Transformer shot! (was scope SMPS/ capacitor venting)

[Dimitrij Klingbeil]

On 09.03.2016 00:10, Jeroni Paul wrote:

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

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

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

 

[JC]

On 3/8/2016 4:31 PM, Cursitor Doom wrote:

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

Waste of breath.

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

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

OMFG

 

[JC]

On 3/8/2016 1:43 PM, Cursitor Doom wrote:

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

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

Yes, they're fine. It's not *my* opinion that this is a poor design! I
posted the schematic to s.e.d and the designers there told me that. I've
said all along I know nothing about this type of PSU so I defer to those
better qualified.

Yes, I read your posts on sed, you didn't tell them this was a Philips
scope, and some replies implied Philips made televisions, jeez.

Philips/Fluke made excellent test equipment and many products beat
Tektronix into the ground but being European never made it across the
pond in large numbers. I've worked in both design and repair facilities
and the folk who couldn't repair ANYTHING were the design guys because
they always spent 3 days criticizing the original design, another week
debating it with their colleagues and then gave it to a repair tech to
fix (in about 30 minutes). You obviously fit into the design mentality,
so good luck and give the scope to someone who will appreciate it.

 

[Cursitor Doom]

On Wed, 09 Mar 2016 00:20:35 +0100, Dimitrij Klingbeil wrote:

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

Hi Dimitrij,
Actually I did unsolder it in the the end as I wanted to inspect the
underside for any clues as its specifications. Sadly there were none
visible all around the device. Clearly all was not well with it and I'm
inclined to believe that it's not an original part; just another poor
replacement.
When I removed it from circuit I was happy to give it 2W worth of power
and see what happened. It just got hotter and hotter until I had to pull
the plug at 120'C because my probe won't go any higher. In contrast, a
known good 20ohm 2W resistor fed with the same 6.3V rose to 70'C and
stopped at that. I have now replaced the old brown 'thing' with a 6W
ceramic power resistor which after half an hour of running hooked up to
the scope and under load only gets up to 40'C which I'm entirely happy
with. The transformer is making a nice gentle, barely audible hiss; not
that rasping sound it used to before your diode was replaced; all the DC
Vouts are fine and there's no acrid scorching smell coming from the board
any more; just a pleasant warm 'smell' of electronics behaving
themselves.
I now feel confident enough to re-insert the board and connect up the VHT
and EHT and heater supplies - then we'll see if there's any life in the
tube! But that will have to wait until tomorrow or I'm looking at divorce
again, apparently.
Later....

 

[Cursitor Doom]

On Wed, 09 Mar 2016 16:16:19 -0500, legg wrote:

From previous traffic on this issue:

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

So you're saying these ARE the original factory parts? Wow!

"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.

I think I can now see how that first diode met its end. Having its own
radiant 1 bar fire juxtaposed to it would have lowered its junction
barrier height - we all remember how temperature affects p-n junctions -
enabling increased current flow and the eventual destruction of the diode
through thermal runaway. It was asking for trouble having that resistor
running at such a high temperature in this particular position.

BTW, there's an identically-sized power resistor at right angles to the
one I've now binned and after a half hour's running, it tops out at only
33'C. I know it's got different working conditions, but in this case it
was correctly rated for its job - unlike the other one. Looks like a
design error after all, then.

Larger bodied resistors with the same power ratings may run at a lower
body surface temperature, but this will be of little value if their bulk
results in reduced air flow or physical contact with nearby components.

Not a problem with the new component.

This position should not be filled with a wire-wound component.
Metal glaze, at certain values and body sizes, offers a reduced self
inductance, allowing them to function in higher frequency snubber
circuitry.

Yes, I'm aware of the perils of parasitic inductance in w/w resistors;
the one I've selected doesn't suffer from it.

Be nice to see some traces tomorrow (one can but hope!)

 

[Cursitor Doom]

On Wed, 09 Mar 2016 18:31:44 -0500, legg wrote:

Unless you've actually cracked one open, you will not be able to tell
what the element construction is.

No need. I have a VNA. Better than x-ray vision.

 

[Cursitor Doom]

On Wed, 09 Mar 2016 23:57:37 +0100, Dimitrij Klingbeil wrote:

> Looks like you've got it going back to normal operation, finally!

I know! It just seems so weird having the thing running like a top for
the first time, not having to worry about overheating, not having that
horrible burning smell and the rasping noise - plus all the voltages
where they should be.
But I couldn't have done it without you, Dimitrij. I'm enormously
grateful for your assistance and your lengthy explanations have been a
huge help to me in getting to grips with this 'alien technology' LOL.

Anyway, it's late here now so I'll report back tomorrow for what will
hopefully be the last time.

 

[Julian Barnes]

On Thu, 10 Mar 2016 00:04:29 +0000, Cursitor Doom wrote:

But I couldn't have done it without you, Dimitrij. I'm enormously
grateful for your assistance and your lengthy explanations have been a
huge help to me in getting to grips with this 'alien technology' LOL.

