Hi all,

I've completed my tests of the main transformer and am now 99% certain
that it is the cause of all the problems I've been experiencing with this
old analogue scope. It's clear there's something very wrong with the
large, multi-tapped output winding. Here's the schematic again:

https://www.flickr.com/photos/128859...in/dateposted-

I removed ALL connections from the transformer. ALL the other output
windings are giving exactly the outputs I would expect from a given
input; it's just the long winding on the lower right hand side that's
giving nonsense outputs. As you can see, the centre tap is grounded and
there are 3 tapping points either side of it. When injected with a 20kHz
sine wave of 50V p-p to the primary winding, the peak-to-peak outputs
from the problem secondary at each tap are as follows (from top to bottom)

13V
13V
3V
0V (gnd)
3V
1.8V
1.8V

I would have expected these voltages to be symmetrical either side of the
0V centre tap, but as you can see, this isn't the case at all. I can only
conclude from this, to use a technical term, that this tranny is ****ed.
If there's something obvious I've overlooked (which I doubt) please feel
free to point it out. Otherwise I'll be opening it up to perform an
autopsy over the weekend.
Thanks again to everyone who tried to help.

On 19.02.2016 20:20, Cursitor Doom wrote:
Hi all,

I've completed my tests of the main transformer and am now 99%
certain that it is the cause of all the problems I've been
experiencing with this old analogue scope. It's clear there's
something very wrong with the large, multi-tapped output winding.
Here's the schematic again:

https://www.flickr.com/photos/128859...in/dateposted-


I removed ALL connections from the transformer. ALL the other
output windings are giving exactly the outputs I would expect from a
given input; it's just the long winding on the lower right hand side
that's giving nonsense outputs. As you can see, the centre tap is
grounded and there are 3 tapping points either side of it. When
injected with a 20kHz sine wave of 50V p-p to the primary winding,
the peak-to-peak outputs from the problem secondary at each tap are
as follows (from top to bottom)

13V 13V 3V 0V (gnd) 3V 1.8V 1.8V

I would have expected these voltages to be symmetrical either side
of the 0V centre tap, but as you can see, this isn't the case at all.
I can only conclude from this, to use a technical term, that this
tranny is ****ed. If there's something obvious I've overlooked
(which I doubt) please feel free to point it out. Otherwise I'll be
opening it up to perform an autopsy over the weekend. Thanks again
to everyone who tried to help.

Hi

If you had a reliable connection to the primary and all secondaries were
in fact free (either the tranny completely removed from the circuit
board or at least no diodes anywhere remaining), then that would mean
the voltages are likely screwed up... But wait:

Are you sure that you have not mixed up the windings? Maybe the two 1.8V
windings are actually the 2 symmetrical "innermost" ones, the 3V ones
are the "medium" ones and the 15V are the "outermost" windings? Your
measured winding voltage ratios "1.8:3.0:13.0 volts" and the schematic
output voltage ratios "6.7:13.4:60.7 volts" (I've added a little
compensation for 0.7V silicon diodes) are (from a purely ratiometric
point of view) not very far away from each other. In fact they are so
close that the differences between the smaller ones can be easily
explained by your measurement errors (how accurate was that 0.8V
measurement anyway?) and the possibly intended uneven loading of the
power rails in the scope.

So, considering the winding connections slightly rearranged, the
transformer looks just fine to me.

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.

Regards
Dimitrij

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

Are you sure that you have not mixed up the windings? Maybe the two 1.8V
windings are actually the 2 symmetrical "innermost" ones, the 3V ones
are the "medium" ones and the 15V are the "outermost" windings? Your
measured winding voltage ratios "1.8:3.0:13.0 volts" and the schematic
output voltage ratios "6.7:13.4:60.7 volts" (I've added a little
compensation for 0.7V silicon diodes) are (from a purely ratiometric
point of view) not very far away from each other. In fact they are so
close that the differences between the smaller ones can be easily
explained by your measurement errors (how accurate was that 0.8V
measurement anyway?) and the possibly intended uneven loading of the
power rails in the scope.

