Transformer shot! (was scope SMPS/ capacitor venting)

[Dimitrij Klingbeil]

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

 

[legg]

On Tue, 23 Feb 2016 18:43:40 -0000 (UTC), Cursitor Doom
<curd@notformail.com> 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

 

[Cursitor Doom]

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.

 

[John-Del]

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.

 

[Cursitor Doom]

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...

 

[Cursitor Doom]

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.

 

[John-Del]

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.

 

[Cursitor Doom]

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.

 

[Cursitor Doom]

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!

 

[legg]

On Wed, 24 Feb 2016 17:22:13 -0000 (UTC), Cursitor Doom
<curd@notformail.com> 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

 

[legg]

On Wed, 24 Feb 2016 04:26:28 -0800 (PST), John-Del <ohger1s@aol.com>
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

 

[Dimitrij Klingbeil]

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

 

[legg]

On Wed, 24 Feb 2016 11:35:48 -0800 (PST), John-Del <ohger1s@aol.com>
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

 

[Dimitrij Klingbeil]

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 procedure:
- 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

 

[Jon Elson]

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

 

[Cursitor Doom]

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?

 

[Cursitor Doom]

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-2-2uF-2-5KV-2500V-DC-HIGH-QUALITY-
PAPER-IN-OIL-CAPACITOR-NOS-/151689743255?
hash=item235169cb97:g:N3sAAOSwKrhVXwzP

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

 

[Cursitor Doom]

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.

 

[Cursitor Doom]

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.

 

[Cursitor Doom]

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.