On Wed, 24 Feb 2016 04:26:28 -0800, John-Del wrote:

Even today on a slow day, I'll spend a lot more time on something that
isn't economically worth the effort just to solve the puzzle. Even us
old grizzled veterans aren't immune to such things.

Well I'm pretty old and grizzled myself! Just getting stuck back into
troubleshooting again after a 30yr. lay-off. So much has changed!
Anyway, the plan was to replace one suspect part after another one at a
time and test in between each replacement so as to identify the specific
part which is at fault (the new caps arrived today, btw). However, I only
got as far as replacing that diode (the by134) with Dimitrij's suggested
4007 and *something* has *definitely* changed.
The 20ohm power resistor is warming up *much* more slowly and the hissing
noise has gone. The limiting factor now is not the 20ohm resistor, but
the improvised dummy load (a 30ohm 40W w/w resistor) which gets too hot
long before the 20ohm circuit-board part. In fact even with the dummy
load disconnected, the 20ohm resistor doesn't get hot in a hurry like it
did before. So like I say, *something* has changed and that something can
only be the diode swap. Can't believe it would make that much difference,
surely?
More testing as soon as I find a better DL...