Transformer shot! (was scope SMPS/ capacitor venting)

[Cursitor Doom]

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/128859641@N02/24535280896/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 fucked.
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.

[Dimitrij Klingbeil]

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

[Cursitor Doom]

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.

[jurb...@gmail.com]

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.

[Cursitor Doom]

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.

[legg]

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

[Cursitor Doom]

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.....

[Cursitor Doom]

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...

[legg]

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

[Dimitrij Klingbeil]

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

[JC]

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?

[Cursitor Doom]

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...

[JC]

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/40466580@N07/shares/H24830
https://www.flickr.com/photos/40466580@N07/shares/J18jga

[Cursitor Doom]

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.

[Cursitor Doom]

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.

[Cursitor Doom]

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. :(

[Cursitor Doom]

On Sat, 20 Feb 2016 15:45:34 -0500, JC wrote:

Have you posted this before on another thread related to this? Either you
have or it's deja vu. Sorry if you did, I didn't note it properly.

> 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)

Yeah, those Stocko connectors. On another Philips manual I have for a
different scope, they actually publish the proper values for a dummy load
which would be really helpful to have on the current problem I face.

> 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.

Not sure why you say pull V1812, but here's the waveform I got between
those two points when I did this test a few weeks ago:

https://www.flickr.com/photos/128859641@N02/24538703002/in/photostream/

But then it goes downhill. Here's V1812's C/E junction:

https://www.flickr.com/photos/128859641@N02/24020257733/in/photostream/

And the B/E junction of the main chopper transistor:

https://www.flickr.com/photos/128859641@N02/24538689342/in/photostream/

Not surprising it doesn't work properly with control voltages like that!!!

[JC]

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.

[JC]

Pulling V1812 lets you see the clean output from the driver chip without
all the crap feedback from the transformers. Your frequency looks good.

I got the same crap and ringing/distortion on my PSU without a load. Put
a load on it. switchers don't work off load.

[Cursitor Doom]

On Sun, 21 Feb 2016 09:46:52 -0500, JC wrote:

> 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.

Will a DVM suffice or should I do this with my faithful old analogue AVO?

> 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.

You're a late-comer to this party, so you will be unaware that even when
tested under full working conditions with all the loads plugged in, this
twitcher/switcher still hisses and the 20 Ohm power resistor R1814 (just
below right from the chopper transistor on the schematic) quickly starts
to burn up.

I take your point on the dummy load, though. I must rig one up before
doing any more live testing.

[legg]

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

[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

[Dimitrij Klingbeil]

On 20.02.2016 13:55, Cursitor Doom wrote:

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 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]

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? :-/

[JC]

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]

FR107G Ebay (UK) #390565307743 cheap enough.

[Cursitor Doom]

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.

[Cursitor Doom]

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]

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]

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! :)

[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.

[legg]

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.

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.

[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!

[Dimitrij Klingbeil]

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

[Dimitrij Klingbeil]

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

[Cursitor Doom]

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]

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.

[legg]

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

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

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

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

RL

[Cursitor Doom]

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

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

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

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

[John-Del]

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

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

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

Even today on a slow day, I'll spend a lot more time on something that isn't economically worth the effort just to solve the puzzle. Even us old grizzled veterans aren't immune to such things.

[Cursitor Doom]

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

> Even today on a slow day, I'll spend a lot more time on something that
> isn't economically worth the effort just to solve the puzzle. Even us
> old grizzled veterans aren't immune to such things.

Well I'm pretty old and grizzled myself! Just getting stuck back into
troubleshooting again after a 30yr. lay-off. So much has changed!
Anyway, the plan was to replace one suspect part after another one at a
time and test in between each replacement so as to identify the specific
part which is at fault (the new caps arrived today, btw). However, I only
got as far as replacing that diode (the by134) with Dimitrij's suggested
4007 and *something* has *definitely* changed.
The 20ohm power resistor is warming up *much* more slowly and the hissing
noise has gone. The limiting factor now is not the 20ohm resistor, but
the improvised dummy load (a 30ohm 40W w/w resistor) which gets too hot
long before the 20ohm circuit-board part. In fact even with the dummy
load disconnected, the 20ohm resistor doesn't get hot in a hurry like it
did before. So like I say, *something* has changed and that something can
only be the diode swap. Can't believe it would make that much difference,
surely?
More testing as soon as I find a better DL...

