Hi all,

I'm returning to the scope smps I was working on some time ago before
fitting a new bathroom intervened. I've removed the smps board from the
scope for testing purposes.
On cranking up the supply voltage (230V here) I get to about 150V on my
variac whereupon I can hear a capacitor venting; a clear hissing sound
like air being let out of a tyre. Trouble is, I can't see which cap it is
out of about the dozen or so electrolytics on the board. There's no
visible steam or smoke whatsoever. I've tried using a piece of thin
hollow plastic pipe as a stethoscope and moving around between caps, but
it's nowhere near precise enough to differentiate the cap responsible.
(I'm guessing it's an electrolytic; can't think what else could make that
sound).
Anyway, should I just crank up the voltage to 230 in the hope that it'll
blow altogether and thus be obvious, or is that going to cause collateral
damage (electrical damage I mean not actual physical damage)? I've tested
all the caps in circuit for ESR and capacitance and they read fine, so
this must be something that only materialises at close to working voltage.
I'm all out of ideas. Any suggestions?

In article , says...

On cranking up the supply voltage (230V here) I get to about 150V on
my
variac whereupon I can hear a capacitor venting; a clear hissing sound
like air being let out of a tyre. Trouble is, I can't see which cap it is
out of about the dozen or so electrolytics on the board. There's no
visible steam or smoke whatsoever. I've tried using a piece of thin
hollow plastic pipe as a stethoscope and moving around between caps, but
it's nowhere near precise enough to differentiate the cap responsible.
(I'm guessing it's an electrolytic; can't think what else could make that
sound).

Could it be an electrical discharge? Try viewing in the dark (with
precautions against contact!)...

Mike.

On Thu, 21 Jan 2016 15:10:21 +0000, MJC wrote:

Could it be an electrical discharge? Try viewing in the dark (with
precautions against contact!)...

I like your thinking so I tried it. But no sign of any arcing at all.
Anyway it really does sound like more of a hissing than a rasping.

On Thu, 21 Jan 2016 18:00:23 -0000 (UTC), Cursitor Doom
wrote:

On Thu, 21 Jan 2016 15:10:21 +0000, MJC wrote:

Could it be an electrical discharge? Try viewing in the dark (with
precautions against contact!)...

I like your thinking so I tried it. But no sign of any arcing at all.
Anyway it really does sound like more of a hissing than a rasping.
I have heard high frequency arcs that sound just like air escaping, at
least to my ears.
Eric

On Thu, 21 Jan 2016 10:49:34 -0800, etpm wrote:

I have heard high frequency arcs that sound just like air escaping, at
least to my ears.
Eric

Interesting. What component was failing? There's a 1.5kV transformer on
this board which is encased in opaque grey plastic so if it's something
arcing inside that it won't show up in the dark. I thought perhaps a old
AM radio next to the board might indicate something useful, but I'm
guessing it would be swamped by all the spurious noise these things make
even when they're running right.
Spectrum analyser, perhaps? Bit drastic but I do have one!

Sometimes an smps run at low voltage (or with a defect) will not osc cleanly and make noise like hissing air. I've seen some supplies sound like that when one or more of the secondaries are feeding a short. Most modern designs shut down completely but a lot of the older ones throttled back to a safe current but otherwise would continue to run.

In article ,
Cursitor Doom wrote:

Interesting. What component was failing? There's a 1.5kV transformer on
this board which is encased in opaque grey plastic so if it's something
arcing inside that it won't show up in the dark.

Try the Grand-Dad's Ear Horn approach. Get a length of 1"-diameter
plastic pipe, hold one end against your ear, and move the other end
around the board. This will let you "listen" to the individual
components, while still maintaining a safe distance from any high
voltage.

Seems to me that you could be hearing either:

- Actual venting. I'd be a bit surprised, though, if this were
happening and you couldn't spot the culprit. Maybe fire it up in a
really cold location, and look for a cloud of vapor?

- Switching noise from a failing cap. I've heard SMPS capacitors
begin to hiss like this when they began failing due to the infamous
"'lytic capacitor electolyte plague" a few years ago. They'd go
high-ESR well before they'd start to leak and vent.

Checking all of the caps on that board with an ESR meter (with the
power off and fully discharged, of course) might locate the
culprits.

- High-voltage leakage. I had a Tek 7904 scope which was suffering
from corona discharge and hiss (and an occasional "snap") due to
some contamination/deterioration on the high-voltage lead from the
power supply to the CRT. Cleaning didn't help. Coating the lead
(from the PSU, through the cage) with a bit of high-voltage
silicone putty made the problem go away.

"Cursitor Doom" wrote in message
...
On Thu, 21 Jan 2016 15:10:21 +0000, MJC wrote:

Could it be an electrical discharge? Try viewing in the dark (with
precautions against contact!)...

I like your thinking so I tried it. But no sign of any arcing at all.
Anyway it really does sound like more of a hissing than a rasping.



I would look for anything shorted on the secondary side of the supply. Pulls
the switcher frequency down into the audible range. (for those of us who can
still hear anyway...)

