On Tue, 23 Feb 2016 15:40:12 -0800, John-Del wrote:

I haven't read all the posts, but way back when I suggested pulling
every cap and checking for value and ESR *out* of circuit. Have you
done that?

No. Normally that would be one of the first things I'd do, but the traces
on this board are old and brittle, so I'm avoiding upsetting them until
I've exhausted other possibilities (drawing ever closer now). The few I
am replacing this week are clearly in sub-prime condition from visual
inspection alone. I'm suspicious of these (metalized polyester types)
more than the "usual suspects" electrolytics in this particular case.

On Tue, 23 Feb 2016 18:43:40 -0000 (UTC), Cursitor Doom
wrote:

On Mon, 22 Feb 2016 08:49:11 -0500, legg wrote:

If it's slow, it looks like a short when the power transistor is trying
to turn on, stressing the current snubber around L1804.

I don't think this diode is the culprit, TBH. Just out of curiosity I
hooked it up and tested it this afternoon. The faster diodes turned up so
I thought it might be instructive to compare them. The main flaw in my
test is that I'm unable to replicate actual working conditions. I just
hooked up each diode in series with a 1k resistor and fed the arrangement
from my 600ohm sig gen using 10VAC p-p. Slow recovery was certainly
visible on the scope with the BY134, but it wasn't *that* bad. In fact it
was still able to function as a viable rectifier right up to nearly
600kHz. There were no signs of slow recovery with the UF4007 of course,
but the difference at 20kHz, whilst still noticeable, is unlikely to be
causing the issues I've experienced.
But as I say, it was in no way a scientific test and only when the new
diode is in circuit will we know for sure. I won't be holding my breath!

If you've got UF4007s, then what are you waiting for? Polyester film
caps in low voltage circuitry are the last things to suspect. Their
perfomance is most easily assessed in the working unit.

After making whatever node tests are made convenient by the
transformer's absence, stop screwing around and reassemble the unit.

No benefit is obtained by running the unit unloaded unless the loaded
outputs produce non-typical loading effects, as measured on the
transformer output windings and rectified outputs.

Unstable waveforms will produce the same voltage ratios as a steady
signal. The present switching circuitry is an excellent signal
generator for the application, having survived all insults so far.

RL

On Tue, 23 Feb 2016 22:46:21 -0500, legg wrote:

If you've got UF4007s, then what are you waiting for? Polyester film
caps in low voltage circuitry are the last things to suspect.

Even if there are bits flaking off them?? That's the case here!

Obviously a professional technician just wants to get each unit fixed as
soon as possible so as to get on to the next one and maximise his income.
But I'm just a hobbyist and my motivations are not at all the same. Of
course I'd like to get this up and running, but if I don't *learn*
something from the experience, then it'll be next to worthless AFAIC. So
you might see it as screwing around to run these side-by-side diode tests
from your perspective, but I really don't. This unit is beyond economic
repair, but I'm still working on it - for a little while longer anyway -
whereas a professional service person could not afford the time on what
he would see as a basket-case.

On Wednesday, February 24, 2016 at 5:22:37 AM UTC-5, Cursitor Doom wrote:


Obviously a professional technician just wants to get each unit fixed as
soon as possible so as to get on to the next one and maximise his income.
But I'm just a hobbyist and my motivations are not at all the same.

Well, that's how I look at it. But I do remember back when I was a teenager working on what was then new technology (transistorized TVs) and the boss trying to get me to check the "Goldenrods" (RCAs service bulletins back then). I didn't want to because I wanted to track the problem down myself.

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.

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...

Decided to use the scope itself as the 'dummy load' by plugging the psu
back into it. It now takes 1m 25s for the power resistor to reach 50'C
whereas previously it was just under 15s., so an unmistakable
improvement.
Does anyone know what temp I should expect this resistor to run at, BTW?
I mean if they're good for 70'C I could leave it powered up longer and
see if it tops out before reaching that.

On Wed, 24 Feb 2016 17:22:13 -0000 (UTC), Cursitor Doom
wrote:

Decided to use the scope itself as the 'dummy load' by plugging the psu
back into it. It now takes 1m 25s for the power resistor to reach 50'C
whereas previously it was just under 15s., so an unmistakable
improvement.
Does anyone know what temp I should expect this resistor to run at, BTW?
I mean if they're good for 70'C I could leave it powered up longer and
see if it tops out before reaching that.

If these are the maroon-colored parts, they are Philips flame-proof
parts designed to run with body surface temperatures in excess of
175C.

The long preformed leads are thin dia steel, with poor thermal
conductivity, in order to reduce thermal conduction to the printed
wiring.

Your real concern should be the temperature of film caps and
insulators in the immediate viscinity, which have a lower tolerance to
overtemperatures. They should not touch.

RL

It's been many years since I worked on power supplies that had large wattage resistors in it, but I do remember some 10 watters running hot enough to sizzle water or spit off them, and that's when they were running normally. That would put it over 100C I guess.

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

On Wednesday, February 24, 2016 at 5:22:37 AM UTC-5, Cursitor Doom wrote:


Obviously a professional technician just wants to get each unit fixed as
soon as possible so as to get on to the next one and maximise his income.
But I'm just a hobbyist and my motivations are not at all the same.

