Transformer shot! (was scope SMPS/ capacitor venting)

[Cursitor Doom]

On Sun, 21 Feb 2016 19:36:29 +0100, Dimitrij Klingbeil wrote:

[...]

First of all, did you really use 100 kHz as written? Most LCR meters
have 100 Hz, 1 kHz, 10 kHz and 100 kHz signals. Did you perchance use
the 100 Hz one instead? 100 Hz would be so low that the inductive part
might not even register properly.

Yes, definitely 100kHz. Not my preferred choice, but the only option
given the meter I used which was actually a capacitor ESR meter.

[SRF remarks noted]

So your 100 kHz measurements indicated very low impedances, like some
winding was shorted out. But then you also have 2 high voltage windings
in there, which would have been way above SRF at 100 kHz frequency, so
they will effectively behave like shorted even if they were perfectly
fine otherwise. At 100 kHz they're no longer inductors, they're likely
just capacitors instead.

Very good point. I admit I never considered that possibility.

Your transformer is probably supposed to run at something like 20 kHz in
normal resonant operation, so 20 kHz should be ok. But because it has
high voltage windings, it may be very close to the HV winding's SRF.
Indeed Philips engineers may even have chosen to run the transformer not
below, but essentially right at SRF. They may have selected the
resonance capacitors for the primary in such a way that the primary
(together with the resonance capacitors) would have a resonant frequency
which closely matches the self-resonance of one of the high voltage
windings, being just a little bit below to account for tolerances.

If that's the case, you should use a lower frequency for testing
impedances. Most LCR meters don't offer 20 kHz anyway, just 10 kHz and
100 kHz as "nearest neighbors". 100 kHz won't do, so use 10 kHz. That
should give you realistic impedances (which you can manually multiply by
2 to get to the in-circuit conditions).

Yes, it might be illuminating to sweep a range of frequencies and note
any resonances, I can see the value of that. Unfortunately, an LCR meter
is one item of test equipment I don't have, so it would have to be sig
gen and scope in combination. Anyway, it's do-able.
Many thanks for your observations as always.

 

[Cursitor Doom]

On Sun, 21 Feb 2016 20:09:47 +0100, Dimitrij Klingbeil wrote:

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:

Recheck pin function before jumping to conclusions.

Right; now re-checked. DC measurments proved (unsurprisingly) too
close together so I re-tested using 100khz instead. These are the
impedances WRT ground of the output taps of the long winding in the
order they actually come out of the transformer: GND, 0.17ohms,
0.17ohms, 0.26ohms, 0.28ohms, 3.7ohms, 3.8ohms. So this doesn't seem
to tally up with the schematic. Or does it? I need a pint of strong
coffee to kick-start my head on this one. :-/ Anyway, later...
... In one of your pictures there are a couple of diodes that look
messed up (V1809 and V1811) near the bridge. They are supposed to be
BY208-1000 (1000v rectifiers), I can see "40" on one, maybe 1N4007?

If you can't find a BY208-1000 replacement, a MUR4100E(G) should work.

Sorry, that may be physically too big to fit. A MUR1100EG or something
similar should work and fit in the available space too.

Dimitrij

Thank you. If you've had the chance to read my follow up to JC (I think
it was) then you'll be aware that one of those BY208 diodes was replaced
by a BY134. If the design is that critical of the speed of the diodes it
uses then maybe it won't function properly as you suggest. I can only
imagine the technician who replaced it was unaware of the critical nature
of the part concerned.
I'm kind of unhappy with this design overall, to be honest. It was
critically appraised on s.e.d and found generally unsatisfactory. I'm
strongly tempted to just save the transformers, junk everything else and
start afresh with a modern design. The rest of the scope is mint and
untouched, it's only the psu section that's been butchered around and
shows signs of burning and scorching in places. Maybe the best thing to
do would be to bin it? :-/

 

[legg]

On Sun, 21 Feb 2016 13:36:49 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

<snip>

In one of your pictures there are a couple of diodes that look messed up
(V1809 and V1811) near the bridge. They are supposed to be BY208-1000
(1000v rectifiers), I can see "40" on one, maybe 1N4007?

I like your thinking! But no, the one nearest the bridge is a BY208-1000
alright, the other one to the side of it is a BY134. They both tested
fine out of circuit.

