Well, I've replaced all the flaking capacitors and still no improvement.
A number of people have been suggesting I remove the two resonant caps
(the 30n ones) from the primary circuit and test them. I didn't have any
expectation that this would achieve anything since they tested good in-
circuit, but as we're running out of ideas now I did remove them this
afternoon and they both tested at 31nF a piece and no signs of any
physical damage. I then subbed a couple of common-or-garden mylars of the
same value in their places and re-swept for changes in resonance. Result
was no change in resonance - but a slightly better Q(!!) Also checked the
two 10Meg resistors whilst I was at it and they were fine, too. So I can
only think of making up Dimitrij's winding tester and looking for signs
of any turns shorting in the main transformer.
I'm coming to the end of the amount of time I'm prepared to spend on this
psu as it stands. I'm more and more tempted to mothball the key parts of
it til next year then rebuild it as a conventional non-res converter to a
fresh design. TBH, I'm not prepared to still be testing this thing after
the end of this week, so if anyone has any last-ditch ideas, now's the
time to toss 'em into the mix. Speak now or forever hold your peace.
Thanks, all.