Well, that's how I look at it. But I do remember back when I was a teenager working on what was then new technology (transistorized TVs) and the boss trying to get me to check the "Goldenrods" (RCAs service bulletins back then). I didn't want to because I wanted to track the problem down myself.

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.

It's important to remember what hat you're wearing, when you're
performing specific tasks.

If this guy had originally stated that he had a Philips scope and that
he just wanted to sniff it's perfume, I would never have bothered
responding.

RL

On 24.02.2016 00:23, Cursitor Doom wrote:
On Wed, 24 Feb 2016 00:04:13 +0100, Dimitrij Klingbeil wrote:

On 23.02.2016 19:43, Cursitor Doom wrote:
On Mon, 22 Feb 2016 08:49:11 -0500, legg wrote:

If it's slow, it looks like a short when the power transistor
is trying to turn on, stressing the current snubber around
L1804.

I don't think this diode is the culprit, TBH. Just out of
curiosity I hooked it up and tested it this afternoon. The faster
diodes turned up so I thought it might be instructive to compare
them. The main flaw in my test is that I'm unable to replicate
actual working conditions. I just hooked up each diode in series
with a 1k resistor and fed the arrangement from my 600ohm sig gen
using 10VAC p-p. Slow recovery was certainly visible on the scope
with the BY134, but it wasn't *that* bad. In fact it was still
able to function as a viable rectifier right up to nearly 600kHz.
There were no signs of slow recovery with the UF4007 of course,
but the difference at 20kHz, whilst still noticeable, is unlikely
to be causing the issues I've experienced. But as I say, it was
in no way a scientific test and only when the new diode is in
circuit will we know for sure. I won't be holding my breath!

Well, I don't think that it's the main culprit either. But it may
impair the working of the energy recovery circuit far enough to
make it inefficient, forcing it to dump too much power into the
resistor. If everything else was well, that might still have worked
to some extent.

But you're trying to troubleshoot it, and something is obviously
wrong that causes the resonant circuit to appear as too low
impedance. Either the transformer is broken or the output circuits
(rectifiers) or the whole thing is operated on wrong frequency too
far out of resonance.

If the energy recovery circuit was working well, it should be able
to protect the resonant circuit, even at some overload, by
diverting the energy back into the main capacitor. That would allow
you more time to "probe around", checking what is the cause of the
overload.

Slow diodes usually become worse with rising currents, so one that
is able to drive an 1k resistor from a signal generator may just as
well behave like an RF short circuit if one tries to push
significant amps through it. So it's really difficult to compare.

Anyway, while I don't thing that it's enough, I was hoping that
making that part work efficiently again would at least lower the
load on the resistor to some extent, and give you more time for
such more complex things like resonance frequency measurements or
even adjustments.

Also, as for testing the transformer (out-of-circuit, with a poor
man's IWT equivalent), see my other post.

Regards Dimitrij

Many thanks as ever for your thoughts, Dimitrij. One question on
your other post before I forget: your schematic shows pulsing the
transformer input at 100Hz, so we're just testing the primary winding
in this instance, right? We're not concerned in this test about
what's coming out of the secondaries? I assume so because 100Hz is so
far off its intended frequency range but would be grateful if you'd
confirm if I have this right.

I fully agree with your observations on my diode test's
shortcomings.

The only other thing I'm waiting for is some replacement caps for
the original tropical fish types that don't look very healthy. They
test okay at low voltage but may be misbehaving badly at closer to
their working conditions. They're in really poor shape visually and I
could certainly believe THEY might be responsible for the issues I've
had. They should be here tomorrow or Thursday so by the end of this
week, I should have some firm results one way or the other.

As for the transformer test: This is actually quite a generic test that
is often used to test inter-turn winding isolation under high voltage
conditions by the manufacturers of motors, inductors and transformers.
Normally they use a specialized piece of equipment called an IWT, and
the test is routinely performed in production. Unfortunately an IWT is
expensive (like $2500 and up), and rather specialized, so the typical
repairman won't have access to one unless he works at a place where they
are commonly used.

The test frequency actually doesn't matter, and most common IWTs won't
go all that high. In the past, when IWTs still had a tube screen, they
needed a steady repetition rate in order to display the trace. So 50
Hz (or whatever your country's line frequency is) was not unusual.
Nowadays they all have flat screens and lots of sample memory, so the
repeat rates are usually from "single shot" to maybe a dozen a second.

Since you've told us in the past that you have a CRT oscilloscope (you
posted a picture of a noisy signal on a switching transistor that was
shown on such an instrument), a test circuit should have some impulse
rate that is reasonably fast that you'll be able to see a steady trace
on the screen. That's where my 100 Hz came from. You can obviously go
lower, the circuit has no lowest limit, but because of the resistor in
the charging circuit it won't likely be able to go much faster. 10k is
already a low resistance for 300-something volts and 10W is also quite
considerable, so making it faster would mean making it beefy and power
hungry too, and these side effects would outweigh the benefits.

For the actual test, one single impulse would theoretically be enough.

In practice however you'll want a repeating pulse train for 2 reasons.
First, in order to see it (unless you also have a digital scope to
capture a single pulse), and second, in order to see the state of the
isolation properly (usually broken isolation will arc in some sort of
semi-irregular fashion and that may not be visible with only one try).