[Cursitor Doom]

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

[legg]

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

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

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

RL

[John-Del]

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

[Dimitrij Klingbeil]

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

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

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

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

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

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

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

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

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

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

Regards
Dimitrij

[legg]

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

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

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

RL

[legg]

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

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

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

RL

[Cursitor Doom]

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

[...]

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

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

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

OK, I follow that...

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

[...]

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

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

[Cursitor Doom]

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

[...]

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

[Dimitrij Klingbeil]

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

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

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

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

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

Now my guess, what may have happened:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Regards
Dimitrij

[Jon Elson]

Lots of stuff in commercial gear runs at 70 C or even hotter. I don't like
to see parts running that hot. Especially in something that might get
buried in a shield housing deep in the bowels of some piece of gear like a
scope. But, 70C is not insanely hot for a power resistor. Of course, I
have NO IDEA how hot it is actually supposed to get.


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

Jon

[Cursitor Doom]

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

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

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

> So, does the scope actually run correctly?

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

[legg]

On Thu, 25 Feb 2016 12:53:04 -0000 (UTC), Cursitor Doom
<cu...@notformail.com> wrote:

<snip>

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

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

Troubleshoot the (unidentified?) signal board.

RL

[MJC]

In article <nalek2$k2e$1...@dont-email.me>, nos...@no-address.com says...

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

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

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

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

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

Mike.

[Cursitor Doom]

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

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

I'm guessing you mean like this:

http://www.ebay.co.uk/itm/SIC-SAFCO-2-2uF-2-5KV-2500V-DC-HIGH-QUALITY-
PAPER-IN-OIL-CAPACITOR-NOS-/151689743255?
hash=item235169cb97:g:N3sAAOSwKrhVXwzP

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

[Cursitor Doom]

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

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

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

> Troubleshoot the (unidentified?) signal board.

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

[Cursitor Doom]

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

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

61.7
12.7
5.8
0
-5.8
-12.7
-62.4

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

The correct figures according to the manual should be:

60
12.7
6
0
-6
-12.7
-60

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

[Cursitor Doom]

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

[...]

Dimitrij, I will have to run these latest checks you suggest next week
now as I have to leave on family matters and have no choice other than
divorce. The PSU is now putting out near-enough the correct voltages in
the 6-60VDC range as required when connected up to the scope and the
transformer is virtually silent. It's just the power resistor heating
that's causing concern. If you think of anything else, please leave your
thoughts here. If not, I'll proceed with your checks on my return.
many thanks again.

[legg]

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

RL

[jurb...@gmail.com]

Are the chopper transistors getting hot ?

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

[Cursitor Doom]

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

> Are the chopper transistors getting hot ?

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

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

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

[jurb...@gmail.com]

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

Then you start disconnecting things.

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

[Cursitor Doom]

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

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

[Mark Zacharias]

"legg" <le...@nospam.magma.ca> wrote in message
news:k32ucb9jaaa5t2ndn...@4ax.com...

> On Thu, 25 Feb 2016 12:53:04 -0000 (UTC), Cursitor Doom
> <cu...@notformail.com> wrote:
>
> <snip>

> Sounds more like you're getting closer to root cause.
>
> Troubleshoot the (unidentified?) signal board.
>
> RL
>

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

mz

[Cursitor Doom]

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

[...]

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

Making progress! :)

[Dimitrij Klingbeil]

Hi

Since you were planning to be away for a while, I was in no hurry to
reply right away. I've seen your other post too, and obviously the
transformer must be ok.

I think that, from the major power-carrying components point of view,
your power supply is now "almost ok". The "power train" clearly works,
otherwise you couldn't get correct output voltages under load.

But the fact that the power resistor still overheats, hints to some
timing being slightly wrong. It can no longer be "completely" wrong, as
was the case with the slow diode, but it's not yet "right" either.


1.

There is still the question with V1808. You said it looks ok, and it
tested ok with a multimeter, but that's not really indicative of its
true behavior under full load at high frequencies. If it has degraded
for any reason ("lost its switching speed") then the resistor R1814
would be running at a higher load than normal. Not many times higher,
but about double or triple. That would be somewhat consistent with your
observation of it running too hot after a few minutes. You should now
have (hopefully) a few spare UF4007s, so if in doubt, replace V1808.

