On 28.02.2016 22:31, Cursitor Doom wrote:
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.

That's definitely not "a little bit". By very far, not!

Muscling around a scope chassis (not the probe tip, but the probe ground
and the big scope chassis connected to it on the other end of the cable)
from zero to some 800 V in several dozen microseconds is no small feat,
much less doing that 20000 times a second repetitively.

Not many power supplies will do that on an internal node without running
into major stability issues (unless you have a very small
battery-operated "pocket" scope, sitting on a wooden table far away from
any earthed metal, thereby being a "light" load).

The probe tip is not the issue, but the scope itself, hanging from the
probe ground, that is.

"Hhmmm. As I've said before, I'm reluctant to replace that power resistor
with anything higher rated. "

It is NOT higher rated. A 100 watt incandescent in that spot will limit the current even lower and there is less chance of blowing anything else.

I did not mean to suggest that you change your plan of troubleshooting, just that next time when it comes to a hot test, use the bulb. If something is still shorted you have a hell of alot more time to figure out what, rather than having overheat in seconds. When the light dims, you probably found the problem. the light goes down in resistance as all the filters charge and get almost to full voltage, once it does that it works without a net. Regular fuse and all that.

I consider a dim bulb tester a must for this type of work.

On 28.02.2016 22:33, Cursitor Doom wrote:
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!

That's not a problem. It only ever sees 320 V from the mains, plus any
little remains of the mains surges that may come its way past C1802+3.

Even with surges and such, 450 V is likely the highest thing it will
ever see, so a 630 V rating is a good and conservative one.

It won't ever see the 800 V. But the transistor V1806 (collector) will.

Basically, the input caps will "see" only normal rectified mains (320
V). The resonant caps will also see some 300 to 320 V, but because the
sinewave resonance signal is bipolar, and one end is tied to the
positive end of the input caps, there will be times (each half cycle)
where the voltages will add and the result (referenced to the emitter)
will reach some 600 - 620 V. At these same times during the cycle, L1806
will also be reset via V1811, and the reset voltage (some 200 V, also
being in series) will also add to this, plus any little remains (50 V or
less) from the L1804 circuit. So the collector of V1806 will "see" quite
a lot of voltage when V1806 is in the "off" phase. But this high voltage
only applies to the V1806 collector, not to the other parts / signals.

Dimitrij

On 28.02.2016 22:59, Dimitrij Klingbeil wrote:
On 28.02.2016 22:31, Cursitor Doom wrote:
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.

That's definitely not "a little bit". By very far, not!

Muscling around a scope chassis (not the probe tip, but the probe
ground and the big scope chassis connected to it on the other end of
the cable) from zero to some 800 V in several dozen microseconds is
no small feat, much less doing that 20000 times a second
repetitively.

Not many power supplies will do that on an internal node without
running into major stability issues (unless you have a very small
battery-operated "pocket" scope, sitting on a wooden table far away
from any earthed metal, thereby being a "light" load).

The probe tip is not the issue, but the scope itself, hanging from
the probe ground, that is.

P.S. Since you indicated that you have some background with radio...

Consider the collector of V1806 as a signal source. As a signal source
that can basically drive a 800 V peak-to-peak square wave.

Consider the whole power supply board (including any cables and the
isolation transformer or variac that you are using to feed it) as one
half of a dipole antenna.

Consider the scope (the whole metal chassis) and the probe cable as the
other half of the same dipole antenna.

Consider the two halves connected in the middle by the probe ground
clip, at that overheating power resistor in the supply.

What you get, is a center-fed dipole, sitting on your table, and being
driven with a 800 V peak to peak fast square wave. Not a light load.

An isolated high voltage differential probe would "separate the halves",
so that the big (parasitic) dipole would no longer exist.

Regards
Dimitrij

On Sun, 28 Feb 2016 22:50:19 +0100, Dimitrij Klingbeil wrote:

[...]

OK, Dimitrij, a *huge* quantity of info to digest here and in your other
new postings on the subject and more detail than I can assimilate in a
single sitting! Many thanks as ever; that will certainly be all I need to
know for the time being. I'll get to work on the checks when I return to
base after midweek.
Laterz...

On Sun, 28 Feb 2016 14:02:33 -0800, jurb6006 wrote:

[...]
I consider a dim bulb tester a must for this type of work.

Ah, that makes much more sense now; many thanks for that clarification.

On 28.02.2016 22:59, Dimitrij Klingbeil wrote:
On 28.02.2016 22:31, Cursitor Doom wrote:
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.

That's definitely not "a little bit". By very far, not!

Muscling around a scope chassis (not the probe tip, but the probe
ground and the big scope chassis connected to it on the other end of
the cable) from zero to some 800 V in several dozen microseconds is
no small feat, much less doing that 20000 times a second
repetitively.

Not many power supplies will do that on an internal node without
running into major stability issues (unless you have a very small
battery-operated "pocket" scope, sitting on a wooden table far away
from any earthed metal, thereby being a "light" load).

The probe tip is not the issue, but the scope itself, hanging from
the probe ground, that is.

P.S.

There is a simple though unwritten rule about power supply testing:

"Never connect the ground (common, chassis etc.) of any test equipment
to the switching node (power transistor collector, drain or power IC
output pin and its associated signals) of a switching power supply!"