The test is done in such a way that the coil (under test) and the
resonance capacitor (inside the IWT) are connected together while at the
same time a very fast charging circuit "charges" this LC resonant
circuit to a preset voltage and then immediately disconnects itself. The
LC tank is then allowed to "ring down" naturally without outside
interference and the ringing waveform is observed.

The frequency of the ring wave is determined by L and C, the duration by
the coil's resistive losses. A defective coil (shorted or with arcing
isolation) will have a very low "Q", so there will be basically no ring
wave, just a fast decay from the charging peak down to zero.

In your case, you can use 15 nF for the cap, so that it will match the
same conditions like in the actual power supply. This means that the
ring wave will have not only the full starting voltage but also the
actual "correct" resonance frequency. With these conditions you can of
course take wave shape measurements with an oscilloscope on any output
of the transformer, not just on the primary. They all should show the
same shape and the voltages should all be realistic (but of course
brief, since each pulse doesn't last very long). The cap and charging
circuit should of course be connected only to the primary, to make for
realistic test conditions.

With enough pulses per second you should get a bright steady trace.

The 10 W resistor should allow for continuous duty operation, so you can
take your time looking at the scope. Lower wattages will also do, but
will need limited test duration with cool-down periods for the resistor.

Regards
Dimitrij

On Wed, 24 Feb 2016 11:35:48 -0800 (PST), John-Del
wrote:

It's been many years since I worked on power supplies that had large wattage resistors in it, but I do remember some 10 watters running hot enough to sizzle water or spit off them, and that's when they were running normally. That would put it over 100C I guess.

Book hot spot limits for Philips PR01, PR02 and PR03 is between 220
and 250C, depending on the series. This is typical for later metal
glaze films. Book derating for normal use is linear, to zero watts at
150C ambient.

RL

On Wed, 24 Feb 2016 21:15:08 +0100, Dimitrij Klingbeil wrote:

[...]
For the actual test, one single impulse would theoretically be enough.

Ah! I kind of suspected it would be, hence my query...

In practice however you'll want a repeating pulse train for 2 reasons.
First, in order to see it (unless you also have a digital scope to
capture a single pulse), and second, in order to see the state of the
isolation properly (usually broken isolation will arc in some sort of
semi-irregular fashion and that may not be visible with only one try).

OK, I follow that...

With enough pulses per second you should get a bright steady trace.

[...]

The 10 W resistor should allow for continuous duty operation, so you can
take your time looking at the scope. Lower wattages will also do, but
will need limited test duration with cool-down periods for the resistor.

Thanks for the details, Dimitrij. I think I can cover for all of that
without any trouble. You do explain things with remarkable clarity I must
say. It may yet not be necessary if my component replacements succeed,
but it's good to already have the steps to follow should they fail.

On Wed, 24 Feb 2016 21:15:08 +0100, Dimitrij Klingbeil wrote:

[...]

Oh, Dimitrij, I meant to say your diode replacement has made a bigger
improvement to the psu than we expected. The hissing noise has almost
gone and the power resistor now takes almost a minute and a half to reach
50'C instead of less than 15 seconds using the old incorrect BY134 diode.
I now have sufficient time to do some probing around under mains power!
First up I plan to test the rectified outputs from the long secondary
winding to see if they are anywhere near the 6V-60V range they should be.
I'll report back with the results tomorrow.
Many thanks for that!

On 24.02.2016 22:37, Cursitor Doom wrote:
On Wed, 24 Feb 2016 21:15:08 +0100, Dimitrij Klingbeil wrote:

[...]

Oh, Dimitrij, I meant to say your diode replacement has made a bigger
improvement to the psu than we expected. The hissing noise has almost
gone and the power resistor now takes almost a minute and a half to
reach 50'C instead of less than 15 seconds using the old incorrect
BY134 diode. I now have sufficient time to do some probing around
under mains power! First up I plan to test the rectified outputs from
the long secondary winding to see if they are anywhere near the
6V-60V range they should be. I'll report back with the results
tomorrow. Many thanks for that!

Wow, that's quite something! I wonder how this thing ever worked in its
previous state. Given the change you describe, that square-to-sine wave
circuit must have been as "good" as completely non-operational.

Now that it looks like it's "almost working", the supply might even run
again in the scope (to some extent at least), but the fact that the
resistor still slowly heats up a little, may indicate that it's slightly
out of resonance now. I mean, the frequency is not completely wrong, but
it might be just somewhat off-center.

No exact idea yet, but I think I'm beginning to see a pattern:

Here's my guess, not sure wild or not, so take it with a grain of salt
and the usual precautions of a power supply repairman

It looks like the thing may have drifted a little bit out of resonance
over the years. That can happen, electronic parts age and tolerances
slowly increase. Running out-of-resonance, the power factor of the
resonant circuit was probably no longer close to one, but instead the
resonant circuit began to pull reactive power. If you try to drive an LC
circuit with a frequency that is slightly wrong, the driving source will
still force the LC into its frequency, but there will be a phase shift
between voltage and current. The further off the frequency, the larger
the phase shift will become. A phase shift means that a load is no
longer purely resistive, but also reactive (either capacitive or
inductive depending on direction) and so the power factor gets lower. As
the power factor gets lower, the total current draw increases (imagine a
constant current due to resistive load plus an additional current due to
the reactive part of the load, which increases).

