting right now.

If there was any danger of something blowing up by setting the frequency
lower, they would not be recommending to set it to the absolute minimum
before slowly adjusting it back "up" again.

Indeed. That seems to be the key point I have to observe.

Can you adjust the pulse rate to 17.35 kHz and then test the supply with
a dummy load?

Well I could.... But that's spot on resonance. I was under the impression
that they're not supposed to run actually directly at resonance?

Can you test it with a variac and see if it still maintains output in
regulation down to 175 V "mains" (adjusted to 17.35 kHz, that is)?

Yes, no problem. It seems the key regulated output is the 12.7V one and
if that's correct, the rest should follow. There's a trimmer for 12.7V on
the underside of the board.

Thanks again, Dimitrij. I'll report back tomorrow...