Amen to that. Been quietly following this from the start and have learned
a lot.
Dimitri, are you a professional SMPS designer by any chance? I admire
your carefully-considered 'measure twice, cut once' approach.

 

[legg]

On Wed, 9 Mar 2016 17:48:50 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

On Wed, 09 Mar 2016 00:20:35 +0100, Dimitrij Klingbeil wrote:

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


Hi Dimitrij,
Actually I did unsolder it in the the end as I wanted to inspect the
underside for any clues as its specifications. Sadly there were none
visible all around the device. Clearly all was not well with it and I'm
inclined to believe that it's not an original part; just another poor
replacement.
When I removed it from circuit I was happy to give it 2W worth of power
and see what happened. It just got hotter and hotter until I had to pull
the plug at 120'C because my probe won't go any higher. In contrast, a
known good 20ohm 2W resistor fed with the same 6.3V rose to 70'C and
stopped at that. I have now replaced the old brown 'thing' with a 6W
ceramic power resistor which after half an hour of running hooked up to
the scope and under load only gets up to 40'C which I'm entirely happy
with. The transformer is making a nice gentle, barely audible hiss; not
that rasping sound it used to before your diode was replaced; all the DC
Vouts are fine and there's no acrid scorching smell coming from the board
any more; just a pleasant warm 'smell' of electronics behaving
themselves.
I now feel confident enough to re-insert the board and connect up the VHT
and EHT and heater supplies - then we'll see if there's any life in the
tube! But that will have to wait until tomorrow or I'm looking at divorce
again, apparently.
Later....

From previous traffic on this issue:

"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.

"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."

Larger bodied resistors with the same power ratings may run at a
lower body surface temperature, but this will be of little value
if their bulk results in reduced air flow or physical contact
with nearby components.

This position should not be filled with a wire-wound component.
Metal glaze, at certain values and body sizes, offers a reduced
self inductance, allowing them to function in higher frequency
snubber circuitry.

RL

 

[Dimitrij Klingbeil]

On 09.03.2016 18:48, Cursitor Doom wrote:

On Wed, 09 Mar 2016 00:20:35 +0100, Dimitrij Klingbeil wrote:

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

When I removed it from circuit I was happy to give it 2W worth of
power and see what happened. It just got hotter and hotter until I
had to pull the plug at 120'C because my probe won't go any higher.
In contrast, a known good 20ohm 2W resistor fed with the same 6.3V
rose to 70'C and stopped at that. I have now replaced the old brown
'thing' with a 6W ceramic power resistor which after half an hour of
running hooked up to the scope and under load only gets up to 40'C
which I'm entirely happy with. The transformer is making a nice
gentle, barely audible hiss; not that rasping sound it used to before
your diode was replaced; all the DC Vouts are fine and there's no
acrid scorching smell coming from the board any more; just a pleasant
warm 'smell' of electronics behaving themselves.

Hi

Looks like you've got it going back to normal operation, finally!

The whole thing might just have been victim of a resistor with a "hot
channel" (old carbon resistors will sometimes do this without reason).

Or an improperly replaced one - likely the same result. No matter if it
broke down with a low resistance path or if it got replaced improperly.

You can check if that was the case (if you still kept the resistor) by
hooking it up to a power supply with an ammeter. If you see it heat up
and then suddenly the amperage rising and the resistor starting to heat
even faster (as if that was an NTC in there) then it very likely has
developed a hot channel (a small area not quite unlike an arc path that
conducts lots of current as it overheats).

If that's the case, it may explain several things. For once, this
resistor was intended to provide a field reset function for the
inductor. The circuit would likely require the reset to be quite
"complete" (the field decaying to zero) between switching cycles.

If the resistance is lower than normal, the field will decay slower, and
a resistor with a hot channel would likely slow the winding reset long
enough to be "incomplete" - that is, by the time that the next switching
cycle arrives, there would be still significant remaining current
flowing through the inductor and the diode.

This can be a condition that the power circuit might not like. There
is only a moderately fast diode in there - the original BY208 is not
really fast. As long as the winding reset is complete, that might not
matter because the diode would already be "out of charge carriers" when
the next pulse arrives, so there would not be much reverse recovery.

If the reset happens to be incomplete because of the resistor breaking
down, then the diode would still be conducting when the pulse comes and
that would bring its (quite long) reverse recovery time into equation.
During that reverse recovery time the diode would pass current and the
resistor (and subsequent circuitry too, up to the resonance LC) would be
getting "hammered" with large 800 V voltage spikes from the switcher,
thus it would be subjected to high peak currents because of the spikes.

Of course, hammering the resistor with enormous voltage spikes won't be
very good for its reliability, and since it was already breaking down,
would also accelerate the process a lot (and stress other parts too).

Dimitrij

 

[legg]

On Wed, 9 Mar 2016 22:37:40 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

On Wed, 09 Mar 2016 16:16:19 -0500, legg wrote:
snip

This position should not be filled with a wire-wound component.
Metal glaze, at certain values and body sizes, offers a reduced self
inductance, allowing them to function in higher frequency snubber
circuitry.

