On Sun, 28 Feb 2016 18:55:47 +0100, Dimitrij Klingbeil wrote:

[...]
I think that the most realistic test would be to sweep the resonant
circuit with a signal generator and watch the waveform. If the resonance
frequency looks right (in the 20 kHz ballpark) and a signal generator is
able to drive it from a high 600 Ohm source impedance to a significant
amplitude without much "sagging" (that is, the resonant circuit presents
little load to the generator), it's probably OK.

Thanks again, Dimitrij. You're obviously an expert on the little
understood world of resonant converters so when you say try this or that,
I make a point of paying extra attention. I liked your theory on the
resistor heating due to this supply running out of resonance as a result
of component values changing over time; in fact I'm currently pinning my
hopes on it. It's a pity I'm stuck here for a few more days with my
revolting in-laws but it'll be the first thing I do on my return!

Somewhere I have a big old valve/tube capacitor tester capable of
simulating realistic high voltage working conditions. It'd be interesting
to know what kind of checks it's capable of performing if it's still in
working order and if I can find it among the towering piles of obsolete
test equipment I have here (a couple of million pounds worth of gear at
new prices adjusted for inflation) I may possibly hook it up and give it
a shot.

How about those 'Octopus' component testers? They subject the part under
examination to sweeping test voltages over the expected working range and
you look for any signs of breakdown on an oscilloscope in X=Y mode. I
guess this method is about as good as it gets?