On 06.03.2016 14:35, Cursitor Doom wrote:
On Sun, 06 Mar 2016 13:26:33 +0000, Cursitor Doom wrote:

Under power, with frequency counter connected between T1 and T2
with V1812 removed from circuit shows the PWM chip pulsing at
22.55kHz.

Sorry, ignore that; copied the wrong piece of paper. It should be
20.64kHz. (This is with the load connected.) I then tried again with
V1812 re-inserted and got 20.62kHz. Apologies for the earlier
error...


Okay, that makes the difference somewhat less. Still higher than 17.35
kHz obviously.

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.)

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.

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

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)?

Regards
Dimitrij