I follow what you're saying, Dimitrij, but for that to be the case, the
tranformer's internal wiring would have to be twisted and I can't see why
they would do that. Admittedly the ground pin is in 'real life' at the
far end of pinouts rather than the centre, but... well, I don't know. If
you're right you must be some kind of genius, that's all I can say.

So, considering the winding connections slightly rearranged, the
transformer looks just fine to me.

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.

OK, you're the boss! I'll carry out that investigation tomorrow and
report back. Maybe I can work out if the internal taps are out of
sequence compared to the schematic by measuring the DC resistance of the
winding at the various taps and... well you get what I mean. Intriguing
idea certainly deserves to be fully explored. Many thanks.

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.

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...ds//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

A short that would drop the voltage on that side of the winding should drop the voltage on the other side due to coupling. Recheck everything, that's what I say.

In fact if you got a power amp that can throw some current maybe inject into the winding that is giving the low voltage and see if it steps up in the other windings. Of course heavy current, but VERY low voltage.

Interestingly I just used my Phase Linear 400 Series Two as such an amp to inject a signal into, believe it or not, the SMPS transformer of a Phillips scope ! Bunch of coincidence, but of no help to you at the moment as it is a totally different model. This one kicks the voltage down to 24 VDC and then feeds the SPMS. I see no connection for a battery but I imagine it could be made to run on batteries.

Wouldn't be bad to have a scope run on like two laptop batteries...

Anyway, I learned to be very careful about condemning transformers. We learn by mistakes and some of us are pretty fart smellers. I hit 191 on an IQ test once, damn, how can I even be alive ?

Umm, I KNOW my IQ is not that high, it was the top I hit when I went on a kick to take alot of online IQ tests. In fact my average was so good out of the about 20 of them I took, I doubt their validity. It was over 135 which is 1 % of the world so really, I doubt it.

At any rate, I would take an audio amp and feed that thing until it runs. You got nothing to lose. reconnect it al and feed it from some nice maybe 100 WPC audio amp with a 10 KHz square wave or something and see what happens.. You have no current limiting now so you follow the smoke.

And BTW, that hosting you're using SUCKS. It does not respond right to my zoom command and it also nags about my browser. I suggest a Dropbox account, AND USING THE /PUBLIC directory. What's more, on Dropbox all your stuff is private, no browsing nor web crawling can find it, you MUST give out the URL by using "Copy Public URL". I highly recommend it. No ads or anything at all. Point the browser and the picture shows up, download it and I can zoom like all hell.

The limitation is like 2GB. You can have really high res photos there. I have had full length movies in mine.

Anyway, you need better resolution to see the diode circuit symbol numbers to know which winding is which because it does not appear to have pin numbers on the transformer.

On Fri, 19 Feb 2016 14:49:34 -0800, jurb6006 wrote:

A short that would drop the voltage on that side of the winding should
drop the voltage on the other side due to coupling. Recheck everything,
that's what I say.

In fact if you got a power amp that can throw some current maybe inject
into the winding that is giving the low voltage and see if it steps up
in the other windings. Of course heavy current, but VERY low voltage.

I must admit the fact that Zs on my most powerful (voltage-wise) sig gen
is 600 ohms was a concern. I would ideally like to have zapped the tranny
with the same voltage and current as its working conditions would expect.
There's always that nagging doubt in my mind about 'what if I'd had more
power to throw at it? Would that show up something useful?'

Umm, I KNOW my IQ is not that high, it was the top I hit when I went on
a kick to take alot of online IQ tests. In fact my average was so good
out of the about 20 of them I took, I doubt their validity. It was over
135 which is 1 % of the world so really, I doubt it.

One can train for an IQ test. A lot of people don't know that, though!

At any rate, I would take an audio amp and feed that thing until it
runs. You got nothing to lose. reconnect it al and feed it from some
nice maybe 100 WPC audio amp with a 10 KHz square wave or something and
see what happens. You have no current limiting now so you follow the
smoke.

That's a very good idea. Must admit I hadn't thought of that dodge!

And BTW, that hosting you're using SUCKS.

Sorry to hear that. I didn't chose Flickr (or whatever it is) I inherited
it from an old Yahoo mail account. If it's that crap, I'll ditch it.