The BY134 is a lower frequency part with 2uS recovery time and is
probably unsuited to replacement of BY208-1000 in any of the snubber
or conversion positions indicated on the schematic primary. It should
be soft recovery, medium speed (200-600nS) avalanche-rated part with a
minimum 800Vprv.

I'd avoid the use of anything advertised as 'ultrafast' (ie UF4007),
as this circuit may need a modest recovery time in order to reduce
power loss and EMI, but they could be used temporarily in
troubleshooting.

RL

 

[Cursitor Doom]

On Sun, 21 Feb 2016 16:28:50 -0500, JC wrote:

On 2/21/2016 3:59 PM, Cursitor Doom wrote:
On Sun, 21 Feb 2016 20:09:47 +0100, Dimitrij Klingbeil wrote:

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:


Thank you. If you've had the chance to read my follow up to JC (I think
it was) then you'll be aware that one of those BY208 diodes was
replaced by a BY134. If the design is that critical of the speed of the
diodes it uses then maybe it won't function properly as you suggest. I
can only imagine the technician who replaced it was unaware of the
critical nature of the part concerned.
I'm kind of unhappy with this design overall, to be honest. It was
critically appraised on s.e.d and found generally unsatisfactory. I'm
strongly tempted to just save the transformers, junk everything else
and start afresh with a modern design. The rest of the scope is mint
and untouched, it's only the psu section that's been butchered around
and shows signs of burning and scorching in places. Maybe the best
thing to do would be to bin it? :-/

Switching supplies are all designed with the diode (and other component)
parameters in mind, its how they function efficiently. Bunging any old
diode in is asking for trouble. Same as the ESR and temp rating of the
caps used. Seriously you need to sit back and chill. These scopes were
very well designed and I would say exceptionally reliable. I'd like to
see you build a replacement. I've designed switching supplies,
everything needs to be right or it goes wrong fast.

FR107G seems to be the current equivalent for the BY208-1000

Well, the modern ones may well be super reliable, but this old thing is
very dated and shows many signs of its age and the scars of previous
faults and questionable repairs. I wouldn't attempt another resonant
converter; there must be something simpler with fewer critical
parameters, surely.

 

[Dimitrij Klingbeil]

On 20.02.2016 15:36, JC wrote:

On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:

Recheck pin function before jumping to conclusions.

Right; now re-checked. DC measurments proved (unsurprisingly) too
close together so I re-tested using 100khz instead. These are the
impedances WRT ground of the output taps of the long winding in the
order they actually come out of the transformer: GND, 0.17ohms,
0.17ohms, 0.26ohms, 0.28ohms, 3.7ohms, 3.8ohms. So this doesn't
seem to tally up with the schematic. Or does it? I need a pint of
strong coffee to kick-start my head on this one. :-/ Anyway,
later...
...
... In one of your pictures there are a couple of diodes that look
messed up (V1809 and V1811) near the bridge. They are supposed to be
BY208-1000 (1000v rectifiers), I can see "40" on one, maybe 1N4007?

Well, if that is true then beware! V1808, V1809 and V1811 are supposed
to be very fast. Any slow (more than a microsecond) diode in these
positions will likely cause symptoms akin to a heavy overload.

Particularly V1811, if replaced with any 1N400x, is likely to render the
energy recovery circuit around L1806 as good as inoperative, thereby
dumping the entire energy from the switcher harmonics into R1814, which
will cause it to overheat fast.

Please recheck L1806 (both windings) for turn-to-turn shorts (with a
signal generator), and if any of the 3 diodes (V1808, V1809, V1811)
looks like it had previously been replaced (possibly improperly
replaced), consider replacing all 3 of them together, using the proper
parts.

Use fast soft-recovery diodes rated for 1kV here. If you can't find any,
use ultrafast ones. They're maybe not optimal from an EMI standpoint
here, but at least they should work well enough for testing.

If you can't find a BY208-1000 replacement, a MUR4100E should work.

Check C1806 for dielectric breakdown. It should be able to withstand at
least 500 V (or something in that ballpark). If it doesn't, replace.

Don't underestimate that L1806 energy recovery circuit. Although it
doesn't by itself transfer any power to the load, this supply heavily
relies on it for proper resonant operation of the main transformer. It
must be working properly before you can test the main transformer
waveform and have any chance of making correct measurements.