It is valid for all types, no matter if flyback, forward or resonant.

The reason for this rule is that a "switching node" usually drives a
square wave with high voltages (some 500 to 600 V in a flyback, may
happen to be as much as 800 or 1000 V in a resonant one), and that a
significant amperage is readily "available" at that node too, due to the
output transistor's low impedance. Neither is the supply designed to
safely drive that into "RF ground" nor is the test equipment made for
being "muscled around" at that sort of voltages and dV/dt rise times.

Grounding the test equipment would mean that the whole power supply
(plus any safety isolation transformer) is being swung around and
letting the test equipment "float" would mean to also swing around the
test equipment. Apart from the obvious safety hazard, this can also
damage the test equipment and even compromise the test equipment's
electrical safety by frying the "Y" capacitors between mains and
secondary or stressing the isolation barrier in the test equipment's
power supply and / or mains transformer, possibly beyond the level of
stress that it was rated for.

So, whenever you troubleshoot some switcher, take heed of this rule.

It's simple to remember, and it can save lives, test equipment,
and some power supplies under test too

Regards
Dimitrij

On Mon, 29 Feb 2016 21:47:38 +0100, Dimitrij Klingbeil wrote:

The probe tip is not the issue, but the scope itself, hanging from the
probe ground, that is.

P.S.

There is a simple though unwritten rule about power supply testing:

"Never connect the ground (common, chassis etc.) of any test equipment
to the switching node (power transistor collector, drain or power IC
output pin and its associated signals) of a switching power supply!"

It is valid for all types, no matter if flyback, forward or resonant.

The reason for this rule is that a "switching node" usually drives a
square wave with high voltages (some 500 to 600 V in a flyback, may
happen to be as much as 800 or 1000 V in a resonant one), and that a
significant amperage is readily "available" at that node too, due to the
output transistor's low impedance. Neither is the supply designed to
safely drive that into "RF ground" nor is the test equipment made for
being "muscled around" at that sort of voltages and dV/dt rise times.

Grounding the test equipment would mean that the whole power supply
(plus any safety isolation transformer) is being swung around and
letting the test equipment "float" would mean to also swing around the
test equipment. Apart from the obvious safety hazard, this can also
damage the test equipment and even compromise the test equipment's
electrical safety by frying the "Y" capacitors between mains and
secondary or stressing the isolation barrier in the test equipment's
power supply and / or mains transformer, possibly beyond the level of
stress that it was rated for.

So, whenever you troubleshoot some switcher, take heed of this rule.

It's simple to remember, and it can save lives, test equipment,
and some power supplies under test too

I'm grateful for that expansion, to be honest. I was kind of struggling
to get my head around what you were getting at in your earlier postings;
didn't make much sense to me on the first read through and although after
a second read I was beginning to sense your meaning, it still wasn't 100%
clear.
At least now I think I can finally see where you're coming from.
Naturally I read up on safety precautions when dealing with switchers
from books I have and all sorts of diverse sources on the net, but I can
honestly say that what you have outlined above has NOT been covered by
anything I've seen or read up until now. This would seem to be a glaring
omission on the part of those who we rely on to prime us up on the hidden
dangers and pitfalls of troubleshooting such equipment.
Another good reason for me to avoid dealing with switchers in future if
at all possible!!

On Mon, 29 Feb 2016 21:47:38 +0100, Dimitrij Klingbeil wrote:

P.S.

There is a simple though unwritten rule about power supply testing:

Oh, I just noticed you did in fact actually state it's an "unwritten
rule" - well my personal experience of searching on the subject can
certainly confirm that!

On 01.03.2016 00:21, Cursitor Doom wrote:
On Mon, 29 Feb 2016 21:47:38 +0100, Dimitrij Klingbeil wrote:

P.S.

There is a simple though unwritten rule about power supply
testing:

Oh, I just noticed you did in fact actually state it's an "unwritten
rule" - well my personal experience of searching on the subject can
certainly confirm that!

At least myself, I have not seen it being being explicitly explained or
written anywhere yet, but it's sort of "common knowledge" in a way...

Among engineers who design power supplies, this seems to be taken for
granted - too self-evident to warrant explanation apparently. Others,
among them the many who design low voltage circuits and prefer to buy
their power supplies off the shelf, rarely get to see switching nodes
driven with significant fractions of a kV with fast rise times. That
leaves their awareness of the "tricks of the trade" rather limited.

Power supply design is both a science and an art, and the power supply
"artists"' rites of initiation can sometimes involve strange things

Anyway, never fear, but always exercise logical thinking, conservative
judgement and be aware of side effects - that would be my advice here.

Dimitrij

"
There is a simple though unwritten rule about power supply testing:

"Never connect the ground (common, chassis etc.) of any test equipment
to the switching node (power transistor collector, drain or power IC
output pin and its associated signals) of a switching power supply!" "

Well now that you wrote it, it is no longer unwritten. :-)

But I know what you mean. It is pretty much RF and if the caps don't short it out it can burn you some. If the voltage is high enough you don't even have to touch it. I got burned by the cathode of a damper tube in a color TV set once. That is half of about a 70 KHz sine wave clocked at 15.734 KHz. Arced to my finger, burnt and cauterized all the way to the bone. On blood, but it sure did smart. And then it felt funny for like six months after.