On Fri, 19 Feb 2016 19:20:35 -0000 (UTC), Cursitor Doom
wrote:

Hi all,

I've completed my tests of the main transformer and am now 99% certain
that it is the cause of all the problems I've been experiencing with this
old analogue scope. It's clear there's something very wrong with the
large, multi-tapped output winding. Here's the schematic again:

https://www.flickr.com/photos/128859...in/dateposted-

I removed ALL connections from the transformer. ALL the other output
windings are giving exactly the outputs I would expect from a given
input; it's just the long winding on the lower right hand side that's
giving nonsense outputs. As you can see, the centre tap is grounded and
there are 3 tapping points either side of it. When injected with a 20kHz
sine wave of 50V p-p to the primary winding, the peak-to-peak outputs
from the problem secondary at each tap are as follows (from top to bottom)

13V
13V
3V
0V (gnd)
3V
1.8V
1.8V

I would have expected these voltages to be symmetrical either side of the
0V centre tap, but as you can see, this isn't the case at all. I can only
conclude from this, to use a technical term, that this tranny is ****ed.
If there's something obvious I've overlooked (which I doubt) please feel
free to point it out. Otherwise I'll be opening it up to perform an
autopsy over the weekend.
Thanks again to everyone who tried to help.

Well, at last there is a serious effort to actually record and report
real measurements. However, you may be misleading yourself.

Are you sure of the pin locations and their function on the
transformer? They will not likely correspond to the schematic
arrangement - which is arranged for functional clarity alone.

The transformer pin numbers are not identified on the schematic.

This is why it is much easier to make accurate winding voltage
measurements when the transformer is in-circuit, connecting to easily
identifiable schematic components and circuit nodes.

The voltages you report would be normal if the pin functions were as
listed below

13V.......60VAC
13V.......60VAC
3V........12V5
0V (gnd)
3V........12V5
1.8V......5V
1.8V......5V

The nonlinearity of the ratio is due to the increasing influence of
forward diode drop at lower voltage and the proportional loading
effects of differing currents on rectifiers, windings and output
filtering components.

Recheck pin function before jumping to conclusions.

RL

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

Are you sure of the pin locations and their function on the transformer?

Well I *was* until Dimitrij pointed out this possibility. He changed the
thread title in his follow-up so I guess you missed it. So yes, it's
something I need to further investigate and I shall report back here in
due course with my findings.....

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...

On Sat, 20 Feb 2016 12:55:19 -0000 (UTC), 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...
If the transformer is removed from the board, which seems to be the
case, you can probe the PCB for continuity between known component
leads/schematic nodes and empty PCB transformer pin lands.

Using a logical physical numbering scheme (if one is not allready
present on the actual transformer body), you can assign numbers to the
board and the schematic, for reference.

RL

On Sat, 20 Feb 2016 09:10:13 -0500, legg wrote:

On Sat, 20 Feb 2016 12:55:19 -0000 (UTC), 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...
If the transformer is removed from the board, which seems to be the
case, you can probe the PCB for continuity between known component
leads/schematic nodes and empty PCB transformer pin lands.

Using a logical physical numbering scheme (if one is not allready
present on the actual transformer body), you can assign numbers to the
board and the schematic, for reference.

RL

I was really struggling trying to match up the pins to their particular
outputs; fortunately JC has has posted the pin-outs for this transformer
and saved me some brain cells (I can't afford to lose any more). Seems
the voltages I'm getting are not far off what they should be after all.

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...

Inductive reactance goes up with the square of the winding's nominal
voltage, so you take the square roots of your impedance values.

If you then take into account that the lower impedance values are
strongly dominated by the DC resistance (which stays linear and does not
square) and the upper one is mostly dominated by the AC reactance (which
does square), the ratios seem to look just fine (well, so far as I can
see, within a reasonable margin of error).

But the ratios don't tell the whole story. Even if there is a winding
short, all impedances will be very low (which they sort-of are, I would
have expected higher values everywhere, but then 100 kHz is maybe too
high, try testing at 10 kHz and see...), but the ratios between the
windings would be still be mostly correct.

Try to run it on higher voltage (like 15 V applied to 12.7 V secondary),
and see if it pulls excessive current and warms up. That would indicate
damage more clearly.

Dimitrij

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...


The transformer secondaries go (on the left side viewed from the top of
the transformer).