Besides, your description of heavy switching noise on V1806 (when you
tried to measure the base drive waveform), up to the point of the
waveform being unrecognizable in the noise, seems to indicate that the
L1806 circuit is shorted at high frequencies. This can be a result of
either a winding short in L1806 or a breakdown in one of its diodes or
some of these diodes being replaced by a generic slow silicon diode.

Dimitrij

 

[Cursitor Doom]

On Sun, 21 Feb 2016 17:13:55 -0500, JC wrote:

On 2/21/2016 4:28 PM, JC wrote:
On 2/21/2016 3:59 PM, Cursitor Doom wrote:
On Sun, 21 Feb 2016 20:09:47 +0100, Dimitrij Klingbeil wrote:

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:


Thank you. If you've had the chance to read my follow up to JC (I
think it was) then you'll be aware that one of those BY208 diodes was
replaced by a BY134. If the design is that critical of the speed of
the diodes it uses then maybe it won't function properly as you
suggest. I can only imagine the technician who replaced it was unaware
of the critical nature of the part concerned.
I'm kind of unhappy with this design overall, to be honest. It was
critically appraised on s.e.d and found generally unsatisfactory. I'm
strongly tempted to just save the transformers, junk everything else
and start afresh with a modern design. The rest of the scope is mint
and untouched, it's only the psu section that's been butchered around
and shows signs of burning and scorching in places. Maybe the best
thing to do would be to bin it? :-/

Switching supplies are all designed with the diode (and other
component)
parameters in mind, its how they function efficiently. Bunging any old
diode in is asking for trouble. Same as the ESR and temp rating of the
caps used. Seriously you need to sit back and chill. These scopes were
very well designed and I would say exceptionally reliable. I'd like to
see you build a replacement. I've designed switching supplies,
everything needs to be right or it goes wrong fast.

FR107G seems to be the current equivalent for the BY208-1000

FR107G Ebay (UK) #390565307743 cheap enough.

Thank you, JC. You and Dimitrij have both given me some subs for suitable
replacements which I'm quite happy to go along with. But I'm not prepared
to spend much more time on this repair, to be honest. I'd relish the
prospect of a comprehensive re-design. Even if it's beyond me at this
stage I'd learn a lot from it.

 

[Dimitrij Klingbeil]

On 20.02.2016 13:55, Cursitor Doom wrote:

On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:

Recheck pin function before jumping to conclusions.

Right; now re-checked. DC measurments proved (unsurprisingly) too
close together so I re-tested using 100khz instead. These are the
impedances WRT ground of the output taps of the long winding in the
order they actually come out of the transformer: GND, 0.17ohms,
0.17ohms, 0.26ohms, 0.28ohms, 3.7ohms, 3.8ohms. So this doesn't seem
to tally up with the schematic. Or does it? I need a pint of strong
coffee to kick-start my head on this one. :-/ Anyway, later...

Hi

As you say, this really doesn't tally up. The ratios look ok (see my
other post), but the absolute values are obviously junk.

After all, Ohm's law still holds, even for reactive impedances, and 60
volts divided by 3.7 Ohms is 16 amps, which would be WAY too much for a
small transformer winding's magnetization current. That would indicate
very heavy overload, most probably due to a short circuit inside one of
the windings.

But there's something that makes me distrust those impedance numbers -
and that is your use of 100 kHz as the testing frequency.

First of all, did you really use 100 kHz as written? Most LCR meters
have 100 Hz, 1 kHz, 10 kHz and 100 kHz signals. Did you perchance use
the 100 Hz one instead? 100 Hz would be so low that the inductive part
might not even register properly.

Second, 100 kHz is not a good choice. The reason for this is the
self-resonance frequency (SRF) of your transformer. All coils and
transformers in the real world are not just coils, but LC circuits. The
L part is (obviously) provided by the winding itself and the C part is
the stray capacitance between the wires in the winding. Being really an
LC circuit, a winding has a resonance frequency, like any "true" LC
circuit would have. This is called the SRF of the winding. To make
matters worse, a transformer that has multiple windings wound with
different geometries and wire diameters has multiple SRFs, one for each
winding.

Windings with few turns of loosely packed thick wire have high SRF
values, while windings with many turns of densely packed thin wire will
have much lower SRFs.

Your particular transformer has 2 high-voltage windings for the kV
outputs. They have lots of densely packed thin wire, so their SRFs will
be very low. I don't know exactly how low, but I'm sure that they will
be much lower than 100 kHz, and that's what makes 100 kHz an unsuitable
choice for testing.