Now my guess, what may have happened:

The thing drifted over the years, and the total RMS current was slowly
increasing because the frequency wandered away from resonance and the
power factor of the resonant circuit was going down.

With the RMS current becoming larger, the load on the diode also became
larger (it depends on the total RMS current of the LC circuit, no matter
whether that's resistive or reactive), and the diode heated up more.

Sooner or later, after many years, the diode finally overheated and
shorted out, immediately blowing the fuse. Somebody saw this and
replaced it. But he did not know, with what speed grade to replace it
properly, so he put in a particularly slow one without thinking.

Now with the slow diode in place, it no longer blew the fuse, but
instead the energy recovery circuit became barely operational.

It still ran for a while, but without the square-to-sine conversion, it
was driving the (now no longer really resonant) main transformer with a
rather square-ish looking waveform.

That waveform caused large current spikes in the resonant capacitors
(you know, capacitors don't like being driven with a square wave, charge
current peaks go through the roof if one tries to do that).

These peaks, plus possibly the low power factor and reactive current
(the frequency may or may not have been re-adjusted after the diode
repair) were now stressing the "new" diode (that was wrong anyway) and
also the resonance capacitors. The capacitors did not like this
additional stress (and they may have drifted over the years already).

When you stress film capacitors with large repetitive current spikes, it
slowly erodes and embrittles the foil electrodes inside. The capacitor
still tests OK on an LCR meter and even on an ESR meter, it may still
look like working, but under full load conditions it can no longer
sustain large currents. It becomes like as if someone has put an inrush
current limiting device on it, it can no longer supply peak loads
(people who repair photoflash units often find this fault in the HV
trigger capacitor, it tests with a correct capacitance, but can no
longer supply a strong current pulse for triggering).

Now there were probably two "processes" going on, accelerating each
other. The resonant capacitors (C1807, C1808) were degrading and letting
the frequency drift ever more out of resonance. The wrong diode degraded
too. When you try to feed a slow diode with large high frequency current
peaks, it can also degrade even more and become even slower and more
like a high-frequency short circuit. Both things probably started
slowly, but were accelerating each other until something really broke.

Now you've replaced the diode, so it should be OK again from the diode
point of view. But the resonant capacitors may have degraded and may now
be in a pitiable state. They are difficult to test because the problem
usually means good LCR meter readings, but much reduced power handling
capability only when running at full power.

My advice would be to replace them anyway. This sort of degradation is
difficult to test, so better safe now than sorry later.

With new capacitors, the resonance frequency will somewhat change (you
know, component tolerances, degradation of old ones, slightly different
values of new ones...). The change won't be drastic, but it may be
significant. Therefore I would advise you to measure the new resonant
frequency and then readjust the power supply's working frequency if it
happens to be different (R1827 is the FREQ trimmer).

To measure, you sweep the primary winding with a signal generator (with
the transformer in circuit and connected, but the transistor V1806
disconnected and no loads attached to the outputs). Look for maximum
amplitude with a scope and measure the frequency with a counter. Compare
the measured value with the one that the circuit runs at (fully
reassembled, with dummy load attached to avoid "light-load" mode).

If they deviate, VERY SLOWLY and VERY CAREFULLY readjust R1827*. The
service manual says how to do it. See chapter 3.4.4.2.1

"This potentiometer is a factory adjustment control. THE SETTING OF THIS
POTENTIOMETER MUST NOT BE DISTURBED UNLESS IT IS ABSOLUTELY IMPOSSIBLE
TO SET THE 12.7 V WITH THE AID OF POTENTIOMETER R1826* (FEEDBACK).
Adjusting procedu
- Set the main input voltage to 220 V.
- Turn R1827* (FREQ) fully anti-clockwise.
- Check that the voltage on the positive pole of C1831* is 12.7 V +/-
100 mV; if necessary; readjust potentiometer R1826* (FEEDBACK).
- Set the main input voltage to 170 V.
- Check that the voltage on the positive pole of C1831* is 12.7 V +/-
100 mV; if necessary; readjust potentiometer R1827* (FREQ)."

If you've had to readjust "FREQ", better re-test with 220 V afterwards,
when you are finished, and double-check the "FEEDBACK" setting again.
Readjust "FEEDBACK" again if there is a voltage mismatch on 12.7 V.

Note that in the service manual that you linked to, the part names are
different, like C1843 instead of C1831 on the schematic. But the
descriptions are reasonable and the meaning seems to be the same. I've
changed the text a little and written the schematic numbers instead.
I've marked the changed item numbers with an "*" and written the
descriptive names (FREQ and FEEDBACK) next to them.

Regards
Dimitrij

Cursitor Doom wrote:

Decided to use the scope itself as the 'dummy load' by plugging the psu
back into it. It now takes 1m 25s for the power resistor to reach 50'C
whereas previously it was just under 15s., so an unmistakable
improvement.
Does anyone know what temp I should expect this resistor to run at, BTW?
I mean if they're good for 70'C I could leave it powered up longer and
see if it tops out before reaching that.
Lots of stuff in commercial gear runs at 70 C or even hotter. I don't like
to see parts running that hot. Especially in something that might get
buried in a shield housing deep in the bowels of some piece of gear like a
scope. But, 70C is not insanely hot for a power resistor. Of course, I
have NO IDEA how hot it is actually supposed to get.