Usually the root cause is a regulation problem (bad caps mostly) causing the
monitored voltage to "appear" low, the switcher duty cycle increases, the
actual DC output rails out, usually shorting a zener diode which gives up
it's life to save components downstream.

Example: an 18 volt zener across a 14 volt line shorts because now that line
has shot up to 35 volts.

Common problem in older consumer vcr's, but 'scopes etc often do the same
thing.

My .02 worth...


Mark Z.

On Fri, 22 Jan 2016 06:45:32 -0600, Mark Zacharias wrote:

I would look for anything shorted on the secondary side of the supply.
Pulls the switcher frequency down into the audible range. (for those of
us who can still hear anyway...)

Usually the root cause is a regulation problem (bad caps mostly) causing
the monitored voltage to "appear" low, the switcher duty cycle
increases, the actual DC output rails out, usually shorting a zener
diode which gives up it's life to save components downstream.

My .02 worth...

Thanks for that. Worth more than 2c I'd say.
In that case I'll check the duty cycle unloaded by scoping the B/E
junction of the chopper transistor. That should be a good indicator of
anything having gone low-res on the secondary side from what you say. I'm
still curious as to why that power resistor is getting so hot. Even if
the tranny is fine something's still amiss somewhere.

"Cursitor Doom" wrote in message
...
what you say. I'm
still curious as to why that power resistor is getting so hot. Even if
the tranny is fine something's still amiss somewhere.

Power resistors often get hot. Can you measure the voltage drop across it
and knowing the resistance calculate the wattage it is using.

"Cursitor Doom" wrote in message
...
Anyway, should I just crank up the voltage to 230 in the hope that it'll
blow altogether and thus be obvious, or is that going to cause collateral
damage (electrical damage I mean not actual physical damage)? I've tested
all the caps in circuit for ESR and capacitance and they read fine, so
this must be something that only materialises at close to working voltage.
I'm all out of ideas. Any suggestions?

Feel the capacitors and find which one is getting hot. An IR heat gun may
be better to keep the shock hazzard down.

On Thu, 21 Jan 2016 10:30:23 -0500, Ralph Mowery wrote:

Feel the capacitors and find which one is getting hot. An IR heat gun
may be better to keep the shock hazzard down.

I'm not really happy about prodding around this thing while it's running
for that reason. I've tried feeling for hot caps after switching it off
again, but at 150V I'm guessing it's not enough to heat the dud cap up
enough to be able to feel the difference. I have tried using a temp
sensor & DVM to check for heating but that proved inconclusive as well.

Cursitor Doom wrote:


On cranking up the supply voltage (230V here) I get to about 150V on my
variac whereupon I can hear a capacitor venting; a clear hissing sound
like air being let out of a tyre.

Well, that could just be the switching action of the power supply. Some
generate pure ultrasonics, but some have lots of other frequencies in them
and the part we can hear sounds like hissing. I've heard lots of SMPS make
hissing sounds under some particular condition (no load, full load,
intermittent current pulses to the load, etc.) Undervoltage might have the
PS jumping back and forth between start-up mode and normal switching mode
very fast. MANY SMPS' will make a hissing sound as the main input cap runs
down, which could be exactly what you are seeing with your variac test.

If you do just put full AC to it, wear ear protectors and don't poke your
face near it, just in case it IS a cap getting ready to blow.

Jon

On Thu, 21 Jan 2016 13:19:44 -0600, Jon Elson wrote:

I've never been in a position to work on a switcher before owing to the
lack of an isolation transformer until very recently, so I was unaware of
that. Well, if push comes to shove and there's no alternative, I'll have
to jack up the juice to 230V and we'll find out then one way or another.

Cursitor Doom wrote:
Hi all,

I'm returning to the scope smps I was working on some time ago before
fitting a new bathroom intervened. I've removed the smps board from the
scope for testing purposes.
On cranking up the supply voltage (230V here) I get to about 150V on my
variac whereupon I can hear a capacitor venting; a clear hissing sound
like air being let out of a tyre. Trouble is, I can't see which cap it is
out of about the dozen or so electrolytics on the board. There's no
visible steam or smoke whatsoever. I've tried using a piece of thin

You're running a switcher at way out of spec input voltage. The magnetics
making awful noisy or hissing sounds is not unexpected under those
conditions as it fails to startup correctly.

On Thu, 21 Jan 2016 22:06:02 +0000, Cydrome Leader wrote:

You're running a switcher at way out of spec input voltage. The
magnetics making awful noisy or hissing sounds is not unexpected under
those conditions as it fails to startup correctly.

Well I've now wound it up to the correct supply volts and it's still
hissing. Nothing's blown that I can see; there's just one power resistor
that's running too hot to touch (because the psu is unloaded?)everything
else, including the transformer is cool. Also, by use of better gauge
pipe, I've been able to detect the sound is coming from the grey 1.5kV
transformer in the middle of the board:

https://www.flickr.com/photos/128859...2/16727519128/

Could this be just normal behaviour that we don't normally hear since
it's normally buried deep within the scope casing??

BTW, those burnt areas are previous problems that someone else fixed
years ago and not relevant to the current issue.