If fact, if you try to operate a winding above its SRF (let's say the
winding has a 20 kHz SRF and you try to apply 100 kHz), then the winding
will no longer behave like an inductor, but it will behave like a
capacitor instead. I know, this seems crazy, but that's how a winding
behaves above its SRF.

In a transformer, where there are multiple windings, there are also
multiple SRFs, so at some test frequency, some windings may happen to be
below their respective SRFs, while some other windings may be above
their respective SRFs, depending on how you choose the test frequency.

If any winding happens to be above its SRF, then it will behave like a
capacitor. As you know, capacitors behave more or less like a short
circuit at high frequencies, and an above-SRF winding will behave like
that too. That is, it will look (from an impedance measurement) like if
it was heavily overloaded or even shorted out altogether.

So your 100 kHz measurements indicated very low impedances, like some
winding was shorted out. But then you also have 2 high voltage windings
in there, which would have been way above SRF at 100 kHz frequency, so
they will effectively behave like shorted even if they were perfectly
fine otherwise. At 100 kHz they're no longer inductors, they're likely
just capacitors instead.

Now, to test transformer winding impedances, you need to select a
reasonable test frequency. It must obviously be lower than any SRF of
any winding - otherwise the transformer will appear overloaded. If you
don't know the SRFs' values, you can measure them out with a signal
generator and an oscilloscope. But you don't need to. Normally no
transformer is operated above its SRF (it would not work very well if
one tried), so you can assume the normally intended frequency of
operation to be a "safe" choice that is unlikely to hit SRF limits.

Your transformer is probably supposed to run at something like 20 kHz in
normal resonant operation, so 20 kHz should be ok. But because it has
high voltage windings, it may be very close to the HV winding's SRF.
Indeed Philips engineers may even have chosen to run the transformer not
below, but essentially right at SRF. They may have selected the
resonance capacitors for the primary in such a way that the primary
(together with the resonance capacitors) would have a resonant frequency
which closely matches the self-resonance of one of the high voltage
windings, being just a little bit below to account for tolerances.

If that's the case, you should use a lower frequency for testing
impedances. Most LCR meters don't offer 20 kHz anyway, just 10 kHz and
100 kHz as "nearest neighbors". 100 kHz won't do, so use 10 kHz. That
should give you realistic impedances (which you can manually multiply by
2 to get to the in-circuit conditions).

Regards
Dimitrij

 

[Dimitrij Klingbeil]

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:

On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:

Recheck pin function before jumping to conclusions.

Right; now re-checked. DC measurments proved (unsurprisingly)
too close together so I re-tested using 100khz instead. These are
the impedances WRT ground of the output taps of the long winding
in the order they actually come out of the transformer: GND,
0.17ohms, 0.17ohms, 0.26ohms, 0.28ohms, 3.7ohms, 3.8ohms. So this
doesn't seem to tally up with the schematic. Or does it? I need a
pint of strong coffee to kick-start my head on this one. :-/
Anyway, later...
... In one of your pictures there are a couple of diodes that
look messed up (V1809 and V1811) near the bridge. They are supposed
to be BY208-1000 (1000v rectifiers), I can see "40" on one, maybe
1N4007?

If you can't find a BY208-1000 replacement, a MUR4100E(G) should
work.

Sorry, that may be physically too big to fit. A MUR1100EG or something
similar should work and fit in the available space too.

Dimitrij

 

[Cursitor Doom]

On Mon, 22 Feb 2016 00:52:28 +0100, Dimitrij Klingbeil wrote:

As for the BY134,
[...]
Get rid of it, and while you're at it, consider the condition of the
other two (V1808+9) identical ones. Sometimes a person who does an
improper repair will try swapping nearby components hoping that another
one might be "less critical". So if you see signs of unprofessional
manual soldering on them, take that whole trinity and replace them.

Will do. I'm guessing the tech who replaced that diode was solely
concerned with its voltage rating. In all honesty, I'd have been the same
before this speed importance was drawn to my attention in this thread.

Same with C1806. If it looks suspicious, does not pass a withstand test
at some 105% of its rated voltage or shows high ESR, change it too.

That one actually looks fine appearance-wise, but I'll test it
electrically of course.

