On Sun, 28 Feb 2016 03:01:02 +0100, Dimitrij Klingbeil wrote:

1.

There is still the question with V1808. You said it looks ok, and it
tested ok with a multimeter, but that's not really indicative of its
true behavior under full load at high frequencies. If it has degraded
for any reason ("lost its switching speed") then the resistor R1814
would be running at a higher load than normal. Not many times higher,
but about double or triple. That would be somewhat consistent with your
observation of it running too hot after a few minutes. You should now
have (hopefully) a few spare UF4007s, so if in doubt, replace V1808.

Yes, I bought 20 of those faster diodes to be on the safe side.

If you find out that the replacement of V1808 makes a (little) change
for the better (slightly lower load on R1814), then replace V1809 too.
It would in this case be likely that those BY208-1000s have all degraded
and became out-of-spec. They all have the same type and age.

Actually it's possible to test the condition of V1808 in circuit,
without replacing it, but the test is tricky. You would need to see, on
an oscilloscope, the voltage waveform across R1814.

[live power resistor procedure testing snipped]

Actually I did do this a while back without knowing the risks! As you can
see, I survived to tell the tale. All I was seeing was about 30V of noise
across that resistor but that was before I was informed of the importance
of hooking the supply up to a load, so the test was probably invalid.

Ok, so much for the other BY208s in snubber circuits. Replace and see.

Certainly can do that, yes.


2.

The other open question is that of the resonance capacitors (C1807 and
C1808). As I noted in another post, they may be degraded and it may be
difficult to test for this condition properly (LCR meter won't likely
show the problem).

Is there any way of *definitively* testing such a capacitor against all
its possible failure modes? And I'd be interested to know where you get
this figure of 800V you mention from?

A resonant circuit driven too slow (below resonance), will pull reacive
power (will have a power factor below unity), but the direction of the
phase shift will be inductive.

Fortunately this is one aspect I pretty much totally understand. As an
old-style radio ham of more decades than I care to recall, the concepts
of resonance, reactance, impedance, power factor and phase shift are like
second nature so please don't go to any trouble explaining the finer
points in extreme detail; there's absolutely no need. BTW, your
explanations are unusually clear and thorough, I've noticed. If you don't
already, you really should edit or author technical manuals. It's an all-
too rare talent nowadays.