On Fri, 22 Jan 2016 11:30:10 +0000, Cursitor Doom wrote:

Could this be just normal behaviour that we don't normally hear since
it's normally buried deep within the scope casing??

It can be so hard to tell sometimes. Try scoping the output from the
transformer (taking suitable precautions of course!) and look for any
irregularities that might indicate internal arcing.

On Fri, 22 Jan 2016 12:11:31 +0000, Chris wrote:

It can be so hard to tell sometimes. Try scoping the output from the
transformer (taking suitable precautions of course!) and look for any
irregularities that might indicate internal arcing.

Well I've just scoped the B/E junction of the chopper transistor and it's
just ALL noise; no discernable control pulses at all, so clearly this
thing has issues beyond what this medium can assist with, I'm sorry to
say.
Anyway, must do some shopping right now or there'll be hell to pay.

Cydrome Leader wrote:
Cursitor Doom wrote:
Hi all,

I'm returning to the scope smps I was working on some time ago before
fitting a new bathroom intervened. I've removed the smps board from the
scope for testing purposes.
On cranking up the supply voltage (230V here) I get to about 150V on my
variac whereupon I can hear a capacitor venting; a clear hissing sound
like air being let out of a tyre. Trouble is, I can't see which cap it is
out of about the dozen or so electrolytics on the board. There's no
visible steam or smoke whatsoever. I've tried using a piece of thin

You're running a switcher at way out of spec input voltage. The magnetics
making awful noisy or hissing sounds is not unexpected under those
conditions as it fails to startup correctly.

No load could also cause weird noises- that board looks really old. If
that transformer is burned or arcing it's already trashed- more testing
won't make the problem worse, but giving it with no load or with no load
and at a weird input voltage won't help troubleshoot anything easier.

On Fri, 22 Jan 2016 22:28:23 +0000, Cydrome Leader wrote:

No load could also cause weird noises- that board looks really old. If
that transformer is burned or arcing it's already trashed- more testing
won't make the problem worse, but giving it with no load or with no load
and at a weird input voltage won't help troubleshoot anything easier.

Yes, the world of vintage has its pros and cons. The plus side is
everything is big enough to work with - handy if you are half-blind and
clumsy like me. But the con is things get very fragile so you have to
think thrice before disturbing anything (and there's quite a bit on this
board that's been disturbed before due to previous issues.)

On Friday, January 22, 2016 at 5:34:38 PM UTC-5, Cursitor Doom wrote:
On Fri, 22 Jan 2016 22:28:23 +0000, Cydrome Leader wrote:

No load could also cause weird noises- that board looks really old. If
that transformer is burned or arcing it's already trashed- more testing
won't make the problem worse, but giving it with no load or with no load
and at a weird input voltage won't help troubleshoot anything easier.

Yes, the world of vintage has its pros and cons. The plus side is
everything is big enough to work with - handy if you are half-blind and
clumsy like me. But the con is things get very fragile so you have to
think thrice before disturbing anything (and there's quite a bit on this
board that's been disturbed before due to previous issues.)

Interesting problem. Many great suggestions have been put on the table from different members so there is not much to add. The resister that is running too hot is telling us something. Why is that resister not a happy camper? What is it connected to. There is a hiss or possibly plasma arcing and a resister that is not happy. looking at a diagram to see what the unhappy resistor does in the power supply could provide a clue.

On Fri, 22 Jan 2016 15:45:32 -0800, John Heath wrote:

Interesting problem. Many great suggestions have been put on the table
from different members so there is not much to add. The resister that is
running too hot is telling us something. Why is that resister not a
happy camper? What is it connected to. There is a hiss or possibly
plasma arcing and a resister that is not happy. looking at a diagram to
see what the unhappy resistor does in the power supply could provide a
clue.

Yes, a schematic would be extremely useful but I don't have the precise
one; just a similar model of scope with a similar ps section. Some parts
are correct and in the right place, others aren't. You have to work with
what you've got. Hopefully tomorrow I'll discover a few more clues...

On Fri, 22 Jan 2016 15:45:32 -0800 (PST), John Heath
wrote:

On Friday, January 22, 2016 at 5:34:38 PM UTC-5, Cursitor Doom wrote:
On Fri, 22 Jan 2016 22:28:23 +0000, Cydrome Leader wrote:

No load could also cause weird noises- that board looks really old. If
that transformer is burned or arcing it's already trashed- more testing
won't make the problem worse, but giving it with no load or with no load
and at a weird input voltage won't help troubleshoot anything easier.

Yes, the world of vintage has its pros and cons. The plus side is
everything is big enough to work with - handy if you are half-blind and
clumsy like me. But the con is things get very fragile so you have to
think thrice before disturbing anything (and there's quite a bit on this
board that's been disturbed before due to previous issues.)

Interesting problem. Many great suggestions have been put on the table from different members so there is not much to add. The resister that is running too hot is telling us something. Why is that resister not a happy camper? What is it connected to. There is a hiss or possibly plasma arcing and a resister that is not happy. looking at a diagram to see what the unhappy resistor does in the power supply could provide a clue.

It will run hot whenever operation is abnormal.
Just doing its job.

RL