|
Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
|
Thread Tools |
18th Feb 2020, 12:47 pm | #1 |
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.
__________________
Thermionic Emission, warms the cockles of your tubes. |
18th Feb 2020, 12:55 pm | #2 |
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, 1:04 pm | #3 |
Heptode
Join Date: Nov 2017
Location: London, UK. Bury, Lancashire quite regularly :)
Posts: 611
|
Re: Murphy A146CM Output Transformer damping.
Yes. 14.89K four band markings plus tolerance band (1%.)
__________________
Thermionic Emission, warms the cockles of your tubes. |
18th Feb 2020, 1:17 pm | #4 |
Moderator
Join Date: Dec 2010
Location: Exeter, Devon and Poole, Dorset UK.
Posts: 6,875
|
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 |
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, 1:58 pm | #6 | |
Heptode
Join Date: Nov 2017
Location: London, UK. Bury, Lancashire quite regularly :)
Posts: 611
|
Re: Murphy A146CM Output Transformer damping.
Quote:
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.
__________________
Thermionic Emission, warms the cockles of your tubes. |
|
18th Feb 2020, 3:14 pm | #7 |
Nonode
Join Date: Nov 2016
Location: Aberaeron, Ceredigion, Wales, UK.
Posts: 2,887
|
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 |
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.
__________________
Thermionic Emission, warms the cockles of your tubes. |
18th Feb 2020, 4:18 pm | #9 |
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 |
Dekatron
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,724
|
Re: Murphy A146CM Output Transformer damping.
19kHz pilot-tone from the source perhaps?
__________________
-- Graham. G3ZVT |
18th Feb 2020, 4:26 pm | #11 |
Heptode
Join Date: Nov 2017
Location: London, UK. Bury, Lancashire quite regularly :)
Posts: 611
|
Re: Murphy A146CM Output Transformer damping.
The thought had crossed my mind that perhaps the construction of the overheating resistor was inductive...
__________________
Thermionic Emission, warms the cockles of your tubes. |
18th Feb 2020, 5:00 pm | #12 | |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
|
Re: Murphy A146CM Output Transformer damping.
Quote:
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. Last edited by PJL; 18th Feb 2020 at 5:07 pm. |
|
18th Feb 2020, 6:08 pm | #13 |
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 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,536
|
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 |
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 |
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 |
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 |
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.
|