If you find out that the replacement of V1808 makes a (little) change
for the better (slightly lower load on R1814), then replace V1809 too.
It would in this case be likely that those BY208-1000s have all degraded
and became out-of-spec. They all have the same type and age.

Actually it's possible to test the condition of V1808 in circuit,
without replacing it, but the test is tricky. You would need to see, on
an oscilloscope, the voltage waveform across R1814. It should be
basically a flat line, with short surge-like spikes at some 20 kHz
intervals. All the pulses must be polarized in one direction only. The
left-hand pin (on the schematic) of R1814 must be positive. There must
be no spikes in the reverse direction. If there are any (the polarity
would be alternating), then V1808 is degraded and no longer operable at
full speed and needs replacement(, and so does V1809 likely as well).

Unfortunately this test is difficult, because you can't connect a scope
ground to R1814! This is a very fast switching signal that runs at high
power and reaches voltages of some 800 V in normal operation! Even if
you disconnect both mains grounds and "float" both the scope AND the
power supply, and even if you power both the scope AND the power supply
from two SEPARATE isolation transformers in order to increase isolation
and minimize the stray capacitance via mains, this test would still be
very dangerous and I would definitely not advise trying. Using two scope
channels in "subtract" mode might work, but only if you have two high
voltage probes rated for 1 kV, and only if both probes are exactly
identical and the compensation of both channels is precisely matched to
each other (a rather unlikely condition that requires some effort to
achieve). To be honest, to do this test properly, you would need an
isolated high-voltage differential probe. Unless you have one, don't
even bother trying, to replace the diode is easier and much safer.

Ok, so much for the other BY208s in snubber circuits. Replace and see.


2.

The other open question is that of the resonance capacitors (C1807 and
C1808). As I noted in another post, they may be degraded and it may be
difficult to test for this condition properly (LCR meter won't likely
show the problem). Again, if you can get known good spares, they can
easily be replaced, but the spares must be rated for resonant operation.
"Typical" film capacitors are not designed for this use.

Foil capacitors with Polypropylene isolation rated for continuous
resonant duty like the "FKP 1" type should work well here, and so may
"MKP 4C" type too, to some extent, but only the 630 V DC rated ones, and
only if two are used in series like in the original schematic ("MKP 4C"
with lower ratings would hit its high frequency AC limits).

So, "FKP 1" rated at 400 V or 630 V DC (two 33 nF in series) or rated at
1000 V DC (one single 15 nF) or "MKP 4C" rated at 630 V DC (two 33 nF in
series), would be feasible replacement candidates, but not many others
due to the high loading requirement in resonant operation.

If yours turn out to be degraded, and you replace the 30 nF originals
(now probably unobtainable) with 33 nF, you may need to re-adjust the
resonance frequency somewhat.


3.

Also, the frequency adjustment may be slightly out of resonance (maybe
the previous repairer has misadjusted it and component parameters can
also drift over the years). Again, a misadjusted frequency, especially
if it has been set too high rather than too low (compared to the true
resonance frequency of the LC circuit) can cause the dissipation
resistor to overheat (so a little low is better than a little high).

A resonant circuit driven too slow (below resonance), will pull reacive
power (will have a power factor below unity), but the direction of the
phase shift will be inductive. If driven a too fast (above resonance),
it will appear capacitive instead. Please note that the square-to-sine
conversion circuitry, especially the snubbers, will have lower stress
from peak currents when driving an inductive load than when driving a
capacitive load, so an inductive load is "easier" on them.

Please read the instructions in the service manual (I've also copied the
relevant part in my other post), and also note that the service manual
clearly advises to always adjust the frequency "from below" and never
"from above" ("use 170 V mains, then set the trimmer to lowest possible
frequency, and slowly raise it until the output voltage regulation can
just be obtained, but no more than this"). So the designers from Philips
must have preferred this design to run rather slightly below resonance
than slightly above it, and they must have had good reasons to write the
adjustment instructions in such a way, as to prevent an accidental "too
high" frequency setting.

Note that any frequency adjustment should be done with the correct dummy
load connected in order to avoid entering a "light-load" mode.