Can you imagine that on the chassis and probes of your scope ?

On Mon, 29 Feb 2016 16:29:47 -0800, jurb6006 wrote:

Can you imagine that on the chassis and probes of your scope ?

Not really. It's too weird. Maybe I could simulate it in spice to get a
better idea of what's going on here. Anyone got a model suggestion for a
'hanging scope'?

On Mon, 29 Feb 2016 16:29:47 -0800, jurb6006 wrote:

Can you imagine that on the chassis and probes of your scope ?

It's very simple really. Just remember to keep the scope grounded to the
DUT ground AND the probe clipped to the power node (X1 setting on the
probe) at ALL times and bob's your uncle, you can't go wrong. That way
you are only placing about 15pf || 1M loading on the DUT.
HTH.

On 01.03.2016 22:29, Chris wrote:
On Mon, 29 Feb 2016 16:29:47 -0800, jurb6006 wrote:

Can you imagine that on the chassis and probes of your scope ?

It's very simple really. Just remember to keep the scope grounded to
the DUT ground AND the probe clipped to the power node (X1 setting on
the probe) at ALL times and bob's your uncle, you can't go wrong.
That way you are only placing about 15pf || 1M loading on the DUT.
HTH.

You must be meaning X10 setting. Common scope inputs as well as probes
are not designed to handle the typical peaks from power supplies at X1.

On Wed, 02 Mar 2016 22:28:47 +0100, Dimitrij Klingbeil wrote:

You must be meaning X10 setting. Common scope inputs as well as probes
are not designed to handle the typical peaks from power supplies at X1.

Fair point! But I use an externally selectable decade attenuator for
anything over 400V so X1 is good for me. But yes, in the absence of that,
X10 would be the way to go.

On Mon, 29 Feb 2016 21:47:38 +0100, Dimitrij Klingbeil wrote:

There is a simple though unwritten rule about power supply testing:

"Never connect the ground (common, chassis etc.) of any test equipment
to the switching node (power transistor collector, drain or power IC
output pin and its associated signals) of a switching power supply!"

As an aside, I'm just a bit mystified as to why anyone would want to do
this anyway?

Now, apologies for the delay, but I had the usual accumulation of
pressing things to deal with on my return so have only now got around to
carrying out the checks last suggested here.

OK, I measured the resonant frequency of the primary circuit (with the
chopper NOT disconnected, see notes below) by sweeping a frequency range
across the main tranformer's primary input terminals. It's not
particularly peaky, so there's a Khz or so on either side of Fo before we
get to the -3db shoulders. Fo, with no load connected came out as
17.35kHz.

Under power, with frequency counter connected between T1 and T2 with V1812
removed from circuit shows the PWM chip pulsing at 22.55kHz.

Unfortunately I have no idea what the factory figures should be and
whilst it seems like there's a big difference between the PWM chip's
output and the primary circuit's resonance, AIUI, they're not supposed to
be in sync at any time anyway. But are they supposed to be this far apart?

Notes:

1. I know somewhere it was stated that the chopper transistor should be
removed for the resonance test, but I couldn't see the harm in leaving it
in. If it invalidates the test, of course, then I'll whip it out and re-
do it. If you think it's relevant let me know.

2. I pulled V1812 as someone suggested because the noise coming back
down its collector from L1803 might have interfered with the frequency
counter's ability to read the clock pulses.

On Sun, 06 Mar 2016 13:26:33 +0000, Cursitor Doom wrote:

Under power, with frequency counter connected between T1 and T2 with
V1812 removed from circuit shows the PWM chip pulsing at 22.55kHz.

Sorry, ignore that; copied the wrong piece of paper. It should be
20.64kHz. (This is with the load connected.) I then tried again with V1812
re-inserted and got 20.62kHz. Apologies for the earlier error...

On Wednesday, March 2, 2016 at 4:28:56 PM UTC-5, Dimitrij Klingbeil wrote:
On 01.03.2016 22:29, Chris wrote:
On Mon, 29 Feb 2016 16:29:47 -0800, jurb6006 wrote:

Can you imagine that on the chassis and probes of your scope ?

It's very simple really. Just remember to keep the scope grounded to
the DUT ground AND the probe clipped to the power node (X1 setting on
the probe) at ALL times and bob's your uncle, you can't go wrong.
That way you are only placing about 15pf || 1M loading on the DUT.
HTH.

You must be meaning X10 setting. Common scope inputs as well as probes
are not designed to handle the typical peaks from power supplies at X1.

The quote function musta got screwed up. I always use 10X unless I really need the gain, which is rare. I also recommend others use the 10X at all times as well. Not only does it reduce circuit loading, it also protects the scope to some extent.

Not the first time Usenet quoting got screwed up. I expect to see on a quote of a quote and on a direct quote but it seems not to work that way all the time.

When you've got this thing plugged in and running, what is visible in
the display? Can you get a locator dot? Traces in free-run?

RL

On Sun, 06 Mar 2016 13:28:52 -0500, legg wrote:

When you've got this thing plugged in and running, what is visible in
the display? Can you get a locator dot? Traces in free-run?