So, does the scope actually run correctly? That would probably indicate the
transformer is fine, and maybe there is some load somewhere in the scope
that is excessive, maybe a bad electrolytic? I've got a B&K scope here that
blows a $13 power module after 15 minutes or so, and I've gotten tired of
fixing it. Since every time the module popped, the interval got shorter,
I'm strongly suspecting a bad electrolytic, but a quick visual inspection
does not show anything obvious. I've long since replaced it with a Tek
scope, so I'm just going to take it to the surplus shop.

Jon

On Wed, 24 Feb 2016 17:51:21 -0600, Jon Elson wrote:

Lots of stuff in commercial gear runs at 70 C or even hotter. I don't
like to see parts running that hot. Especially in something that might
get buried in a shield housing deep in the bowels of some piece of gear
like a scope.

I couldn't agree more. Coming from the germanium semiconductor generation
where even slightly too much heat was terminal, I still like to go by the
rule of burnt thumb: if if it burns your thumb it's too hot. In which
case derate, derate, derate.

So, does the scope actually run correctly?

I didn't get the chance to find out! Began this morning trying to get
some voltage readings off the psu outputs and there was nothing there to
read. To cut a long story short, further investigation reveals something
has gone short-circuit on one of the signal boards. When the psu is
removed and run from my make-shift dummy load, it's still 'fine' with its
new diode (not quite right, but functioning to high degree). So clearly I
jumped the gun slotting it back in the scope when it still wasn't 100%
and now it's damaged something - typical!
I'm running out of time now as we have to leave later to spend a few days
with 'er mother 300 miles away and whilst I shall still have internet
access there, I'm not allowed to take any test gear with me. Ain't life
great?

On Thu, 25 Feb 2016 12:53:04 -0000 (UTC), Cursitor Doom
wrote:

snip
I didn't get the chance to find out! Began this morning trying to get
some voltage readings off the psu outputs and there was nothing there to
read. To cut a long story short, further investigation reveals something
has gone short-circuit on one of the signal boards. When the psu is
removed and run from my make-shift dummy load, it's still 'fine' with its
new diode (not quite right, but functioning to high degree). So clearly I
jumped the gun slotting it back in the scope when it still wasn't 100%
and now it's damaged something - typical!

Sounds more like you're getting closer to root cause.

Troubleshoot the (unidentified?) signal board.

RL

In article , says...

It looks like the thing may have drifted a little bit out of resonance
over the years. That can happen, electronic parts age and tolerances
slowly increase. Running out-of-resonance, the power factor of the
resonant circuit was probably no longer close to one, but instead the
resonant circuit began to pull reactive power. If you try to drive an LC
circuit with a frequency that is slightly wrong, the driving source will
still force the LC into its frequency, but there will be a phase shift
between voltage and current. The further off the frequency, the larger
the phase shift will become. A phase shift means that a load is no
longer purely resistive, but also reactive (either capacitive or
inductive depending on direction) and so the power factor gets lower. As
the power factor gets lower, the total current draw increases (imagine a
constant current due to resistive load plus an additional current due to
the reactive part of the load, which increases).

Your wonderful description of resonant circuits reminds me of an
experience I had as an apprentice. I was given the job of trying to lay
down a silicone insulating film generated by polymerising a silicone
vapour in a high voltage AC plasma. (I don't remember why it had to be
AC.)

We didn't have any special HV AC supplies but stores did have a signal
generator and powerful audio amplifier. I managed to scrounge a large
open-centred coil (meant to generate a magnetic field around a bell-jar)
and a collection of high voltage capacitors, waxed paper in steel cans
with ceramic terminals on top.

With these I built a series LC circuit which generated a satisfactory
plasma. I'm afraid I don't recollect any measurements. Health-and-safety
consisted of large hand-written warning notices!

However despite being distracted by my plasma I also noticed that the
poor capacitors, excellent with DC no doubt, didn't like the AC current
they were subjected to, and bulged, leaked and fizzed. The experiment
was terminated abruptly!

Mike.

On Thu, 25 Feb 2016 14:18:07 +0000, MJC wrote:

We didn't have any special HV AC supplies but stores did have a signal
generator and powerful audio amplifier. I managed to scrounge a large
open-centred coil (meant to generate a magnetic field around a bell-jar)
and a collection of high voltage capacitors, waxed paper in steel cans
with ceramic terminals on top.

I'm guessing you mean like this:

http://www.ebay.co.uk/itm/SIC-SAFCO-...-HIGH-QUALITY-
PAPER-IN-OIL-CAPACITOR-NOS-/151689743255?
hash=item235169cb97:g:N3sAAOSwKrhVXwzP

I still have a couple of dozen of this type here.

On Thu, 25 Feb 2016 09:00:07 -0500, legg wrote:

Sounds more like you're getting closer to root cause.

I'm afraid not. It didn't happen yesterday when I first hooked the psu
back into the scope so this is a fresh fault - and probably my fault for
not testing the psu's output voltages properly before plugging it back
in.