Regards
Dimitrij

[Dimitrij Klingbeil]

Hi

Noted your progress :)

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

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

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

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

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

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

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

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

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

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

Regards
Dimitrij

[Cursitor Doom]

On Sun, 28 Feb 2016 03:01:02 +0100, Dimitrij Klingbeil wrote:

> 1.
>
> There is still the question with V1808. You said it looks ok, and it
> tested ok with a multimeter, but that's not really indicative of its
> true behavior under full load at high frequencies. If it has degraded
> for any reason ("lost its switching speed") then the resistor R1814
> would be running at a higher load than normal. Not many times higher,
> but about double or triple. That would be somewhat consistent with your
> observation of it running too hot after a few minutes. You should now
> have (hopefully) a few spare UF4007s, so if in doubt, replace V1808.

Yes, I bought 20 of those faster diodes to be on the safe side. :)

> If you find out that the replacement of V1808 makes a (little) change
> for the better (slightly lower load on R1814), then replace V1809 too.
> It would in this case be likely that those BY208-1000s have all degraded
> and became out-of-spec. They all have the same type and age.
>
> Actually it's possible to test the condition of V1808 in circuit,
> without replacing it, but the test is tricky. You would need to see, on
> an oscilloscope, the voltage waveform across R1814.

[live power resistor procedure testing snipped]

Actually I did do this a while back without knowing the risks! As you can
see, I survived to tell the tale. All I was seeing was about 30V of noise
across that resistor but that was before I was informed of the importance
of hooking the supply up to a load, so the test was probably invalid.

> Ok, so much for the other BY208s in snubber circuits. Replace and see.

Certainly can do that, yes.

>
> 2.
>
> The other open question is that of the resonance capacitors (C1807 and
> C1808). As I noted in another post, they may be degraded and it may be
> difficult to test for this condition properly (LCR meter won't likely
> show the problem).

Is there any way of *definitively* testing such a capacitor against all
its possible failure modes? And I'd be interested to know where you get
this figure of 800V you mention from?

> A resonant circuit driven too slow (below resonance), will pull reacive
> power (will have a power factor below unity), but the direction of the
> phase shift will be inductive.

Fortunately this is one aspect I pretty much totally understand. As an
old-style radio ham of more decades than I care to recall, the concepts
of resonance, reactance, impedance, power factor and phase shift are like
second nature so please don't go to any trouble explaining the finer
points in extreme detail; there's absolutely no need. BTW, your
explanations are unusually clear and thorough, I've noticed. If you don't
already, you really should edit or author technical manuals. It's an all-
too rare talent nowadays.

[Cursitor Doom]

On Sun, 28 Feb 2016 15:33:02 +0100, Dimitrij Klingbeil wrote:
>
> But could you please make a complete list of found faults and your
> replacements, and post it here:
>
> I mean, you posted at the very beginning (long before finding the slow
> diode) that you've found and replaced some obviously defective parts,
> but I can't remember if you ever posted, exactly which ones they were.

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

[Dimitrij Klingbeil]

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

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

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

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

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

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

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

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

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

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

I think that the most realistic test would be to sweep the resonant
circuit with a signal generator and watch the waveform. If the resonance
frequency looks right (in the 20 kHz ballpark) and a signal generator is
able to drive it from a high 600 Ohm source impedance to a significant
amplitude without much "sagging" (that is, the resonant circuit presents
little load to the generator), it's probably OK.

Dimitrij

[Dimitrij Klingbeil]

On 28.02.2016 16:18, Cursitor Doom wrote:
>

> [live power resistor procedure testing snipped]
>
> Actually I did do this a while back without knowing the risks! As you
> can see, I survived to tell the tale. All I was seeing was about 30V
> of noise across that resistor but that was before I was informed of
> the importance of hooking the supply up to a load, so the test was
> probably invalid.

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

Dimitrij

[Dimitrij Klingbeil]

On 28.02.2016 19:06, Dimitrij Klingbeil wrote:
> On 28.02.2016 16:18, Cursitor Doom wrote:
>>
>> [live power resistor procedure testing snipped]
>>
>> Actually I did do this a while back without knowing the risks! As
>> you can see, I survived to tell the tale. All I was seeing was
>> about 30V of noise across that resistor but that was before I was
>> informed of the importance of hooking the supply up to a load, so
>> the test was probably invalid.
>
> Also, even with a dummy load connected, the stray capacitance of an
> oscilloscope, when hanging off the loose end of a power circuit with
> some 800 to 900 V worth of HF on it, would probably cause so much
> undue capacitive loading that the power supply circuitry would hardly
> handle it.