I haven't tried this yet as I can guess sod's law making it the case that
I'd have to pull the plug just at the point where the CRT has warmed up
sufficiently. The other slight problem is to test this requires the board
to be completely inserted with every connection made plus a temp probe to
the power resistor which is all rather fiddlesome and not to be done
repeatedly if it can be avoided. I can see a situation arising (sod's law
again) where someone here will post saying - "oh, whilst you still have
the board out, just check this..."
Nevertheless, if nothing is said in the next 18 hours, I will test it all
reconnected and post the outcome here.

On 06.03.2016 14:35, Cursitor Doom wrote:
On Sun, 06 Mar 2016 13:26:33 +0000, Cursitor Doom wrote:

Under power, with frequency counter connected between T1 and T2
with V1812 removed from circuit shows the PWM chip pulsing at
22.55kHz.

Sorry, ignore that; copied the wrong piece of paper. It should be
20.64kHz. (This is with the load connected.) I then tried again with
V1812 re-inserted and got 20.62kHz. Apologies for the earlier
error...


Okay, that makes the difference somewhat less. Still higher than 17.35
kHz obviously.

If I understood the service manual correctly, they seem to suggest to
start from the lowest frequency when performing an adjustment and going
up until output regulation is reached. (They write from "fully
counter-clockwise" actually, referring to the "FREQ" trimmer, and
looking at the schematic that would likely be from the "lowest" position
of the wiper, meaning starting at the highest resistance and going
towards lower resistance.)

It's only my guess, but I think that they intended this supply to run
rather somewhere below resonance than somewhere above. This would mean
that adjusting the pulse frequency down to 17.35 kHz should do no harm
as that value would be lower than the setting right now.

If there was any danger of something blowing up by setting the frequency
lower, they would not be recommending to set it to the absolute minimum
before slowly adjusting it back "up" again.

Can you adjust the pulse rate to 17.35 kHz and then test the supply with
a dummy load?

Can you test it with a variac and see if it still maintains output in
regulation down to 175 V "mains" (adjusted to 17.35 kHz, that is)?

Regards
Dimitrij

On Sun, 06 Mar 2016 23:34:55 +0100, Dimitrij Klingbeil wrote:

If I understood the service manual correctly, they seem to suggest to
start from the lowest frequency when performing an adjustment and going
up until output regulation is reached. (They write from "fully
counter-clockwise" actually, referring to the "FREQ" trimmer, and
looking at the schematic that would likely be from the "lowest" position
of the wiper, meaning starting at the highest resistance and going
towards lower resistance.)

OK, well I can easily establish that safely by popping V1812 out of
circuit temporarily so sweeping the frequency adjustment pot won't have
any effect.

It's only my guess, but I think that they intended this supply to run
rather somewhere below resonance than somewhere above. This would mean
that adjusting the pulse frequency down to 17.35 kHz should do no harm
as that value would be lower than the setting right now.

If there was any danger of something blowing up by setting the frequency
lower, they would not be recommending to set it to the absolute minimum
before slowly adjusting it back "up" again.

Indeed. That seems to be the key point I have to observe.

Can you adjust the pulse rate to 17.35 kHz and then test the supply with
a dummy load?

Well I could.... But that's spot on resonance. I was under the impression
that they're not supposed to run actually directly at resonance?

Can you test it with a variac and see if it still maintains output in
regulation down to 175 V "mains" (adjusted to 17.35 kHz, that is)?

Yes, no problem. It seems the key regulated output is the 12.7V one and
if that's correct, the rest should follow. There's a trimmer for 12.7V on
the underside of the board.

Thanks again, Dimitrij. I'll report back tomorrow...

On Sun, 6 Mar 2016 21:18:58 -0000 (UTC), Cursitor Doom
wrote:

On Sun, 06 Mar 2016 13:28:52 -0500, legg wrote:

When you've got this thing plugged in and running, what is visible in
the display? Can you get a locator dot? Traces in free-run?

I haven't tried this yet as I can guess sod's law making it the case that
I'd have to pull the plug just at the point where the CRT has warmed up
sufficiently. The other slight problem is to test this requires the board
to be completely inserted with every connection made plus a temp probe to
the power resistor which is all rather fiddlesome and not to be done
repeatedly if it can be avoided. I can see a situation arising (sod's law
again) where someone here will post saying - "oh, whilst you still have
the board out, just check this..."
Nevertheless, if nothing is said in the next 18 hours, I will test it all
reconnected and post the outcome here.

As long as you haven't been fooling with psu trimpots, you can forget
about the resistor.

If you have been fooling with trim pots, then you'll have to follow
the manual adjustment procedure first...assuming similarity to PM3262.
(check pot rotation effects expected between models).

Forget about the resistor while you're doing this.

Continue to forget about this resistor until (and if ever) V1811 dies
again.

RL

On Sun, 06 Mar 2016 19:27:19 -0500, legg wrote:

[...]

I am happy to assure you that I have not touched a single trimmer so far
and neither will I be dismissing the heating of the power resistor, which
is clearly indicating that something is still amiss here.

On Sun, 06 Mar 2016 23:34:55 +0100, Dimitrij Klingbeil wrote:

It's only my guess, but I think that they intended this supply to run
rather somewhere below resonance than somewhere above. This would mean
that adjusting the pulse frequency down to 17.35 kHz should do no harm
as that value would be lower than the setting right now.