Troubleshoot the (unidentified?) signal board.

May have to wait til the latter part of next week; I have to leave later
today for a 5-6 days due to family-in-law commitments.

Final update for the time being as I have to leave soon now:

That short turned out to be intermittent. I hope it was just due to
something shorting out on the bench that won't happen when the casing is
back on because you all know what a bitch it can be to trace intermittent
faults. Anyway, that fault has now disappeared, so I took some voltage
measurements before the 20W resistor got to hot (from 19'C to 60'C takes
about 1.50s now) and I have:

61.7
12.7
5.8
0
-5.8
-12.7
-62.4

This is with the psu board plugged into the scope and all power
connections made except for the VHT stuff.

The correct figures according to the manual should be:

60
12.7
6
0
-6
-12.7
-60

So very close! Looks like the main transformer may be ok after all.

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.

On Thu, 25 Feb 2016 14:46:50 -0000 (UTC), Cursitor Doom
wrote:

On Thu, 25 Feb 2016 09:00:07 -0500, legg wrote:

Sounds more like you're getting closer to root cause.

I'm afraid not. It didn't happen yesterday when I first hooked the psu
back into the scope so this is a fresh fault - and probably my fault for
not testing the psu's output voltages properly before plugging it back
in.

Troubleshoot the (unidentified?) signal board.

May have to wait til the latter part of next week; I have to leave later
today for a 5-6 days due to family-in-law commitments.


Missing voltages don't necessarily indicate shorts. They can also
indicate open circuit to the source. Review solder joints and
connections around the transformer pins. Possible damage in recent
removal activity.

RL

Are the chopper transistors getting hot ?

Did you actually check the resistance of that resistor that is getting hot ?

On Thu, 25 Feb 2016 23:48:16 -0800, jurb6006 wrote:

Are the chopper transistors getting hot ?

The main chopper is a TO-3 cased BJT with a closely-finned heatsink
bolted to the top of it. By the time the resistor starts to emit a
scorching smell, the chopper hasn't even had the chance to get barely
warm.

Did you actually check the resistance of that resistor that is getting
hot ?

Yes, it's exactly 20 ohms as specified. But please don't ask me to do any
other checks for the next few days as I'm staying over 300 miles away at
present.

OK, when you get back to it, put together a bulb tester. Take the 20 ohm straight out and put a 100 watt lightbulb (incandescent) there.

Then you start disconnecting things.

That is probably the only way to troubleshoot this. You ain't finding anything with the ohmmeter, but it isn't shutting down. Something is not showing up unless under voltage. Ohmmeters can't detect that.

On Fri, 26 Feb 2016 00:47:32 -0800, jurb6006 wrote:

OK, when you get back to it, put together a bulb tester. Take the 20 ohm
straight out and put a 100 watt lightbulb (incandescent) there.

Then you start disconnecting things.

That is probably the only way to troubleshoot this. You ain't finding
anything with the ohmmeter, but it isn't shutting down. Something is not
showing up unless under voltage. Ohmmeters can't detect that.

Hhmmm. As I've said before, I'm reluctant to replace that power resistor
with anything higher rated. At the moment it's acting as a robust
detector that something isn't right. I don't want to replace it and then
find the excess energy has burned out the transformer primary instead!

Dimitrij has already given me some steps to follow for resonance checks
when I get back and I'm trying to keep the suggestions made here in an
orderly queue, so thanks for your input which is appreciated, but no
further test ideas from anyone for the time being, please!!!

"legg" wrote in message
...
On Thu, 25 Feb 2016 12:53:04 -0000 (UTC), Cursitor Doom
wrote:

snip

Sounds more like you're getting closer to root cause.

Troubleshoot the (unidentified?) signal board.

RL



Yes! Once you get it fired up again, start looking for parts getting HOT on
those other boards!

mz

On Thu, 25 Feb 2016 00:36:16 +0100, Dimitrij Klingbeil wrote:

[...]

Dimitrij, I think you may have missed this I posted elsewhere so I'm re-
posting it here now for you personally:

"Final update for the time being as I have to leave soon now:

That short turned out to be intermittent. I hope it was just due to
something shorting out on the bench that won't happen when the casing is
back on because you all know what a bitch it can be to trace intermittent
faults. Anyway, that fault has now disappeared, so I took some voltage
measurements before the 20W resistor got to hot (from 19'C to 60'C takes
about 1.50s now) and I have:

61.7 12.7 5.8 0
-5.8 -12.7 -62.4

This is with the psu board plugged into the scope and all power
connections made except for the VHT stuff.

The correct figures according to the manual should be:

60 12.7 6
0
-6 -12.7 -60

So very close! Looks like the main transformer may be ok after all."

Making progress!

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

On 27.02.2016 01:42, Cursitor Doom wrote:
On Thu, 25 Feb 2016 00:36:16 +0100, Dimitrij Klingbeil wrote:

[...]

Dimitrij, I think you may have missed this I posted elsewhere so I'm
re- posting it here now for you personally:

"Final update for the time being as I have to leave soon now:

That short turned out to be intermittent. I hope it was just due to
something shorting out on the bench that won't happen when the
casing is back on because you all know what a bitch it can be to
trace intermittent faults. Anyway, that fault has now disappeared, so
I took some voltage measurements before the 20W resistor got to hot
(from 19'C to 60'C takes about 1.50s now) and I have:

61.7 12.7 5.8 0 -5.8 -12.7 -62.4

This is with the psu board plugged into the scope and all power
connections made except for the VHT stuff.

