Pye P76F. Transformer query. - UK Vintage Radio Repair and Restoration Discussion Forum

longneck90 · 31st Jul 2014, 8:30 pm

Hello to everyone. I have recently brought this Pye P76F radio back to life. It is going very well. But... as I was moving chassis back into the cabinet, (it was switched off) I put my hand on the mains transformer, and thought it seemed warmer than it should be. The radio had been switched on for about 2 hours beforehand. I was able to hold the transformer by hand as I manouvred the chassis into the cabinet. During the repair of this radio, I changed the 2 core mains cable for a three core cable, earthed to chassis. When I now looked at the circuit diagram, I felt (and not because of any electronic knowledge that I have; I haven't got any) that by the chassis being earthed, there may have been some extra stress or load on the transformer, that was tending to make it run a bit warm. I monitored the current consumption for 2 hours on one day, and 6 hours the next day. It never moved from 150 mA. Also, when I remove the earth from the chassis, reception becomes poor. When I replace earth, reception is perfect. Unfortunately, I don't know how to transfer pictures of circuit onto 'Discussion Forum'. Any advice will be gratefully received. All the best. John.

HamishBoxer · 31st Jul 2014, 8:43 pm

Earthing should not that I am aware of effect the dissipation of the transformer. An earth can very often improve performance of a set.

Think after two hours and not drawing any more current it is possibly all fine.

Nuvistor · 31st Jul 2014, 8:54 pm

The resistive losses in the TX will be about 3 watts plus any heating effects from magnetic loses. If there was any electrical leakage in the transformer adding an earth could cause problems but with the current being stable I don't think this is occurring.
There are members with lots of experience in transformer theory on this site, they will be able to give you a better answer than I can.

To add photos, click on the advanced button and add the photo in the additional options/manage attachments.

Herald1360 · 31st Jul 2014, 9:13 pm

If the transformer temperature is less than about 60C after several hours running (especially on a warm day) there's unlikely to be anything wrong with it. 60C is decidedly "ouch" hot!

longneck90 · 1st Aug 2014, 5:44 pm

Thanks to HamishBoxer, Nuvistor, and Herald1360 for all your useful advice. You have put my mind at ease. The temperature of the transformer is nowhere near 60 degrees F. The weather at the time was also very warm. I'll try to learn how to transfer photos, etc. Thank you all. John.

paulsherwin · 2nd Aug 2014, 1:53 am

It's normal for mains transformers under load to run warm. There's nothing to worry about if the temperature is stable after two hours and there are no nasty smells, bubbling noises, smoke etc.

You are right that earthing a previously floating chassis can increase the voltage stress on the transformer, but this rarely causes problems in practice, and if it does the plug fuse should blow or the consumer unit breaker should trip.

Radio Wrangler · 2nd Aug 2014, 7:22 am

Transformers are designed to run warm. The choices of what gauge wire to use are based on how much temperature rise above ambient is permissible. Similarly the size of the iron core is chosen in a compromise between losses, surface area and temperature rise.

It works out that for a well-designed transformer, you can weigh it and make a good estimate of its power handling capability.

David

longneck90 · 3rd Aug 2014, 2:58 pm

Tranmsformer query. Thanks also to Paulsherwin and Radio Wrangler for your advice.All the best. John.

31st Jul 2014, 8:30 pm	#1
longneck90 Hexode Join Date: Sep 2013 Location: Wembley, London, UK. Posts: 395	Pye P76F. Transformer query. Hello to everyone. I have recently brought this Pye P76F radio back to life. It is going very well. But... as I was moving chassis back into the cabinet, (it was switched off) I put my hand on the mains transformer, and thought it seemed warmer than it should be. The radio had been switched on for about 2 hours beforehand. I was able to hold the transformer by hand as I manouvred the chassis into the cabinet. During the repair of this radio, I changed the 2 core mains cable for a three core cable, earthed to chassis. When I now looked at the circuit diagram, I felt (and not because of any electronic knowledge that I have; I haven't got any) that by the chassis being earthed, there may have been some extra stress or load on the transformer, that was tending to make it run a bit warm. I monitored the current consumption for 2 hours on one day, and 6 hours the next day. It never moved from 150 mA. Also, when I remove the earth from the chassis, reception becomes poor. When I replace earth, reception is perfect. Unfortunately, I don't know how to transfer pictures of circuit onto 'Discussion Forum'. Any advice will be gratefully received. All the best. John.

31st Jul 2014, 8:43 pm	#2
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,931	Re: Pye P76F. Transformer query. Earthing should not that I am aware of effect the dissipation of the transformer. An earth can very often improve performance of a set. Think after two hours and not drawing any more current it is possibly all fine. __________________ G8JET BVWS Archivist and Member V.M.A.R.S

31st Jul 2014, 8:54 pm	#3
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,431	Re: Pye P76F. Transformer query. The resistive losses in the TX will be about 3 watts plus any heating effects from magnetic loses. If there was any electrical leakage in the transformer adding an earth could cause problems but with the current being stable I don't think this is occurring. There are members with lots of experience in transformer theory on this site, they will be able to give you a better answer than I can. To add photos, click on the advanced button and add the photo in the additional options/manage attachments.

31st Jul 2014, 9:13 pm	#4
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Pye P76F. Transformer query. If the transformer temperature is less than about 60C after several hours running (especially on a warm day) there's unlikely to be anything wrong with it. 60C is decidedly "ouch" hot! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

1st Aug 2014, 5:44 pm	#5
longneck90 Hexode Join Date: Sep 2013 Location: Wembley, London, UK. Posts: 395	Re: Pye P76F. Transformer query. Thanks to HamishBoxer, Nuvistor, and Herald1360 for all your useful advice. You have put my mind at ease. The temperature of the transformer is nowhere near 60 degrees F. The weather at the time was also very warm. I'll try to learn how to transfer photos, etc. Thank you all. John.

2nd Aug 2014, 1:53 am	#6
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,932	Re: Pye P76F. Transformer query. It's normal for mains transformers under load to run warm. There's nothing to worry about if the temperature is stable after two hours and there are no nasty smells, bubbling noises, smoke etc. You are right that earthing a previously floating chassis can increase the voltage stress on the transformer, but this rarely causes problems in practice, and if it does the plug fuse should blow or the consumer unit breaker should trip.

2nd Aug 2014, 7:22 am	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Pye P76F. Transformer query. Transformers are designed to run warm. The choices of what gauge wire to use are based on how much temperature rise above ambient is permissible. Similarly the size of the iron core is chosen in a compromise between losses, surface area and temperature rise. It works out that for a well-designed transformer, you can weigh it and make a good estimate of its power handling capability. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

3rd Aug 2014, 2:58 pm	#8
longneck90 Hexode Join Date: Sep 2013 Location: Wembley, London, UK. Posts: 395	Re: Pye P76F. Transformer query. Tranmsformer query. Thanks also to Paulsherwin and Radio Wrangler for your advice.All the best. John.