Pye P53 Bad Audio Output Transformer - UK Vintage Radio Repair and Restoration Discussion Forum

OldTechFan96 · 11th May 2018, 4:51 pm

During the initial cold checks of this radio I suspected that the audio output transformer was bad as the resistance measured across the primary was a lot higher that it should have been.

During the first power on test (with only the rectifier in place) a healthy amount of HT was found on the positive terminals of C31 and C32. The next day I powered up the radio again to find HT only on C32. The voltage on C31 was 1.6V.

R19 was not open and tested fine so I decided to remove the audio output transformer.

The primary winding was open circuit between pins 2 and 4. It was also open between pins 3 and 4. The resistance between pins 2 and 3 was 43k ohms.

According to the trader sheet the primary should be 485 ohms with a 15 ohm tapping.

Another thing that made me suspect a bad output transformer was that both the transformer and EL41 output valve looked to have been getting hot when the radio was last in service.

How do I go about finding a replacement? Are new output transformers available? Or should I look for a used equivalent?

Thanks

ukcol · 11th May 2018, 5:32 pm

This is not a valuable set (early 50s woody) so unless this particular one has sentimental value you won't want to go to the expense of having the transformer rewound.

Your best bet is to try and find one from a scare P53. You could post a request in the parts wanted section.

Failing finding a scrape P53 any "hum-buck" output transformer from a set that uses an EL41 would be fine. The P53 is quite a large set and so physical size shouldn't be a problem.

If you are still struggling to source one any hum bucking type transformer designed for different output valves is likely to work quite well as optimum load is not critical.

You could even experiment with a non hum bucking transformer by increasing the values of C31 and C32 BUT don't go above 50uF for C32 or you may damage the rectifier.

You will need to identify the cause of the failure, C34 being the most likely culprit. Also change the EL41 and its cathode components R18 and C35.

Ed_Dinning · 11th May 2018, 8:51 pm

Hi, if all else fails I can do a rewind. To get the set going you can use a resistor of about 1K in place of the upper section of the transformer primary that connects HT to the rest of the set. Any output transformer can then be lashed in to prove the set.
If you find it works and want to keep it going at low cost then the resistor dodge will work OK, but the hum level will be higher.

Ed

OldTechFan96 · 12th May 2018, 5:26 pm

I'll keep an eye out online for a replacement transformer. How close of a match does the primary resistance need to be? I missed out on a transformer on eBay that had a primary resistance of 530 ohms with a 30 ohms tap.

UKcol, the transformer is likely to be toast because of the components you mentioned. The radio was fitted with a modern 3 pin plug. I am certain that it was tested without any regard for its age.

Ed, the resistor dodge does seem interesting for the time being. Would any resistor do? Would I need a particular wattage?

Ed_Dinning · 12th May 2018, 7:34 pm

Try about 1K 5W. That may be the wrong value but it will cause no harm. The worst thing will be increased hum levels.

Not that the resistance ratio pri/sec of an output transformer does not tell you much.
You need to measure its voltage ratio to get the correct figure for matching purposes.

Ed

kestrelmusic · 12th May 2018, 7:44 pm

Can I just add, replace the capacitor which couples to the grid of the output valve (C34 on the Trader sheet). It is almost certainly defective, and unless you replace it the output valve will be severely over-run and the output transformer primary will burn out.
Athough this is by no means a rare set, it was a nice design and will repay a little time and trouble with repairs. Good luck!

11th May 2018, 4:51 pm	#1
OldTechFan96 Octode Join Date: Jul 2017 Location: Newcastle Upon Tyne, Tyne & Wear, UK. Posts: 1,186	Pye P53 Bad Audio Output Transformer During the initial cold checks of this radio I suspected that the audio output transformer was bad as the resistance measured across the primary was a lot higher that it should have been. During the first power on test (with only the rectifier in place) a healthy amount of HT was found on the positive terminals of C31 and C32. The next day I powered up the radio again to find HT only on C32. The voltage on C31 was 1.6V. R19 was not open and tested fine so I decided to remove the audio output transformer. The primary winding was open circuit between pins 2 and 4. It was also open between pins 3 and 4. The resistance between pins 2 and 3 was 43k ohms. According to the trader sheet the primary should be 485 ohms with a 15 ohm tapping. Another thing that made me suspect a bad output transformer was that both the transformer and EL41 output valve looked to have been getting hot when the radio was last in service. How do I go about finding a replacement? Are new output transformers available? Or should I look for a used equivalent? Thanks Attached Thumbnails

11th May 2018, 5:32 pm	#2
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Pye P53 Bad Audio Output Transformer This is not a valuable set (early 50s woody) so unless this particular one has sentimental value you won't want to go to the expense of having the transformer rewound. Your best bet is to try and find one from a scare P53. You could post a request in the parts wanted section. Failing finding a scrape P53 any "hum-buck" output transformer from a set that uses an EL41 would be fine. The P53 is quite a large set and so physical size shouldn't be a problem. If you are still struggling to source one any hum bucking type transformer designed for different output valves is likely to work quite well as optimum load is not critical. You could even experiment with a non hum bucking transformer by increasing the values of C31 and C32 BUT don't go above 50uF for C32 or you may damage the rectifier. You will need to identify the cause of the failure, C34 being the most likely culprit. Also change the EL41 and its cathode components R18 and C35.

11th May 2018, 8:51 pm	#3
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: Pye P53 Bad Audio Output Transformer Hi, if all else fails I can do a rewind. To get the set going you can use a resistor of about 1K in place of the upper section of the transformer primary that connects HT to the rest of the set. Any output transformer can then be lashed in to prove the set. If you find it works and want to keep it going at low cost then the resistor dodge will work OK, but the hum level will be higher. Ed

12th May 2018, 5:26 pm	#4
OldTechFan96 Octode Join Date: Jul 2017 Location: Newcastle Upon Tyne, Tyne & Wear, UK. Posts: 1,186	Re: Pye P53 Bad Audio Output Transformer I'll keep an eye out online for a replacement transformer. How close of a match does the primary resistance need to be? I missed out on a transformer on eBay that had a primary resistance of 530 ohms with a 30 ohms tap. UKcol, the transformer is likely to be toast because of the components you mentioned. The radio was fitted with a modern 3 pin plug. I am certain that it was tested without any regard for its age. Ed, the resistor dodge does seem interesting for the time being. Would any resistor do? Would I need a particular wattage?

12th May 2018, 7:34 pm	#5
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: Pye P53 Bad Audio Output Transformer Try about 1K 5W. That may be the wrong value but it will cause no harm. The worst thing will be increased hum levels. Not that the resistance ratio pri/sec of an output transformer does not tell you much. You need to measure its voltage ratio to get the correct figure for matching purposes. Ed

12th May 2018, 7:44 pm	#6
kestrelmusic Hexode Join Date: Feb 2008 Location: Wrexham, North Wales, UK. Posts: 466	Re: Pye P53 Bad Audio Output Transformer Can I just add, replace the capacitor which couples to the grid of the output valve (C34 on the Trader sheet). It is almost certainly defective, and unless you replace it the output valve will be severely over-run and the output transformer primary will burn out. Athough this is by no means a rare set, it was a nice design and will repay a little time and trouble with repairs. Good luck!