Pye Fenman 1. - UK Vintage Radio Repair and Restoration Discussion Forum

Ex Greeny · 19th Oct 2014, 7:52 pm

I have an unusual problem when switched to FM. As the volume pot is turned from minimum to maximum, the volume decreases about halfway throughout its travel, then continues to increase as the pot reaches its maximum travel. The volume pot has been stripped down, cleaned and tested. This problem does not occur when switched to the AM ranges, or indeed when switched to Gram, and injecting an audio signal into the Gram socket, a smooth increase in volume is achieved throughout its travel.
Several faulty capacitors and resistors have been replaced in the process of renovating this set, and apart from the volume not being as good as I would like, it seems to be working quite well.
Can anyone shed some light on this problem?

G6Tanuki · 19th Oct 2014, 8:13 pm

is there any DC appearing across the control?

Ex Greeny · 19th Oct 2014, 8:25 pm

Yes,around 4v, top end of pot to chassis.

Refugee · 19th Oct 2014, 8:26 pm

Discriminator capacitor. It is the one with the positive to chassis just before the audio goes to the wave change switch.

Silicon · 20th Oct 2014, 12:40 am

The volume control has a fixed centre tap. If the components connected to it (C58 and R31) were faulty you might get a dip in the volume.

However I would expect it to affect all wavebands on most radios.

This radio passes the FM audio through the triode of the ECH81. The volume control is separated from the 80v anode voltage by capacitor C57. If C57 (and maybe C58) was leaky you would get DC on the grid of the EABC80 triode which would affect the bias.

Ex Greeny · 20th Oct 2014, 10:39 am

Thanks for your helpful suggestions. Alarm bells should have been ringing when getting a DC voltage across the volume pot, the culprit being C57 (Trader Sheet) having gone leaky. Since replacing it, the original problem has been solved and the volume has increased considerably on all ranges. This set has probably been modified sometime in its life, as the volume pot R32 is a conventional 3 terminal pot, and components R31 and C58 are not fitted.
Thanks once again, I can now get on with renovating the cabinet!

19th Oct 2014, 7:52 pm	#1
Ex Greeny Tetrode Join Date: Jan 2013 Location: Mansfield, Nottinghamshire, UK Posts: 65	Pye Fenman 1. I have an unusual problem when switched to FM. As the volume pot is turned from minimum to maximum, the volume decreases about halfway throughout its travel, then continues to increase as the pot reaches its maximum travel. The volume pot has been stripped down, cleaned and tested. This problem does not occur when switched to the AM ranges, or indeed when switched to Gram, and injecting an audio signal into the Gram socket, a smooth increase in volume is achieved throughout its travel. Several faulty capacitors and resistors have been replaced in the process of renovating this set, and apart from the volume not being as good as I would like, it seems to be working quite well. Can anyone shed some light on this problem?

19th Oct 2014, 8:13 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Pye Fenman 1. is there any DC appearing across the control?

19th Oct 2014, 8:25 pm	#3
Ex Greeny Tetrode Join Date: Jan 2013 Location: Mansfield, Nottinghamshire, UK Posts: 65	Re: Pye Fenman 1. Yes,around 4v, top end of pot to chassis.

19th Oct 2014, 8:26 pm	#4
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Pye Fenman 1. Discriminator capacitor. It is the one with the positive to chassis just before the audio goes to the wave change switch.

20th Oct 2014, 12:40 am	#5
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,169	Re: Pye Fenman 1. The volume control has a fixed centre tap. If the components connected to it (C58 and R31) were faulty you might get a dip in the volume. However I would expect it to affect all wavebands on most radios. This radio passes the FM audio through the triode of the ECH81. The volume control is separated from the 80v anode voltage by capacitor C57. If C57 (and maybe C58) was leaky you would get DC on the grid of the EABC80 triode which would affect the bias.

20th Oct 2014, 10:39 am	#6
Ex Greeny Tetrode Join Date: Jan 2013 Location: Mansfield, Nottinghamshire, UK Posts: 65	Re: Pye Fenman 1. Thanks for your helpful suggestions. Alarm bells should have been ringing when getting a DC voltage across the volume pot, the culprit being C57 (Trader Sheet) having gone leaky. Since replacing it, the original problem has been solved and the volume has increased considerably on all ranges. This set has probably been modified sometime in its life, as the volume pot R32 is a conventional 3 terminal pot, and components R31 and C58 are not fitted. Thanks once again, I can now get on with renovating the cabinet!