BY208-1000s are hard to come by nowadays, so here is a list of some more
modern candidates: MUR1100E, BYV26E, UF4007. They should fit, and even
though they are faster than the original BY208-1000, they should work.

Once you've fixed that botched repair on the energy recovery circuit,
connect a taillight lamp to the 12.7 V output, test it again and tell
your results here (make sure you put all the proper parts back in,
before you switch it on, this supply may be unforgiving if any parts are
missing and it's powered on).

Will do. I'll order the parts tomorrow if I can't find any in my spares
bin.

As for the design being "generally unsatisfactory", let me disagree.
Resonant converters do have a well earned place in the world of power
electronics, but the design of them is, in a way, a black art. They have
lots of pitfalls for the unwary and not so many engineers can actually
design them properly and they tend to use special components (inductive
ones in particular) that would be rather unsuitable for other topologies
too. Yet they do have certain benefits, low noise operation that is
suitable for sensitive measurement instruments, being one of them. They
are not so easy to understand, compared to "simple" flyback topology
supplies - so people go screaming "this is too complex" or "this uses
too many parts". In fact your supply's energy recovery circuit is
actually a little unregulated flyback converter of its own! But so far
(and considering the design's day and age), all the parts that I've seen
in that schematic seem to me to have a good reason for their existence.

I read somewhere that resonant converters are poorly understood by
engineers who don't specialise in them and that accurate, detailed
literature on them is not easy to find. So it's very valuable to have
knowledgeable people like yourself and others here who do understand how
they work; otherwise I'd have nowhere to turn for advice on how to
proceed with this!
I'm going to work through the steps you've outlined here and elsewhere
and hope they work. But if the problem remains, I shall definitely be
mothballing it for the foreseeable future. My patience isn't infinite!

 

[JC]

On 2/21/2016 3:59 PM, Cursitor Doom wrote:

On Sun, 21 Feb 2016 20:09:47 +0100, Dimitrij Klingbeil wrote:

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:


Thank you. If you've had the chance to read my follow up to JC (I think
it was) then you'll be aware that one of those BY208 diodes was replaced
by a BY134. If the design is that critical of the speed of the diodes it
uses then maybe it won't function properly as you suggest. I can only
imagine the technician who replaced it was unaware of the critical nature
of the part concerned.
I'm kind of unhappy with this design overall, to be honest. It was
critically appraised on s.e.d and found generally unsatisfactory. I'm
strongly tempted to just save the transformers, junk everything else and
start afresh with a modern design. The rest of the scope is mint and
untouched, it's only the psu section that's been butchered around and
shows signs of burning and scorching in places. Maybe the best thing to
do would be to bin it? :-/

Switching supplies are all designed with the diode (and other component)
parameters in mind, its how they function efficiently. Bunging any old
diode in is asking for trouble. Same as the ESR and temp rating of the
caps used. Seriously you need to sit back and chill. These scopes were
very well designed and I would say exceptionally reliable. I'd like to
see you build a replacement. I've designed switching supplies,
everything needs to be right or it goes wrong fast.

FR107G seems to be the current equivalent for the BY208-1000

 

[JC]

On 2/21/2016 4:28 PM, JC wrote:

On 2/21/2016 3:59 PM, Cursitor Doom wrote:
On Sun, 21 Feb 2016 20:09:47 +0100, Dimitrij Klingbeil wrote:

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:


Thank you. If you've had the chance to read my follow up to JC (I think
it was) then you'll be aware that one of those BY208 diodes was replaced
by a BY134. If the design is that critical of the speed of the diodes it
uses then maybe it won't function properly as you suggest. I can only
imagine the technician who replaced it was unaware of the critical nature
of the part concerned.
I'm kind of unhappy with this design overall, to be honest. It was
critically appraised on s.e.d and found generally unsatisfactory. I'm
strongly tempted to just save the transformers, junk everything else and
start afresh with a modern design. The rest of the scope is mint and
untouched, it's only the psu section that's been butchered around and
shows signs of burning and scorching in places. Maybe the best thing to
do would be to bin it? :-/

Switching supplies are all designed with the diode (and other component)
parameters in mind, its how they function efficiently. Bunging any old
diode in is asking for trouble. Same as the ESR and temp rating of the
caps used. Seriously you need to sit back and chill. These scopes were
very well designed and I would say exceptionally reliable. I'd like to
see you build a replacement. I've designed switching supplies,
everything needs to be right or it goes wrong fast.