I've just noticed that towards the bottom of (true) page 106, it states
the following:-

"The oscillator frequency is approximately 25kHz, determined by network
C1811, R1823 and is adjustable by means of R1824."

It then goes on to specify the duty cycle. Two things stand out as
requiring further investigation here. Clearly, the 25kHz clock frequency
mentioned is *miles* away from what my clock is running at - and the
frequency adjustment is made with R1827, not R1824 (which is fixed
anyway). I'm guessing the reference to R1824 is just a typo, but can we
say the same for 25kHz??
Since this *completely* changes our former assumptions, I'm going to
confine myself to just replacing the flaky polyester caps for the time
being. Be interested to hear how you think I should proceed now in the
light of this...

And before anyone suggests it: I've frequency swept the primary circuit
just in case there's a second resonance peak at around 25kHz and there
isn't one.

On 07.03.2016 19:08, Cursitor Doom wrote:

I've just noticed that towards the bottom of (true) page 106, it
states the following:-

"The oscillator frequency is approximately 25kHz, determined by
network C1811, R1823 and is adjustable by means of R1824." ...

And before anyone suggests it: I've frequency swept the primary
circuit just in case there's a second resonance peak at around 25kHz
and there isn't one.

Hi

News indeed. Something must be amiss, and quite heavily amiss, that's
for sure.

It looks like the resonant circuit considerably out of tune.

Driving a 17 kHz LC with 25 kHz would not make sense to me, and it looks
like the circuit does not like it too much either, since it overheats.

If it was indeed tuned for 25 kHz, then the currently set 22 (or 20) kHz
pulse rate setting could at least make some sense. It would be slightly
below resonance, but probably not too far away.

I was assuming that the resonant circuit was ok-ish and the frequency
somewhat matched, but this turns out, now, not to be the case.

Now I've looked in the TDA1060 controller datasheet again, and checked
the adjustment range (with the trimpot set from 0 to 10 kOhm and the 11
kOhm fixed resistor and 3.3 nF timing capacitor) and the calculated
resulting range of frequencies happens to be from 17.3 kHz to 33 kHz.

With a 17.3 to 33 kHz range, that would put 25 kHz quite well in the
middle. A 17 kHz resonance frequency does not fit anywhere though. It
can't even reliably be adjusted (even if it were correct, which it
surely isn't) because it's simply out of the trimpot's range.

But it would not make any sense to pulse a 17 kHz LC at 25 kHz either.
The LC will be heavily capacitive, the power factor will be down in the
ditch, and that will overload (-heat) the driver (just as it happens).

Therefore I can only think that 17 kHz is wrong. The LC is out of
resonance. If it were OK, it should never have a 17 kHz SRF.

That leaves either a measurement error (now rather less likely) or a
heavy stray capacitance somewhere, that brings the resonance down.

I can only think of C1806 and C1807. They are in series. If one of them
has an isolation problem, that would leave the other one alone in the
circuit - and therefore double the capacitance.

Also there are the resistors in parallel - R1817 and R1818. They should
be 10 Megaohm. But if one of them is either shorted or improperly
replaced (maybe it formed an isolation breakdown from over-voltage or
someone put in a wrong value like zero Ohm instead), then that would
also short out the corresponding capacitor, and have the same effect.

Usually resistors are reliable, but sometimes, some old ones of the
"carbon composition" variety, do form a "hot channel" and break down.

Doubling the capacitance would almost halve the resonance frequency.
Actually it won't *exactly* halve it, because there is still a third
capacitor in parallel, namely the stray winding capacitance.

This looks quite enough to be realistic. If one resonance cap is shot,
the LC frequency will go down by a great deal, and (assuming it was
initially slightly higher than the pulse frequency), a change from some
25 or 27 kHz to the 17 kHz that you are now seeing, could happen.

It could also explain the overload on the resistor - a heavily
capacitive out-of-resonance load would easily do that.

Can you get these two caps out altogether, connect a known good 15 nF
(or two of 30 nF in series) instead and sweep again?

Also, to avoid unforeseen measurement errors, can you do that out of
circuit? Just the transformer and the capacitor(s) on the primary. This
would also nicely avoid the resistors too, they too may be questionable.

You won't need a high voltage cap for sweeping - any good 15 nF one will
do. But don't power it up with "any 15 nF". To run at full power it
needs something like a FKP1 or MKP-4C with proper ratings (see my
earlier post, there are some suggested models that can work there).

Regards
Dimitrij

On Mon, 7 Mar 2016 13:46:27 -0000 (UTC), Cursitor Doom
wrote:

On Sun, 06 Mar 2016 23:34:55 +0100, Dimitrij Klingbeil wrote:

It's only my guess, but I think that they intended this supply to run
rather somewhere below resonance than somewhere above. This would mean
that adjusting the pulse frequency down to 17.35 kHz should do no harm
as that value would be lower than the setting right now.

I've just noticed that towards the bottom of (true) page 106, it states
the following:-

"The oscillator frequency is approximately 25kHz, determined by network
C1811, R1823 and is adjustable by means of R1824."