The correct figures according to the manual should be:

60 12.7 6 0 -6 -12.7 -60

So very close! Looks like the main transformer may be ok after all."

Making progress!

Hi

Noted your progress

But could you please make a complete list of found faults and your
replacements, and post it he

I mean, you posted at the very beginning (long before finding the slow
diode) that you've found and replaced some obviously defective parts,
but I can't remember if you ever posted, exactly which ones they were.

Also, you have indicated other things that may impair reliability (like
capacitors with pieces of film isolation flaking off), and again, you
didn't seem to indicate the exact schematic part numbers.

As you may well know, to troubleshoot anything properly and reliably,
and to be able to assess the likely chains of cause and effect, one
needs to know the history of the repairs, as completely as possible, and
also anything obviously (visually or otherwise) suspicious too.

Therefore please make some lists, and take particular care to make them
complete, to leave nothing out, and to indicate each and every listed
part's schematic part number (important, since others can't see your
board and need the exact numbers to identify the parts in question).

- one list with all previous repairs that you have found: which parts
were replaced in the past, as visible from manual solder joints, and
where the replacements were of different type from the original, clearly
indicate the exact types of replacements.

- one list with all of your repairs: which parts you found defective and
what exact parts (exact type and manufacturer) you have replaced them with.

- one list with all parts that currently look suspicious or for whatever
reason seem to be of questionable integrity.

It would be nice if you could make a printout of the schematic, and mark
all those items in color (like for example yellow for previous repairs,
circled twice if the repair was inexact, red for those you replaced, and
blue for the suspicious ones), and then scan and post the color-
annotated schematic somewhere for us to see.

To avoid "... and what else was there?" or "... and what about part
XYZ?", please make sure that this annotation is really complete. Trying
to get such information one question at a time can be frustrating.

Regards
Dimitrij

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.

On Sun, 28 Feb 2016 15:33:02 +0100, Dimitrij Klingbeil wrote:

But could you please make a complete list of found faults and your
replacements, and post it he

I mean, you posted at the very beginning (long before finding the slow
diode) that you've found and replaced some obviously defective parts,
but I can't remember if you ever posted, exactly which ones they were.

I think you may possibly be getting mixed up with a different repair
here, Dimitrij. I do have some flaky capacitors to replace when I return
and I'll note which ones I change for your information. As for what
previous technicians may have done, I have no idea what if anything has
been replaced - apart from that one obvious diode. I got absolutely no
background information on this scope, it was given to me for nothing by
some guy who was emigrating so its past will now always remain a mystery.
It's a pity, because this obviously adds another set of unknowns into
troubleshooting the thing, but it's just something I'll have to live with
I guess. In all honesty, this repair is proving to be a 'baptism of fire'
for me in the world of SMPSs of which I admit I know very little (yet a
lot more than I did 3 months ago!)

On 28.02.2016 16:18, Cursitor Doom wrote:
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.

Ok. Usually most people who ask here understand DC parameters well
enough, but rarely get to consider impedance, phase angles and such.

As for the 800 V, that was mostly a guess. Basically I've taken 320 V of
the storage capacitor, added to that another 300 V of the resonant
circuit (when the power transistor is off and it's being swung in the
other direction) plus the voltage rise from the winding reset from the
primary of L1806 (which is actually unknown since I don't know the ratio
between primary and secondary, the secondary being at 320 V), which I
guessed to be somewhere in the 200 V ballpark.

That's 320 V + 300 V + 200 V = 820 V, likely even to be more because the
300 V may reach up to 320 and the 200 is only a guess and may likely end
up higher than that, plus there may be some 50 V from L1804 adding up in
the same polarity, so even a 900 V total won't be out of the question.

That would be consistent with the rating of the BU208 power transistor,
which has a 1500 V absolute maximum collector rating when driven from a
low-impedance base drive signal.

As for definitely testing the resonance caps: I'm somewhat at a loss.

First thing, you can measure the capacitance, that an obvious test. If
the capacitance is wrong, they're can't be working properly.

But reduced current handling ability comes from an increase in ESR and
in the dissipation factor. To measure them, you would need to run the
cap at the intended target frequency (and preferably at a realistic
voltage too).

LCR+ESR meters can measure the dissipation factor and ESR, but those
intended for electrolytics will often measure only ESR and also may have
trouble testing such small foil capacitors like 33 or 15 nF.

Also, I don't know the target numbers for ESR and dissipation here, so
one would need to compare them against a known good pair somehow.

An other way I can think of, would be to run them at resonance with the
transformer, and measure both frequency and "Q". But that's also not
meaningful unless one has a known good reference value for Q.

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.

Dimitrij

On 28.02.2016 16:18, Cursitor Doom wrote:

[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.

Also, even with a dummy load connected, the stray capacitance of an
oscilloscope, when hanging off the loose end of a power circuit with
some 800 to 900 V worth of HF on it, would probably cause so much undue
capacitive loading that the power supply circuitry would hardly handle
it. That may have been the reason why you just got noise (the overload
from the hanging scope may have affected the over-current shutdown of
the power supply controller). As I said, the proper way would be with an
isolated high voltage differential probe (such a probe would present
very little stray parasitics) or maybe with a well matched pair of
(identically compensated) HV probes in subtract mode.

