Murphy A146CM Output Transformer damping. - UK Vintage Radio Repair and Restoration Discussion Forum

Nanozeugma · 18th Feb 2020, 12:47 pm

Morning all

I come in search of insight...
Just finished refurbishing the power chassis of a Murphy A146CM.
All the rubber insulated wiring replaced.
All components replaced (most resistors outside marked tolerance.)

This is the final chapter as I'd previously rebuilt the RF chassis.

I now subjected the receiver to a soak test of a few hours, driving it
from a valve FM tuner via the gram sockets as MW and LW reception is
very poor (AM does work but interference where I live is atrocious.)

All went well and I left it on for several hours without issue.

However, when I inspected the power chassis components later, I noticed that R36 across the output transformer primary appeared distressed.

This is most curious as the original 15K 1W carbon composition was one of the few near their marked value and appeared undamaged. The replacement is a 3W carbon film resistor.

Any thoughts as to why the replacement appears to have overheated?
Images attached. T.I.A.

IanG3XYV · 18th Feb 2020, 12:55 pm

Have you measured the resistor? Think it may be 1K5.

Nanozeugma · 18th Feb 2020, 1:04 pm

Quote:

Originally Posted by IanG3XYV

Have you measured the resistor? Think it may be 1K5.

Yes. 14.89K four band markings plus tolerance band (1%.)

Cobaltblue · 18th Feb 2020, 1:17 pm

Curious the only way I can think this might happen would be if the amplifier were oscillating at a supersonic frequency or there is far higher high frequency content in the source?

I think I would stick a scope on it

Cheers

Mike T

crackle · 18th Feb 2020, 1:52 pm

The colour code follows the body, tip, spot/stripe convention. The silver on this type of resistor was 10% tolerance, I would say the original resistor was easily within tolerance and you may as well put it back into service.
Have you measured the value of the resistor you replaced it with.

Mike

Nanozeugma · 18th Feb 2020, 1:58 pm

Quote:

Originally Posted by crackle

I thought silver on this type of resistor was 10% tolerance, I would say the original resistor was easily with in tolerance and you may as well put it back into service.

Mike

I believe I may not have made myself clear (?)
My observation "Yes. 14.89K four band markings plus tolerance band (1%.)" refers to the replacement fitted, not the original where the silver band does indeed reflect 10% tolerance.

John10b · 18th Feb 2020, 3:14 pm

I think what Mike T,Cobalt Blue, says makes sense. In the quiescent state no current should pass through the resistor, or I’m a I way off, so does it get hot at all times?
As Mike T has indicated the circuit is unstable and oscillating, or have I got it all wrong?
John

Nanozeugma · 18th Feb 2020, 3:34 pm

A couple of points.
I haven't scoped it yet, but there is certainly no audible instability or distortion.
There is about 1.0v difference in cathode voltages between the output valves,
probably different emission values.
I have tried a different type of resistor replacement as an experiment,
so far no sign of distress and it is running cool to the touch.
As before, being driven by a Heathkit FM-4U via the gram sockets.

crackle · 18th Feb 2020, 4:18 pm

Your original replacement resistor may have been a metal film with a spiral element.
Perhaps that was resonating in tune with the cap.
Mike

Graham G3ZVT · 18th Feb 2020, 4:22 pm

19kHz pilot-tone from the source perhaps?

Nanozeugma · 18th Feb 2020, 4:26 pm

Quote:

Originally Posted by crackle

Your original replacement resistor may have been a metal film with a spiral element.
Perhaps that was resonating in tune with the cap.
Mike

The thought had crossed my mind that perhaps the construction of the overheating resistor was inductive...

PJL · 18th Feb 2020, 5:00 pm

Quote:

Originally Posted by Nanozeugma

The thought had crossed my mind that perhaps the construction of the overheating resistor was inductive...

Inductors do not lose energy, they store and release it.

I like the pilot tone idea!

But I do wonder if the resistor is wired in correctly as it would take 122V @ 8mA to cause 1W dissipation in the resistor.

crackle · 18th Feb 2020, 6:08 pm

PLJ, are you saying that an AC current passing through a coil produces no heat?
I didn't realise that.

Mike

Herald1360 · 18th Feb 2020, 6:47 pm

Only if the coil is superconducting. Otherwise IsquaredR applies to the resistance of the coil. If you have a parallel resonant circuit however, the LC circulating current may be many times the current in and out. This can have spectacular effects in poorly designed choke feed circuits in a transmitter PA stage

Nanozeugma · 18th Feb 2020, 9:19 pm

The chassis has been powered and working all day with the replacement resistor. Resistor runs cold to the touch, no sign of distress. Most peculiar...

MotorBikeLes · 18th Feb 2020, 10:46 pm

The first replacement resistor probably spiral metal film as already suggested. Your second replacement looks like metal oxide to me. assuming that is correct, and if that is the only difference then I would assume that the first replacement must be inductive, unlike the original carbon and metal oxide ones. Discuss.
Les.

ajgriff · 18th Feb 2020, 11:00 pm

I know it's not relevant but I can't help commenting on the very impressive macro photography.

Alan

PJL · 18th Feb 2020, 11:50 pm

It seems very unlikely that the very small inductance of a resistor would be sufficient to cause instability but that appears to be the only explanation so far. I would keep an eye on it as it might be intermittent. The FM 19KHz pilot tone is pretty much killed off by the de-emphasis so very unlikely to be that.

