Capacitor Venting

[Cursitor Doom]

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...

 

[Cursitor Doom]

On Fri, 22 Jan 2016 19:24:31 -0500, JC wrote:

For goodness sake, this is out of the PM3264 right? So what happens when
you put it in the scope?

I'm reluctant to do this until I'm sure the psu won't toast it.

I can't find your original posts on this but
these are not very complicated supplies and pretty reliable. Main fail
is the electrolytic caps, all of them will be useless, Philips blue caps
suck, they were never low ESR in the first place and from your photo it
looks like you haven't replaced any of them.

No, because I've tested all of them with one of these:

http://www.peakelec.co.uk/acatalog/jz_esr70.html

and they all check out totally fine.

The hissing is probably the
1.5Kv from the transformer. Its a good scope if you can get it working.
Buy the manual. http://www.jetecnet.com/

Thankfully I've now managed to get a circuit for the psu section off the
net. That will be a *major* help. I'll be happy to buy the complete
manual if I can get the thing at least partly working.

 

[Cursitor Doom]

On Fri, 22 Jan 2016 19:07:54 -0600, Mark Zacharias wrote:

"Cursitor Doom" <curd@notformail.com> wrote in message
news:n7uj7f$9kp$5@dont-email.me...
On Fri, 22 Jan 2016 19:24:31 -0500, JC wrote:

For goodness sake, this is out of the PM3264 right? So what happens
when you put it in the scope?

I'm reluctant to do this until I'm sure the psu won't toast it.

I can't find your original posts on this but these are not very
complicated supplies and pretty reliable. Main fail is the
electrolytic caps, all of them will be useless, Philips blue caps
suck, they were never low ESR in the first place and from your photo
it looks like you haven't replaced any of them.

No, because I've tested all of them with one of these:

http://www.peakelec.co.uk/acatalog/jz_esr70.html

and they all check out totally fine.

The hissing is probably the 1.5Kv from the transformer. Its a good
scope if you can get it working. Buy the manual.
http://www.jetecnet.com/

Thankfully I've now managed to get a circuit for the psu section off
the net. That will be a *major* help. I'll be happy to buy the complete
manual if I can get the thing at least partly working.




Do look for a short to (cold) ground on the secondary, and if you can
isolate and open up that short (like by removing the shorted zener, if
any) then bring it up briefly on a variac and 'scope the dc line(s) for
noise. Remember to use an isolation transformer when servicing the
primary circuit.

Mark Z.

Thanks, Mark. I'll try that in the morning along with a few other things
as it's getting very late here (1.20am) so I'm going to call it a day for
now.

 

[Ralph Mowery]

"Cursitor Doom" <curd@notformail.com> wrote in message
news:n7u5fn$9kp$1@dont-email.me...

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.

 

[JC]

On 1/22/2016 7:03 PM, Cursitor Doom wrote:

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...

For goodness sake, this is out of the PM3264 right? So what happens when
you put it in the scope? I can't find your original posts on this but
these are not very complicated supplies and pretty reliable. Main fail
is the electrolytic caps, all of them will be useless, Philips blue caps
suck, they were never low ESR in the first place and from your photo it
looks like you haven't replaced any of them. The hissing is probably the
1.5Kv from the transformer. Its a good scope if you can get it working.
Buy the manual. http://www.jetecnet.com/

 

[Mark Zacharias]

"Cursitor Doom" <curd@notformail.com> wrote in message
news:n7uj7f$9kp$5@dont-email.me...

On Fri, 22 Jan 2016 19:24:31 -0500, JC wrote:

For goodness sake, this is out of the PM3264 right? So what happens when
you put it in the scope?

I'm reluctant to do this until I'm sure the psu won't toast it.

I can't find your original posts on this but
these are not very complicated supplies and pretty reliable. Main fail
is the electrolytic caps, all of them will be useless, Philips blue caps
suck, they were never low ESR in the first place and from your photo it
looks like you haven't replaced any of them.

No, because I've tested all of them with one of these:

http://www.peakelec.co.uk/acatalog/jz_esr70.html

and they all check out totally fine.