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

[Cursitor Doom]

On Sun, 28 Feb 2016 18:55:47 +0100, Dimitrij Klingbeil wrote:

[...]

> I think that the most realistic test would be to sweep the resonant
> circuit with a signal generator and watch the waveform. If the resonance
> frequency looks right (in the 20 kHz ballpark) and a signal generator is
> able to drive it from a high 600 Ohm source impedance to a significant
> amplitude without much "sagging" (that is, the resonant circuit presents
> little load to the generator), it's probably OK.

Thanks again, Dimitrij. You're obviously an expert on the little
understood world of resonant converters so when you say try this or that,
I make a point of paying extra attention. I liked your theory on the
resistor heating due to this supply running out of resonance as a result
of component values changing over time; in fact I'm currently pinning my
hopes on it. It's a pity I'm stuck here for a few more days with my
revolting in-laws but it'll be the first thing I do on my return!

Somewhere I have a big old valve/tube capacitor tester capable of
simulating realistic high voltage working conditions. It'd be interesting
to know what kind of checks it's capable of performing if it's still in
working order and if I can find it among the towering piles of obsolete
test equipment I have here (a couple of million pounds worth of gear at
new prices adjusted for inflation) I may possibly hook it up and give it
a shot.

How about those 'Octopus' component testers? They subject the part under
examination to sweeping test voltages over the expected working range and
you look for any signs of breakdown on an oscilloscope in X=Y mode. I
guess this method is about as good as it gets?

[Cursitor Doom]

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.

[Cursitor Doom]

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!

[Dimitrij Klingbeil]

On 28.02.2016 20:53, Cursitor Doom wrote:
> On Sun, 28 Feb 2016 18:55:47 +0100, Dimitrij Klingbeil wrote:
>
> [...]
>> I think that the most realistic test would be to sweep the resonant
>> circuit with a signal generator and watch the waveform. If the
>> resonance frequency looks right (in the 20 kHz ballpark) and a
>> signal generator is able to drive it from a high 600 Ohm source
>> impedance to a significant amplitude without much "sagging" (that
>> is, the resonant circuit presents little load to the generator),
>> it's probably OK.
>
> Thanks again, Dimitrij. You're obviously an expert on the little
> understood world of resonant converters so when you say try this or
> that, I make a point of paying extra attention. I liked your theory
> on the resistor heating due to this supply running out of resonance
> as a result of component values changing over time; in fact I'm
> currently pinning my hopes on it. It's a pity I'm stuck here for a
> few more days with my revolting in-laws but it'll be the first thing
> I do on my return!

Hi

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

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

> Somewhere I have a big old valve/tube capacitor tester capable of
> simulating realistic high voltage working conditions. It'd be
> interesting to know what kind of checks it's capable of performing
> if it's still in working order and if I can find it among the
> towering piles of obsolete test equipment I have here (a couple of
> million pounds worth of gear at new prices adjusted for inflation) I
> may possibly hook it up and give it a shot.
>
> How about those 'Octopus' component testers? They subject the part
> under examination to sweeping test voltages over the expected
> working range and you look for any signs of breakdown on an
> oscilloscope in X=Y mode. I guess this method is about as good as it
> gets?

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

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

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

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

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

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

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

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

Regards
Dimitrij

[Dimitrij Klingbeil]

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.

[jurb...@gmail.com]

>"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.

[Dimitrij Klingbeil]

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

[Dimitrij Klingbeil]

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

[Cursitor Doom]

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...

[Cursitor Doom]

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.

[Dimitrij Klingbeil]

On 28.02.2016 22:59, Dimitrij Klingbeil wrote:

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

[Cursitor Doom]

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!!

[Cursitor Doom]

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!

[Dimitrij Klingbeil]

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

[jurb...@gmail.com]

>"
>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 ?

[Cursitor Doom]

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'?

[Chris]

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.

[Dimitrij Klingbeil]

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.

[Chris]

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.

[Cursitor Doom]

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.

[Cursitor Doom]

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...

[jurb...@gmail.com]

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.

[legg]

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

[Cursitor Doom]

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.