Yes, I'm aware of the perils of parasitic inductance in w/w resistors;
the one I've selected doesn't suffer from it.

Unless you've actually cracked one open, you will not be able to tell
what the element construction is.

RL

 

[Cursitor Doom]

On Wed, 09 Mar 2016 23:57:37 +0100, Dimitrij Klingbeil wrote:

> Looks like you've got it going back to normal operation, finally!

Yes! I can confirm it is now back up and running and early checks don't
seem to show any other faults. I've got four nice bright sharp traces and
all the buttons and knobs seem to do what they should (although I haven't
exhaustively tested *every* single function at this time, of course).
The only error I made was switching it back on from 'cold' - it clearly
hasn't been on for a long time and I really should have brought it back
to life on a variac but I forgot. There were some initial crackling
noises and whatnot but they gradually subsided; some noisy knobs
obviously as only to be expected, requiring lubing up but nothing else
noteworthy.

Many thanks again to all who assisted. The kids at the local school
electronics club will soon have a new toy to add to their test gear.

 

[Cursitor Doom]

On Thu, 10 Mar 2016 11:14:39 -0500, JW wrote:

On Wed, 9 Mar 2016 23:57:37 +0100 Dimitrij Klingbeil
nospam@no-address.com> wrote in Message id:
nbq9jc$tkv$1@dont-email.me>:

You can check if that was the case (if you still kept the resistor)

One thing I've learned the hard way. NEVER throw out any replaced
components until the repair is done!

Good advice, but it's all fixed now (see my other post of a moment ago).

 

[Cursitor Doom]

On Thu, 10 Mar 2016 12:00:41 -0500, legg wrote:

> and everyone has a pair of pliers, or a hammer.

Yes, but in the process of investigating the part, you destroy it.

What did the network analyzer say about this particular resistor?
What's the impedance and phase angle between 10MHz and 100MHz?

Why on earth would I need to know that?? The frequency of operation here
as you well know is only 20kHz.

> You didn't use it previously on the caps and transformers.....?

Sorry, was there a cap I needed to know the SRF of? As for the
transformer, what would the VNA have told me that the other metrology
didn't? It was testing under the high working voltage levels I'd have
liked at the time to check for breakdown; no VNA I know of will do that.

 

[Julian Barnes]

On Thu, 10 Mar 2016 12:00:41 -0500, legg wrote:

and everyone has a pair of pliers, or a hammer.

What did the network analyzer say about this particular resistor?
What's the impedance and phase angle between 10MHz and 100MHz?

You didn't use it previously on the caps and transformers.....?

RL

I should quit while you're still only slightly behind because you are
beginning to sound like a troll. You know what they say: "when you're in
a hole, stop digging."

 

[JW]

On Wed, 9 Mar 2016 23:57:37 +0100 Dimitrij Klingbeil
<nospam@no-address.com> wrote in Message id: <nbq9jc$tkv$1@dont-email.me>:

>You can check if that was the case (if you still kept the resistor)

One thing I've learned the hard way. NEVER throw out any replaced
components until the repair is done!

 

[legg]

On Wed, 9 Mar 2016 23:37:00 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

On Wed, 09 Mar 2016 18:31:44 -0500, legg wrote:

Unless you've actually cracked one open, you will not be able to tell
what the element construction is.

No need. I have a VNA. Better than x-ray vision.

and everyone has a pair of pliers, or a hammer.

What did the network analyzer say about this particular resistor?
What's the impedance and phase angle between 10MHz and 100MHz?

You didn't use it previously on the caps and transformers.....?

RL

 

[legg]

On Thu, 10 Mar 2016 18:39:36 -0000 (UTC), Julian Barnes
<jb9889@notformail.com> wrote:

On Thu, 10 Mar 2016 12:00:41 -0500, legg wrote:

and everyone has a pair of pliers, or a hammer.

What did the network analyzer say about this particular resistor?
What's the impedance and phase angle between 10MHz and 100MHz?

You didn't use it previously on the caps and transformers.....?

RL

I should quit while you're still only slightly behind because you are
beginning to sound like a troll. You know what they say: "when you're in
a hole, stop digging."

Seeing as how everything's perfect now, I can only agree with you.

I just hope the little tykes at the orphings home enjoy their new toy
and appreciate all the hard work and brain power went into its
restoration.

Halleluyah!

RL

 

[EdithLeman]

'Cursitor Doom[_4_ Wrote:

Hi all,

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

http://tinyurl.com/gmgxkt6

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 an

there are 3 tapping points either side of it. When injected with a 20kH

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 t
bottom)

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

I would have expected these voltages to be symmetrical either side o
the
0V centre tap, but as you can see, this isn't the case at all. I ca
only
conclude from this, to use a technical term, that this tranny i
****ed.
If there's something obvious I've overlooked (which I doubt) please fee

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.

Are you sure that you have not mixed up the windings? Maybe the tw
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


--
EdithLeman