The hissing is probably the
1.5Kv from the transformer. Its a good scope if you can get it working.
Buy the manual. http://www.jetecnet.com/

Thankfully I've now managed to get a circuit for the psu section off the
net. That will be a *major* help. I'll be happy to buy the complete
manual if I can get the thing at least partly working.

Do look for a short to (cold) ground on the secondary, and if you can
isolate and open up that short (like by removing the shorted zener, if any)
then bring it up briefly on a variac and 'scope the dc line(s) for noise.
Remember to use an isolation transformer when servicing the primary circuit.

Mark Z.

 

[Cursitor Doom]

On Fri, 22 Jan 2016 16:18:21 -0500, Ralph Mowery wrote:

"Cursitor Doom" <curd@notformail.com> wrote in message
news:n7u5fn$9kp$1@dont-email.me...
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.

Sorry for the delay. I'm having to do odd bits towards this project in
between other chores.

OK, it IS a 20 ohm resistor and the voltage across it giving rise to this
heating effect is 3.6V and given that this looks like about a 4W
resistor, makes no sense at all to me. It shouldn't even be getting warm.

The resistor in question is R1814 just to the right of the chopper
transistor on the schematic:

https://www.flickr.com/photos/128859641@N02/24535280896/in/dateposted-
public/

That's all I know right now and I have to dash off again for a couple of
hours....

cheers.

 

[Cursitor Doom]

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.

 

[Cursitor Doom]

On Sat, 23 Jan 2016 21:03:32 +0100, Dimitrij Klingbeil wrote:


Thanks for that, Dimitrij.
Well, I can scope it for a better view of what's happening, because
certainly the voltage readings don't make sense so your explanation rings
true with what I'm seeing here.
I have to say I've never probed one of these before. I'm familiar with
how they work at a block diagram level, but some of the actual circuit
topology is completely alien to me. It's a big obstacle.
So that's another thing to try tomorrow. I'll also recheck the B/E
junction of the chopper with the scope's filtering tweaked; see if I can
get rid of that noise....

 

[Ralph Mowery]

"Cursitor Doom" <curd@notformail.com> wrote in message
news:n808ru$u87$1@dont-email.me...

On Fri, 22 Jan 2016 16:18:21 -0500, Ralph Mowery wrote:

OK, it IS a 20 ohm resistor and the voltage across it giving rise to this
heating effect is 3.6V and given that this looks like about a 4W
resistor, makes no sense at all to me. It shouldn't even be getting warm.

The resistor in question is R1814 just to the right of the chopper
transistor on the schematic:

As the calculated power is less than 3/4 of a watt it should not be getting
hot. It may be that as this looks to be in a high frequency part of the
circuit the meter may not be giving the correct voltage. I would think it
would be some AC value, or maybe puslating DC as there is a diode there.
May show up as a better value if you measure across the coil that the diode
and resistor is across. Might even need a scope to show the true value.

 

[Dimitrij Klingbeil]

On 23.01.2016 17:13, Cursitor Doom wrote:

On Fri, 22 Jan 2016 16:18:21 -0500, Ralph Mowery wrote:

"Cursitor Doom" <curd@notformail.com> wrote in message
news:n7u5fn$9kp$1@dont-email.me...
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.

Sorry for the delay. I'm having to do odd bits towards this project
in between other chores.

OK, it IS a 20 ohm resistor and the voltage across it giving rise to
this heating effect is 3.6V and given that this looks like about a 4W
resistor, makes no sense at all to me. It shouldn't even be getting
warm.

Don't get deceived by a DC voltage measurement on that resistor. It's
very unlikely to be realistic unless you have a true RMS multimeter with
a very wide bandwidth (like hundreds of kHz to lower MHz).

The resistor is in the reset circuit of the inductor L1804. It't there
to "reset" (dissipate the energy of) the field, so it gets to dissipate
whatever this inductor has stored during the switching cycle as soon as
V1806 turns off.

In this circuit, the resistor has a highly uneven load that basically
comes in the form of very narrow and energetic pulses. A DC average
voltage may indeed be low, but the true RMS voltage (and therefore the
equivalent heating value) will not be.