1.5kV
1.0kV
Gap/no pin
HV Common
60
60
12
12
6
6
0

As others have pointed out this PSU will not run happy without a load
and I don't know what would be suitable. When I worked on these I always
just left the psu connected to the scope. Lets face it, the scopes been
turned on at some point with the psu connected so its not going to do
much more damage and at least you will know the loading is correct. The
EHT multipliers on these break down internally on these.

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?

All noted, thank you, gentlemen. I'll have to check those tips out
tomorrow or a divorce will be in the offing.
Until I report back tomorrow then, thanks...

On 2/20/2016 11:52 AM, Cursitor Doom wrote:
All noted, thank you, gentlemen. I'll have to check those tips out
tomorrow or a divorce will be in the offing.
Until I report back tomorrow then, thanks...


I managed to find a PM3264 PSU to try out.

Unloaded it squeals as expected so I tried a makeshift load with what I
had lying around, 6 x 470R 5Watt w/w resistors.

You can pull one of the connectors out of the scope for a connection.
(See photos)

Just for fun (and I'm running this off an isolation transformer), pull
V1812 and scope T2 with T1 as probe ground. you should see a nice drive
waveform for a few seconds and you can check the frequency is 20KHz.

Incidentally the core on L1806 on this board was loose (came apart) and
also caused squealing but of a different note.

If you want me to take any readings let me know, nothing too time
consuming though

Photos of load (It gets hot so take care)

https://www.flickr.com/photos/404665.../shares/H24830
https://www.flickr.com/photos/404665.../shares/J18jga

On Sat, 20 Feb 2016 09:36:18 -0500, JC wrote:

The transformer secondaries go (on the left side viewed from the top of
the transformer).

1.5kV 1.0kV Gap/no pin HV Common 60 60 12 12 6
6
0

Well on that basis there may be nothing wrong after all.

As others have pointed out this PSU will not run happy without a load
and I don't know what would be suitable. When I worked on these I always
just left the psu connected to the scope. Lets face it, the scopes been
turned on at some point with the psu connected so its not going to do
much more damage and at least you will know the loading is correct. The
EHT multipliers on these break down internally on these.

It's not possible to test this board with it connected to the scope. On
this model, it slots inside the two main signal boards which make access
under proper, full working conditions impossible. Just *another* obstacle
I've faced with this repair.
The EHT multiplier has been totally disconnected all through my tests
except where explicitly stated otherwise.


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.

On 2/21/2016 8:36 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 09:36:18 -0500, JC wrote:


It's not possible to test this board with it connected to the scope. On
this model, it slots inside the two main signal boards which make access
under proper, full working conditions impossible. Just *another* obstacle
I've faced with this repair.
The EHT multiplier has been totally disconnected all through my tests
except where explicitly stated otherwise.

Hi, Its been some time since I worked on these but I'm pretty sure we
ran these with the board out, turned round so you can get the connectors
on and I guess without the HT connected. Alternately put a suitable load
on the PSU.


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.


That might be one problem, the sine voltage around T1801 is 800v, your
BY134 is a 600V diode. Also HV diodes can go reverse leaky, try a high
ohmsmeter on it (10-20 meg range). Shouldn't be any reverse leakage.

I guess you saw my next post on this? Try a load on the board before you
do any more work. It will tell you if the PSU runs silent or not under
load. The one I tried was screaming like heck then silent with a load.

On Sun, 21 Feb 2016 13:36:49 -0000 (UTC), Cursitor Doom
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

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

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

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

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.

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.

Are you sure that you have not mixed up the windings? Maybe the two 1.8V
windings are actually the 2 symmetrical "innermost" ones, the 3V ones
are the "medium" ones and the 15V are the "outermost" windings? Your
measured winding voltage ratios "1.8:3.0:13.0 volts" and the schematic
output voltage ratios "6.7:13.4:60.7 volts" (I've added a little
compensation for 0.7V silicon diodes) are (from a purely ratiometric
point of view) not very far away from each other. In fact they are so
close that the differences between the smaller ones can be easily
explained by your measurement errors (how accurate was that 0.8V
measurement anyway?) and the possibly intended uneven loading of the
power rails in the scope.


complete pcb