FR107G seems to be the current equivalent for the BY208-1000

FR107G Ebay (UK) #390565307743 cheap enough.

 

[Dimitrij Klingbeil]

On 21.02.2016 21:59, Cursitor Doom wrote:

On Sun, 21 Feb 2016 20:09:47 +0100, Dimitrij Klingbeil wrote:

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:

Recheck pin function before jumping to conclusions.

Right; now re-checked. DC measurments proved (unsurprisingly)
too close together so I re-tested using 100khz instead. These
are the impedances WRT ground of the output taps of the long
winding in the order they actually come out of the
transformer: GND, 0.17ohms, 0.17ohms, 0.26ohms, 0.28ohms,
3.7ohms, 3.8ohms. So this doesn't seem to tally up with the
schematic. Or does it? I need a pint of strong coffee to
kick-start my head on this one. :-/ Anyway, later...
... In one of your pictures there are a couple of diodes that
look messed up (V1809 and V1811) near the bridge. They are
supposed to be BY208-1000 (1000v rectifiers), I can see "40" on
one, maybe 1N4007?

If you can't find a BY208-1000 replacement, a MUR4100E(G) should
work.

Sorry, that may be physically too big to fit. A MUR1100EG or
something similar should work and fit in the available space too.

Dimitrij

Thank you. If you've had the chance to read my follow up to JC (I
think it was) then you'll be aware that one of those BY208 diodes was
replaced by a BY134. If the design is that critical of the speed of
the diodes it uses then maybe it won't function properly as you
suggest. I can only imagine the technician who replaced it was
unaware of the critical nature of the part concerned. I'm kind of
unhappy with this design overall, to be honest. It was critically
appraised on s.e.d and found generally unsatisfactory. I'm strongly
tempted to just save the transformers, junk everything else and start
afresh with a modern design. The rest of the scope is mint and
untouched, it's only the psu section that's been butchered around and
shows signs of burning and scorching in places. Maybe the best thing
to do would be to bin it? :-/

Hi

As for the BY134, sorry, I must have overlooked that somehow, or maybe
it did not register in my memory right away. Anyway, it's just as bad a
choice as a 1N4007 and its ilk. It's designed for mains rectification
and doesn't even make an attempt at being fast.

No use in an active snubber or energy recovery circuit whose task it is
to "strip out" the high frequency components from a square wave.

Get rid of it, and while you're at it, consider the condition of the
other two (V1808+9) identical ones. Sometimes a person who does an
improper repair will try swapping nearby components hoping that another
one might be "less critical". So if you see signs of unprofessional
manual soldering on them, take that whole trinity and replace them.

Same with C1806. If it looks suspicious, does not pass a withstand test
at some 105% of its rated voltage or shows high ESR, change it too.

BY208-1000s are hard to come by nowadays, so here is a list of some more
modern candidates: MUR1100E, BYV26E, UF4007. They should fit, and even
though they are faster than the original BY208-1000, they should work.

There are also: RGP30M (slightly large, modest speed), UF5408 (slightly
large), MUR4100E (slightly large), STTH112U (smd), US1M (smd) BYG23M
(smd). They may or may not fit due to size and space constraints, and
the SMD ones would likely need some wire leads soldered on (won't look
professional, but hey, if others are too hard to come by, that's ok).

Once you've fixed that botched repair on the energy recovery circuit,
connect a taillight lamp to the 12.7 V output, test it again and tell
your results here (make sure you put all the proper parts back in,
before you switch it on, this supply may be unforgiving if any parts
are missing and it's powered on).

As for the design being "generally unsatisfactory", let me disagree.
Resonant converters do have a well earned place in the world of power
electronics, but the design of them is, in a way, a black art. They have
lots of pitfalls for the unwary and not so many engineers can actually
design them properly and they tend to use special components (inductive
ones in particular) that would be rather unsuitable for other topologies
too. Yet they do have certain benefits, low noise operation that is
suitable for sensitive measurement instruments, being one of them. They
are not so easy to understand, compared to "simple" flyback topology
supplies - so people go screaming "this is too complex" or "this uses
too many parts". In fact your supply's energy recovery circuit is
actually a little unregulated flyback converter of its own! But so far
(and considering the design's day and age), all the parts that I've seen
in that schematic seem to me to have a good reason for their existence.