It's likely that the resistor may be getting pulsed with a couple of
amps worth of current (and corresponding voltages according to Ohm's
law), but the pulses will be narrow. Simple voltmeters won't register
that and even some true RMS voltmeters ("typical cheap" ones) may have
insufficient bandwidth to register the pulses accurately.

Dimitrij

 

[Cursitor Doom]

Yes, the voltage across that power resistor looks very different when
scoped. No clean pulses at all; just constant NOISE and lots of it up to
about 20 or more volts so no wonder it was getting hot.
I also scoped the pwm output from the controller chip (TP.2 on the
diagram) and it looks like it's outputting a pulse train but is totally
overwhelmed by noise which I would imagine must be virtually saturating
V1812 resulting in a close to 100% duty cycle at no load.
Clearly major stability issues here.

 

[John-Del]

On Sunday, January 24, 2016 at 10:16:22 AM UTC-5, Cursitor Doom wrote:

Yes, the voltage across that power resistor looks very different when
scoped. No clean pulses at all; just constant NOISE and lots of it up to
about 20 or more volts so no wonder it was getting hot.
I also scoped the pwm output from the controller chip (TP.2 on the
diagram) and it looks like it's outputting a pulse train but is totally
overwhelmed by noise which I would imagine must be virtually saturating
V1812 resulting in a close to 100% duty cycle at no load.
Clearly major stability issues here.

At the risk of raising the ire of some contributors, I've found over my 45 plus years in the trenches that electrolytic capacitors can pass an in circuit ESR test and still be bad. It is an extraordinarily low percentage to be sure (although in Mitsu DM boards it's typical), but it's still a probability.

If I had that supply on the bench, I'd pull every cap and test for ESR, value, leakage, and dielectric absorption.

 

[John Heath]

On Sunday, January 24, 2016 at 10:46:03 AM UTC-5, John-Del wrote:

On Sunday, January 24, 2016 at 10:16:22 AM UTC-5, Cursitor Doom wrote:
Yes, the voltage across that power resistor looks very different when
scoped. No clean pulses at all; just constant NOISE and lots of it up to
about 20 or more volts so no wonder it was getting hot.
I also scoped the pwm output from the controller chip (TP.2 on the
diagram) and it looks like it's outputting a pulse train but is totally
overwhelmed by noise which I would imagine must be virtually saturating
V1812 resulting in a close to 100% duty cycle at no load.
Clearly major stability issues here.

At the risk of raising the ire of some contributors, I've found over my 45 plus years in the trenches that electrolytic capacitors can pass an in circuit ESR test and still be bad. It is an extraordinarily low percentage to be sure (although in Mitsu DM boards it's typical), but it's still a probability.

If I had that supply on the bench, I'd pull every cap and test for ESR, value, leakage, and dielectric absorption.

Good thing this is a hobby project and not a repair with the customer breathing down your neck. Your hot 20 ohms resister goes through a diode then the collector of the main switching transistor. That transistor has a emitter resister that is of interest. Can not make out its part number or value from the schematic. I believe it is responsible for over current feed back to the controller IC for auto shot down protection. I will venture a guess it is in the 1 to 5 ohms range. Knowing it is okay would be a plus. It is tricky to find a ground with the whole controller IC and main switch transistor connected to the bridge rectifiers directly from the AC plug. One false move and there will be more than a hot 20 ohms resister to worry about. However the secondary side of the switching power supply is nice and isolated for easy measurements. I see at least 6 or 7 DC outputs there ranging from 5 to 60 volts. None of these voltages will be correct however by comparing then a general idea of what percentage of their target. If +60 volts is measuring +20 volts then all the rest of the DC outputs should be 33 percent of their target output voltages. By going through the other 6 or so DC outputs you can see if they are reasonable or possible have a problem. I do not think this will pan out but at least you can say you made the measurements. Another last minute desperate grabbing at a straw , try unplugging the 14 K Volt cup off the CRT just in case it is gassy loading down the power supply. Unlikely as you would have seem in glowing but hey you never know.

 

[legg]

On Fri, 22 Jan 2016 15:45:32 -0800 (PST), John Heath
<heathjohn2@gmail.com> 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