On 25.02.2016 16:51, Cursitor Doom wrote:
On Thu, 25 Feb 2016 00:36:16 +0100, Dimitrij Klingbeil wrote:

[...]

Dimitrij, I will have to run these latest checks you suggest next
week now as I have to leave on family matters and have no choice
other than divorce. The PSU is now putting out near-enough the
correct voltages in the 6-60VDC range as required when connected up
to the scope and the transformer is virtually silent. It's just the
power resistor heating that's causing concern. If you think of
anything else, please leave your thoughts here. If not, I'll proceed
with your checks on my return. many thanks again.

Hi

Since you were planning to be away for a while, I was in no hurry to
reply right away. I've seen your other post too, and obviously the
transformer must be ok.

I think that, from the major power-carrying components point of view,
your power supply is now "almost ok". The "power train" clearly works,
otherwise you couldn't get correct output voltages under load.

But the fact that the power resistor still overheats, hints to some
timing being slightly wrong. It can no longer be "completely" wrong, as
was the case with the slow diode, but it's not yet "right" either.


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.

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. It should be
basically a flat line, with short surge-like spikes at some 20 kHz
intervals. All the pulses must be polarized in one direction only. The
left-hand pin (on the schematic) of R1814 must be positive. There must
be no spikes in the reverse direction. If there are any (the polarity
would be alternating), then V1808 is degraded and no longer operable at
full speed and needs replacement(, and so does V1809 likely as well).

Unfortunately this test is difficult, because you can't connect a scope
ground to R1814! This is a very fast switching signal that runs at high
power and reaches voltages of some 800 V in normal operation! Even if
you disconnect both mains grounds and "float" both the scope AND the
power supply, and even if you power both the scope AND the power supply
from two SEPARATE isolation transformers in order to increase isolation
and minimize the stray capacitance via mains, this test would still be
very dangerous and I would definitely not advise trying. Using two scope
channels in "subtract" mode might work, but only if you have two high
voltage probes rated for 1 kV, and only if both probes are exactly
identical and the compensation of both channels is precisely matched to
each other (a rather unlikely condition that requires some effort to
achieve). To be honest, to do this test properly, you would need an
isolated high-voltage differential probe. Unless you have one, don't
even bother trying, to replace the diode is easier and much safer.

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


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). Again, if you can get known good spares, they can
easily be replaced, but the spares must be rated for resonant operation.
"Typical" film capacitors are not designed for this use.

Foil capacitors with Polypropylene isolation rated for continuous
resonant duty like the "FKP 1" type should work well here, and so may
"MKP 4C" type too, to some extent, but only the 630 V DC rated ones, and
only if two are used in series like in the original schematic ("MKP 4C"
with lower ratings would hit its high frequency AC limits).

So, "FKP 1" rated at 400 V or 630 V DC (two 33 nF in series) or rated at
1000 V DC (one single 15 nF) or "MKP 4C" rated at 630 V DC (two 33 nF in
series), would be feasible replacement candidates, but not many others
due to the high loading requirement in resonant operation.

If yours turn out to be degraded, and you replace the 30 nF originals
(now probably unobtainable) with 33 nF, you may need to re-adjust the
resonance frequency somewhat.


3.

Also, the frequency adjustment may be slightly out of resonance (maybe
the previous repairer has misadjusted it and component parameters can
also drift over the years). Again, a misadjusted frequency, especially
if it has been set too high rather than too low (compared to the true
resonance frequency of the LC circuit) can cause the dissipation
resistor to overheat (so a little low is better than a little high).

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. If driven a too fast (above resonance),
it will appear capacitive instead. Please note that the square-to-sine
conversion circuitry, especially the snubbers, will have lower stress
from peak currents when driving an inductive load than when driving a
capacitive load, so an inductive load is "easier" on them.

Please read the instructions in the service manual (I've also copied the
relevant part in my other post), and also note that the service manual
clearly advises to always adjust the frequency "from below" and never
"from above" ("use 170 V mains, then set the trimmer to lowest possible
frequency, and slowly raise it until the output voltage regulation can
just be obtained, but no more than this"). So the designers from Philips
must have preferred this design to run rather slightly below resonance
than slightly above it, and they must have had good reasons to write the
adjustment instructions in such a way, as to prevent an accidental "too
high" frequency setting.

Note that any frequency adjustment should be done with the correct dummy
load connected in order to avoid entering a "light-load" mode.

Regards
Dimitrij