18th Feb 2020, 12:47 pm	#1
Nanozeugma Heptode Join Date: Nov 2017 Location: London, UK. Bury, Lancashire quite regularly :) Posts: 611	Murphy A146CM Output Transformer damping. Morning all I come in search of insight... Just finished refurbishing the power chassis of a Murphy A146CM. All the rubber insulated wiring replaced. All components replaced (most resistors outside marked tolerance.) This is the final chapter as I'd previously rebuilt the RF chassis. I now subjected the receiver to a soak test of a few hours, driving it from a valve FM tuner via the gram sockets as MW and LW reception is very poor (AM does work but interference where I live is atrocious.) All went well and I left it on for several hours without issue. However, when I inspected the power chassis components later, I noticed that R36 across the output transformer primary appeared distressed. This is most curious as the original 15K 1W carbon composition was one of the few near their marked value and appeared undamaged. The replacement is a 3W carbon film resistor. Any thoughts as to why the replacement appears to have overheated? Images attached. T.I.A. Attached Thumbnails __________________ Thermionic Emission, warms the cockles of your tubes.

18th Feb 2020, 12:55 pm	#2
IanG3XYV Pentode Join Date: Sep 2014 Location: Bedford, Bedfordshire, UK. Posts: 174	Re: Murphy A146CM Output Transformer damping. Have you measured the resistor? Think it may be 1K5.

18th Feb 2020, 1:17 pm	#4
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,857	Re: Murphy A146CM Output Transformer damping. Curious the only way I can think this might happen would be if the amplifier were oscillating at a supersonic frequency or there is far higher high frequency content in the source? I think I would stick a scope on it Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

18th Feb 2020, 1:52 pm	#5
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: Murphy A146CM Output Transformer damping. The colour code follows the body, tip, spot/stripe convention. The silver on this type of resistor was 10% tolerance, I would say the original resistor was easily within tolerance and you may as well put it back into service. Have you measured the value of the resistor you replaced it with. Mike Last edited by crackle; 18th Feb 2020 at 1:59 pm.

18th Feb 2020, 3:14 pm	#7
John10b Nonode Join Date: Nov 2016 Location: Aberaeron, Ceredigion, Wales, UK. Posts: 2,882	Re: Murphy A146CM Output Transformer damping. I think what Mike T,Cobalt Blue, says makes sense. In the quiescent state no current should pass through the resistor, or I’m a I way off, so does it get hot at all times? As Mike T has indicated the circuit is unstable and oscillating, or have I got it all wrong? John

18th Feb 2020, 3:34 pm	#8
Nanozeugma Heptode Join Date: Nov 2017 Location: London, UK. Bury, Lancashire quite regularly :) Posts: 611	Re: Murphy A146CM Output Transformer damping. A couple of points. I haven't scoped it yet, but there is certainly no audible instability or distortion. There is about 1.0v difference in cathode voltages between the output valves, probably different emission values. I have tried a different type of resistor replacement as an experiment, so far no sign of distress and it is running cool to the touch. As before, being driven by a Heathkit FM-4U via the gram sockets. Attached Thumbnails __________________ Thermionic Emission, warms the cockles of your tubes.

18th Feb 2020, 4:18 pm	#9
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: Murphy A146CM Output Transformer damping. Your original replacement resistor may have been a metal film with a spiral element. Perhaps that was resonating in tune with the cap. Mike

18th Feb 2020, 4:22 pm	#10
Graham G3ZVT Dekatron Join Date: May 2010 Location: Greater Manchester, UK. Posts: 18,710	Re: Murphy A146CM Output Transformer damping. 19kHz pilot-tone from the source perhaps? __________________ -- Graham. G3ZVT

18th Feb 2020, 6:08 pm	#13
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: Murphy A146CM Output Transformer damping. PLJ, are you saying that an AC current passing through a coil produces no heat? I didn't realise that. Mike

18th Feb 2020, 6:47 pm	#14
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Murphy A146CM Output Transformer damping. Only if the coil is superconducting. Otherwise IsquaredR applies to the resistance of the coil. If you have a parallel resonant circuit however, the LC circulating current may be many times the current in and out. This can have spectacular effects in poorly designed choke feed circuits in a transmitter PA stage __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

18th Feb 2020, 9:19 pm	#15
Nanozeugma Heptode Join Date: Nov 2017 Location: London, UK. Bury, Lancashire quite regularly :) Posts: 611	Re: Murphy A146CM Output Transformer damping. The chassis has been powered and working all day with the replacement resistor. Resistor runs cold to the touch, no sign of distress. Most peculiar... __________________ Thermionic Emission, warms the cockles of your tubes.

18th Feb 2020, 10:46 pm	#16
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Murphy A146CM Output Transformer damping. The first replacement resistor probably spiral metal film as already suggested. Your second replacement looks like metal oxide to me. assuming that is correct, and if that is the only difference then I would assume that the first replacement must be inductive, unlike the original carbon and metal oxide ones. Discuss. Les. Last edited by MotorBikeLes; 18th Feb 2020 at 10:49 pm. Reason: Rephrased

18th Feb 2020, 11:00 pm	#17
ajgriff Nonode Join Date: May 2004 Location: Halifax, West Yorkshire, UK. Posts: 2,587	Re: Murphy A146CM Output Transformer damping. I know it's not relevant but I can't help commenting on the very impressive macro photography. Alan

18th Feb 2020, 11:50 pm	#18
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Murphy A146CM Output Transformer damping. It seems very unlikely that the very small inductance of a resistor would be sufficient to cause instability but that appears to be the only explanation so far. I would keep an eye on it as it might be intermittent. The FM 19KHz pilot tone is pretty much killed off by the de-emphasis so very unlikely to be that.