Dimitrij

On 28.02.2016 19:06, Dimitrij Klingbeil wrote:
On 28.02.2016 16:18, Cursitor Doom wrote:

[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.

Also, even with a dummy load connected, the stray capacitance of an
oscilloscope, when hanging off the loose end of a power circuit with
some 800 to 900 V worth of HF on it, would probably cause so much
undue capacitive loading that the power supply circuitry would hardly
handle it.

P.S. That voltage estimate has probably surprised you. Unless one looks
at the circuit schematic and adds all the voltages from all the storage
elements (inductors / capacitors), considering timing and phase, it may
not be obvious that the thing was intended to run at such high voltage
levels. But there's a reason why they used a 1500 V transistor in it.

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?

On Sun, 28 Feb 2016 19:06:38 +0100, Dimitrij Klingbeil wrote:

Also, even with a dummy load connected, the stray capacitance of an
oscilloscope, when hanging off the loose end of a power circuit with
some 800 to 900 V worth of HF on it, would probably cause so much undue
capacitive loading that the power supply circuitry would hardly handle
it.

Isn't this just another example of the unsatisfactory nature of this
resonant converter design? If the thing is *that* fussy that a little bit
of stray capacitance can catastrophically destabilise it, then AFAICS
it's a fundamentally unreliable topology and it would be better to have
used one of the non-resonant forms of converter. Unless there's some
compelling reason I may be unaware of not to for oscilloscope power
supplies, of course.

On Sun, 28 Feb 2016 19:23:20 +0100, Dimitrij Klingbeil wrote:

P.S. That voltage estimate has probably surprised you. Unless one looks
at the circuit schematic and adds all the voltages from all the storage
elements (inductors / capacitors), considering timing and phase, it may
not be obvious that the thing was intended to run at such high voltage
levels. But there's a reason why they used a 1500 V transistor in it.

And yet C1804 is rated at 'only' 630V. Weird!

On 28.02.2016 20:53, Cursitor Doom wrote:
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!

Hi

Please don't rely in my advice too much. While I do design electronics,
I'm very far from being an expert in this particular field. I've never
actually designed a resonant power supply, unless you count one little
3W prototype based on a modified Royer / Baxandall structure.

It may be relatively easy to look at a ready-made schematic and try to
guess various upper and lower limits based on parts and topology (like
"signal X cannot be higher than Y volts, otherwise part Z breaks down"
or "ratio of transformer X cannot be above or below A:B, otherwise the
ratings of part Y would be exceeded"), but that's not expertise by any
stretch of the definition. A lot may be intuition, but that's no
expertise either.


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?

I've had to look up, what an "Octopus component tester" is. Apparently a
transformer with some provisions for routing the voltage and current
signals of the load to an oscilloscope, making a simple AC curve tracer.

I don't think that you'll need one here. It can test for breakdown, but
in your case that's unlikely (the capacitor would be buzzing and arcing
and the supply sure wouldn't work "almost normally"). It won't see the
problems that are likely to be important in an LC circuit.

1. The cap must have the correct capacitance. Any LCR meter or any
common pocket multimeter with a capacitance function can measure this.
This is a basic prerequisite that should always be tested first and if
the capacitance is wrong, no further tests will be necessary anyway.

2. The foils inside the cap must have a reliable connection (deviation
manifests itself as ESR, ESL, and the general inability to supply high
impulse currents). This particular curse will sometimes plague the
trigger capacitors from photoflash units (the flash won't trigger or
will only trigger erratically while the capacitance value is still ok).

This is difficult to measure directly, but can be checked with another
capacitor as a reference. You'll need a known good capacitor with the
same value (in your case: 15 nF), but not necessarily with the same
voltage (you can use a known good, but lower voltage one for testing).
The test is only with a signal generator, so the cap won't be subject to
a lot of stress.

Connect the known good capacitor to the original inductor (transformer
primary) with no other loads attached. Sweep with a signal generator
(use as much voltage as the signal generator can provide without much
distortion, that usually won't be a very high voltage anyway) and look
for resonance on a scope. Note the resonance frequency. Disconnect the
known good cap and connect the original one instead. Check where the
resonance is. If it's in the same place and the amplitude has not become
lower, the cap is very likely good. If it disappears and you can only
measure the inductor's SRF instead, (if the inductor has more or less
the same resonance with or without a capacitor connected), then the
capacitor is basically open-circuit or very high ESR. If the resonance
has wandered away somewhere, especially upwards in frequency, then the
cap is most likely degraded and not a good candidate for full power
resonant use either. Same thing if the amplitude has dropped much.

If your resonant caps turn out to be good, that most likely leaves only
the snubber diodes and a possible frequency misadjustment as the likely
causes.

If you check the resonance with a signal generator and scope against a
known good 15 nF, and it suddenly wanders way, or the amplitude drops,
then you'll need to find replacement capacitors. Fortunately, if you put
"WIMA FKP1 33nF" into ebay search, there seem to be many available.

Regards
Dimitrij