It then goes on to specify the duty cycle. Two things stand out as
requiring further investigation here. Clearly, the 25kHz clock frequency
mentioned is *miles* away from what my clock is running at - and the
frequency adjustment is made with R1827, not R1824 (which is fixed
anyway). I'm guessing the reference to R1824 is just a typo, but can we
say the same for 25kHz??
Since this *completely* changes our former assumptions, I'm going to
confine myself to just replacing the flaky polyester caps for the time
being. Be interested to hear how you think I should proceed now in the
light of this...

You are refering to manual component numbers from the 3262 manual and
schematic. The 3264 does not have the same schematic or part numbers.

The schematic is functionally similar, but uses a different control IC
and different components are present/selected to set the IC's
function. Part type for the main transformer/size and pinout, the size
of resonant/snubbing components, along the actual supply power
ratings may vary with model number, as well. One example is the
different resonant cap size used.

3262 PSU adjustment begins with para 3.4.4 on 3262 manual page 115.
(do not ignore preceding setup instructions re scope settings)
You can assume the sequencing and intention of these instructions to
mirror those needed for 3264, however numbers and test conditions that
reflect operating power and typical frequency can be expected to vary.
This includes chip reference pin voltages - TDA1060 internal reference
is 3V62.

The converter runs at a fixed frequency, above resonance. The
frequency only needs adjustment if the output voltages lose full power
low-line voltage regulation. This is also the protective power limit.

The two model control circuits do not limit in a similar manner. The
3262 manual describes a latching power limit that occurs after
repeated continuous overload. This does not appear to be present in in
3264, as it is chip-based feature.

RL

On 07.03.2016 20:21, Dimitrij Klingbeil wrote:
On 07.03.2016 19:08, Cursitor Doom wrote:

I've just noticed that towards the bottom of (true) page 106, it
states the following:-

"The oscillator frequency is approximately 25kHz, determined by
network C1811, R1823 and is adjustable by means of R1824." ...

And before anyone suggests it: I've frequency swept the primary
circuit just in case there's a second resonance peak at around
25kHz and there isn't one.

I can only think of C1806 and C1807. They are in series. If one of
them has an isolation problem, that would leave the other one alone
in the circuit - and therefore double the capacitance.

Also there are the resistors in parallel - R1817 and R1818. They
should be 10 Megaohm. But if one of them is either shorted or
improperly replaced (maybe it formed an isolation breakdown from
over-voltage or someone put in a wrong value like zero Ohm instead),
then that would also short out the corresponding capacitor, and have
the same effect.

Usually resistors are reliable, but sometimes, some old ones of the
"carbon composition" variety, do form a "hot channel" and break
down.

Doubling the capacitance would almost halve the resonance frequency.
Actually it won't *exactly* halve it, because there is still a third
capacitor in parallel, namely the stray winding capacitance.

This looks quite enough to be realistic. If one resonance cap is
shot, the LC frequency will go down by a great deal...

P.S. There may be another failure mode that I did not consider right
away, that can make it a little harder for you to test the caps.

These high voltage impulse-rated capacitors are usually made with three
metal layers and two layers of isolation internally. Mains "X1" and "X2"
rated capacitors are also made in this way. They are like two capacitors
that are connected in series inside. When one isolator breaks down,
the whole capacitor won't be destroyed catastrophically because there's
still the other internal half in series.

But it can happen (depending on the construction of the capacitor, if it
has a continuous metal layer between the isolators) that the capacitance
will "double itself" instead.

If one of your 30 nF caps has failed in this way, it will "become 60 nF"
instead of becoming short-circuit. So you won't see it on an Ohmmeter.

But that would also be reason enough to detune the resonance a lot.

So, my advice would be, do not trust them, and do not trust their
parallel resistors too much either. Take a known working 15 nF cap and
sweep the transformer with it. If you get anything significantly above
17 kHz with a new cap, look for the main problem in this direction.

Dimitrij

On 07.03.2016 20:21, Dimitrij Klingbeil wrote:
On 07.03.2016 19:08, Cursitor Doom wrote:

I've just noticed that towards the bottom of (true) page 106, it
states the following:-

"The oscillator frequency is approximately 25kHz, determined by
network C1811, R1823 and is adjustable by means of R1824." ...

And before anyone suggests it: I've frequency swept the primary
circuit just in case there's a second resonance peak at around
25kHz and there isn't one.
...
... Also, to avoid unforeseen measurement errors, can you do that
out of circuit? Just the transformer and the capacitor(s) on the
primary.

P.P.S. If you decide to sweep the LC part in circuit instead of out of
circuit (because the transformer is difficult to solder out etc...), you
can use a small voltage source (a 9 V block battery) connected to the
power supply's "AC" input. This will precharge the circuit enough to get
the parasitics down. It won't start the power supply controller, but it
will reverse-bias various diodes and also the base-collector junction of
the main switching transistor. That will make these semiconductor parts
non-conducting (at small signal levels) and prevent them from rectifying
the test signal from your sweep generator, and messing it up in various
ways through leakage paths. It's an easier alternative to removing the
switching transistor from the circuit.

Dimitrij

On Mon, 07 Mar 2016 14:23:22 -0500, legg wrote:

You are refering to manual component numbers from the 3262 manual and
schematic. The 3264 does not have the same schematic or part numbers.