Greetings
Dimitrij

 

[Cursitor Doom]

I should perhaps have been more specific and stated that V1811 on the
schematic is the diode that was incorrectly replaced by that lower grade
part.
Anyway, replacements now on order; will report back in a few days.

 

[Cursitor Doom]

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.

Well it seems it *is* far too slow if I understand Dimitrij correctly.
Anyway, I've managed to source one of his suggested substitutes, the
UF4007 type quite cheaply online so we'll find out before the end of this
week if the wrong replacement part has been responsible for the problems
I've experienced.

 

[legg]

On Mon, 22 Feb 2016 10:57:06 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

I should perhaps have been more specific and stated that V1811 on the
schematic is the diode that was incorrectly replaced by that lower grade
part.
Anyway, replacements now on order; will report back in a few days.

A gross failure in this part would blow a fuse, hence it is the least
suspect in that regard only - the fuse doesn't open.

It is actually the only one that is involved in power transfer -
seeing double input voltage stress and peak/average conversion
currents; the other two are snubbers/clamps.

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

RL

 

[Cursitor Doom]

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!

 

[Cursitor Doom]

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.

 

[John-Del]

On Tuesday, February 23, 2016 at 6:26:58 PM UTC-5, 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.

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?

 

[Cursitor Doom]

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.

 

[Dimitrij Klingbeil]

On 21.02.2016 14:46, Cursitor Doom wrote:

On Fri, 19 Feb 2016 22:44:08 +0100, Dimitrij Klingbeil wrote:

But once you have it out and disconnected, please make another
test: apply ca. 15V RMS to the 12.7V winding (to the one where you
measured 3V) instead of to the primary. And check if any isolation
looks like breaking down. Note that the 15V value contains some
compensation for the fact that the power supply uses inductors
after the rectifiers (and therefore the normal winding voltage is
higher than the normal output voltage). That would load the
transformer close to its normal condition and any breakdown should
become apparent.

I did just try this a moment ago, Dimitrij, but doing this just
flattens the output from the sig gen, I'm sorry to say. Hardly
surprising since it's a 600ohm unit and the 12.7V tappings are
0.52ohms 'apart'! To perform this test properly I'd have to adopt
the work-around suggested by another chap here who said use an audio
amp to get the current up. I may well have to do this if it comes to
it. The other problem is, my oscilloscope current probe is lacking a
termination unit so it's readings will be meaningless and I can't
use the true RMS current range on my DVM because it's probably going
to be out of its bandwidth at this frequency range.

Ok, there are other simpler ways to test windings under high voltage

See below for a simple test circuit that would be easily doable with a
few common parts and works like an IWT (impulse winding tester):

<http://imgur.com/2qfjhaX>

It needs a power supply (can be just a mains isolation transformer with
rectifier and capacitor) and it's intended to show the resonance
waveform on an oscilloscope at realistic rated voltage conditions.

The MOSFET (any 400 or 500 V type with less than 1 Ohm Rdson) is driven
with a square wave from a signal generator (frequency should be slow
enough to allow the cap to recharge, some 50 to 100 Hz) and discharges a
capacitor from 320 V (rectified isolated mains) into the inductor under
test. Under discharge conditions the capacitor and the inductor form a
resonant circuit and slowly "ring down".

The resistor heats up with prolonged operation, obviously, since it has
full supply voltage across it, so that's why it's rated 10W.

The waveform is measured (due to the high voltages involved) with a 400
V rated 10:1 oscilloscope probe. It should give a reasonably reliable
indication whether an inductor (or a transformer) is good for use at
full mains voltage or not.

The circuit works similarly to a commercially available IWT and it's
intended to be connected to the primary of a transformer. The waveform
should look like a typical IWT waveform (search for "impulse winding
tester" in Google Images to see what it looks like).

Here's a good looking waveform example:

<http://meguro.com.my/wp-content/uploads/2013/05/Impulse-applied-chart.jpg>

A shorted (or otherwise overloaded) coil will decay very fast or even
hardly resonate at all. A good one will resonate for many cycles.

A failing one with significant corona discharge may look like this:

<http://www.ucetech.com.cn/en/App/Tpl/Home/Uploads//day_150908
/201509081505159156.jpg>

This test should be easy to do, and should be able to settle the
question if the transformer is "shot" with reasonable confidence.

As always, when working with high voltages, pay attention to safety!

Regards
Dimitrij