The schematic is functionally similar, but uses a different control IC
and different components are present/selected to set the IC's function.
Part type for the main transformer/size and pinout, the size of
resonant/snubbing components, along the actual supply power ratings may
vary with model number, as well. One example is the different resonant
cap size used.

OMG you're right. I've had this issue before when looking at manuals
from .pdf files on a screen rather than hard-copy. Well that's just dandy
I must say. Now I'm *really* confused.
Once again I'm really tempted to just scrap this thing as it stands,
salvage the transformer and start afresh in a year's time with a
conventional non-resonant PWM design using a MOSFET instead of a BJT and
a more up-to-date controller.

On Mon, 07 Mar 2016 21:55:56 +0100, Dimitrij Klingbeil wrote:

P.P.S. If you decide to sweep the LC part in circuit instead of out of
circuit (because the transformer is difficult to solder out etc...), you
can use a small voltage source (a 9 V block battery) connected to the
power supply's "AC" input. This will precharge the circuit enough to get
the parasitics down. It won't start the power supply controller, but it
will reverse-bias various diodes and also the base-collector junction of
the main switching transistor. That will make these semiconductor parts
non-conducting (at small signal levels) and prevent them from rectifying
the test signal from your sweep generator, and messing it up in various
ways through leakage paths. It's an easier alternative to removing the
switching transistor from the circuit.

Dimitrij

Sorry, Dimitrij; as you were. Legg has spotted I was mistakenly referring
to the wrong diagram in fact. They look very similar and when you don't
have the physical hard copy manual in front of you then errors like this
are far more easily made, I regret to say.
Sigh. I've had enough for today, I'll take yet another look at it again
tomorrow.

Well, I've replaced all the flaking capacitors and still no improvement.
A number of people have been suggesting I remove the two resonant caps
(the 30n ones) from the primary circuit and test them. I didn't have any
expectation that this would achieve anything since they tested good in-
circuit, but as we're running out of ideas now I did remove them this
afternoon and they both tested at 31nF a piece and no signs of any
physical damage. I then subbed a couple of common-or-garden mylars of the
same value in their places and re-swept for changes in resonance. Result
was no change in resonance - but a slightly better Q(!!) Also checked the
two 10Meg resistors whilst I was at it and they were fine, too. So I can
only think of making up Dimitrij's winding tester and looking for signs
of any turns shorting in the main transformer.
I'm coming to the end of the amount of time I'm prepared to spend on this
psu as it stands. I'm more and more tempted to mothball the key parts of
it til next year then rebuild it as a conventional non-res converter to a
fresh design. TBH, I'm not prepared to still be testing this thing after
the end of this week, so if anyone has any last-ditch ideas, now's the
time to toss 'em into the mix. Speak now or forever hold your peace.
Thanks, all.

On Tue, 8 Mar 2016 16:48:05 -0000 (UTC), Cursitor Doom
wrote:


Well, I've replaced all the flaking capacitors and still no improvement.
A number of people have been suggesting I remove the two resonant caps
(the 30n ones) from the primary circuit and test them. I didn't have any
expectation that this would achieve anything since they tested good in-
circuit, but as we're running out of ideas now I did remove them this
afternoon and they both tested at 31nF a piece and no signs of any
physical damage. I then subbed a couple of common-or-garden mylars of the
same value in their places and re-swept for changes in resonance. Result
was no change in resonance - but a slightly better Q(!!) Also checked the
two 10Meg resistors whilst I was at it and they were fine, too. So I can
only think of making up Dimitrij's winding tester and looking for signs
of any turns shorting in the main transformer.
I'm coming to the end of the amount of time I'm prepared to spend on this
psu as it stands. I'm more and more tempted to mothball the key parts of
it til next year then rebuild it as a conventional non-res converter to a
fresh design. TBH, I'm not prepared to still be testing this thing after
the end of this week, so if anyone has any last-ditch ideas, now's the
time to toss 'em into the mix. Speak now or forever hold your peace.
Thanks, all.

Waste of breath.

RL

On 3/8/2016 11:48 AM, Cursitor Doom wrote:

Well, I've replaced all the flaking capacitors and still no improvement.
A number of people have been suggesting I remove the two resonant caps
(the 30n ones) from the primary circuit and test them. I didn't have any
expectation that this would achieve anything since they tested good in-
circuit, but as we're running out of ideas now I did remove them this
afternoon and they both tested at 31nF a piece and no signs of any
physical damage. I then subbed a couple of common-or-garden mylars of the
same value in their places and re-swept for changes in resonance. Result
was no change in resonance - but a slightly better Q(!!) Also checked the
two 10Meg resistors whilst I was at it and they were fine, too. So I can
only think of making up Dimitrij's winding tester and looking for signs
of any turns shorting in the main transformer.
I'm coming to the end of the amount of time I'm prepared to spend on this
psu as it stands. I'm more and more tempted to mothball the key parts of
it til next year then rebuild it as a conventional non-res converter to a
fresh design. TBH, I'm not prepared to still be testing this thing after
the end of this week, so if anyone has any last-ditch ideas, now's the
time to toss 'em into the mix. Speak now or forever hold your peace.
Thanks, all.


I have no idea what you now consider to be wrong with the PSU. Apart
from a resistor that in your opinion runs too hot even though its well
within its rating I seem to recall amidst your ramblings that the
voltage rails are correct?

It seems to me that the only fault was the diode which was pretty
obvious from the beginning. (Always check for previous repairs).

You persist in not using the correct manual, refuse to test it with an
appropriate load and won't put it in the scope to see if the scope
actually functions.

If you just want to discuss switching supply design go buy some control
chips or an evaluation kit and build some.

On Tue, 08 Mar 2016 12:35:07 -0500, JC wrote:

I have no idea what you now consider to be wrong with the PSU. Apart
from a resistor that in your opinion runs too hot even though its well
within its rating I seem to recall amidst your ramblings that the
voltage rails are correct?

Yes, they're fine. It's not *my* opinion that this is a poor design! I
posted the schematic to s.e.d and the designers there told me that. I've
said all along I know nothing about this type of PSU so I defer to those
better qualified. Having said that, the widespread evidence of charring
and soot residue that appear even worse in real life than in the photos
would seem to indicated that that is not a happy board. The rest of the
scope's boards are still pristine showing no sign of repairs at all.

It seems to me that the only fault was the diode which was pretty
obvious from the beginning. (Always check for previous repairs).

Nothing obvious about it! - apart from the rather poor replacement
technique. The problem with that replacement was very subtle inasmuch as
it wasn't recovering quickly enough at 20kHz. Nevertheless it was still
capable of functioning as a viable rectifier right up to 500kHz.

You persist in not using the correct manual, refuse to test it with an
appropriate load and won't put it in the scope to see if the scope
actually functions.

All the recent tests have been carried out using the actual scope itself
as a load - can't get any better than that. I won't put it in the scope
until that resistor is running cool. It's proximity to other temperature
sensitive components like diodes and its siting deep within the scope
uncooled lead me to believe that it is running far hotter than it should
be - and will be even worse in situ. That *might* just be a design flaw,
but I doubt it.

On Tue, 08 Mar 2016 12:33:50 -0500, legg wrote:

Waste of breath.

Yeah, I know what you're thinking - your suggestion still hasn't been
adopted. I've been giving it more thought and I've come up with a
brainwave.

This is a 2W resistor. If it's trying to dissipate more than 2W as I
strongly believe, something's definitely wrong. The problem up until now
has been measuring the dissipation, because we can't use I^2*R or
variations thereof because of the highly noisy/irregular waveform. So...
Here's the clever bit:
Measure exactly how long it takes at present for the resistor to reach
say 50'C. I believe it's around 1 minute, but get the exact time. Then
let it cool completely back to the room ambient temperature. Remove from
circuit. Attach to bench power supply and by means of trial and error,
set the voltage across the resistor to raise it's temperature to 50'C in
1 minute (will obviously require several attempts, but no matter). Read
off the voltage level which produces this outcome, then just do V^2/R to
find W and see if it exceeds 2W.
I'll do it first thing tomorrow!

On Tue, 08 Mar 2016 21:31:25 +0000, Cursitor Doom wrote:

[snip]
I'll do it first thing tomorrow!

Finally it gets *really* interesting. :-}

Cursitor Doom wrote:
This is a 2W resistor. If it's trying to dissipate more than 2W as I
strongly believe, something's definitely wrong. The problem up until now
has been measuring the dissipation, because we can't use I^2*R or
variations thereof because of the highly noisy/irregular waveform. So...
Here's the clever bit:
Measure exactly how long it takes at present for the resistor to reach
say 50'C. I believe it's around 1 minute, but get the exact time. Then
let it cool completely back to the room ambient temperature. Remove from
circuit. Attach to bench power supply and by means of trial and error,
set the voltage across the resistor to raise it's temperature to 50'C in
1 minute (will obviously require several attempts, but no matter). Read
off the voltage level which produces this outcome, then just do V^2/R to
find W and see if it exceeds 2W.
I'll do it first thing tomorrow!

Good test. It would be more straight to plug the right voltage to make it dissipate 2W, for 20 ohm that would be 6,3V and check what temp results in 1 minute. You could use another identical resistor for the test if you have one lying around to avoid unsoldering.

On 09.03.2016 00:10, Jeroni Paul wrote:
Cursitor Doom wrote:
This is a 2W resistor. If it's trying to dissipate more than 2W as
I strongly believe, something's definitely wrong. The problem up
until now has been measuring the dissipation, because we can't use
I^2*R or variations thereof because of the highly noisy/irregular
waveform. So... Here's the clever bit: Measure exactly how long it
takes at present for the resistor to reach say 50'C. I believe it's
around 1 minute, but get the exact time. Then let it cool
completely back to the room ambient temperature. Remove from
circuit. Attach to bench power supply and by means of trial and
error, set the voltage across the resistor to raise it's
temperature to 50'C in 1 minute (will obviously require several
attempts, but no matter). Read off the voltage level which produces
this outcome, then just do V^2/R to find W and see if it exceeds
2W. I'll do it first thing tomorrow!

Good test. It would be more straight to plug the right voltage to
make it dissipate 2W, for 20 ohm that would be 6,3V and check what
temp results in 1 minute. You could use another identical resistor
for the test if you have one lying around to avoid unsoldering.


There's no need to unsolder, neither to actively avoid unsoldering. That
resistor is connected through a diode on the board. Just apply the
proper polarity signal, and the diode will take care of the isolation.