Cossor 523 hum - UK Vintage Radio Repair and Restoration Discussion Forum

PaulR · 9th May 2018, 12:27 pm

I have done the necessary to get this set working well on all bands. It is quieter on FM than AM but I think that is to be expected.

All that remains is a hum on all bands which is affected by the volume control. It increases at it is turned up to a certain point then decreases to an extent as it reaches full volume. It disappears if I remove the EABC80 and is much improved if I short the wiper connection of the volume pot. The cases of the volume and tone controls are earthed.

The leads from the wafer switch to the volume control are long, but they are screened and the screen is earthed.. I have replaced the mains lead but have followed the placing of the original one.

Can anyone suggest anything else I can try?

Thank you

PaulR · 9th May 2018, 6:44 pm

Just having another look at the set I removed C43 and made a direct connection from the volume pot wiper. The set seems much better without it, but does it perform any useful function?

ms660 · 9th May 2018, 7:05 pm

C43 stops the detectors -ve voltage from reaching the grid, with a suitable signal that voltage could be enough to cut off the valve, has R29 been replaced or checked?

Lawrence.

Cobaltblue · 9th May 2018, 7:05 pm

C43 is a DC blocker

The set develops the DC component from the detector accross the volume control.

By shorting it out you are changing the DC conditions on the triode section of V4.

Has C46 and C47 been replaced or are known to be good?

Check R32 its a high value and liable to go higher.

Also have you checked the EABC80 by substitution?

Cheers

Mike T

Crossed with Lawrence

Nuvistor · 9th May 2018, 7:08 pm

Which service information are you using? C43 is different in Manufactures and Trader sheets?

ukcol · 9th May 2018, 7:09 pm

Hi Paul,

Assuming you are using the Trader Sheet - C43 (0.01uF) along with R29 (6.8M) provide grid biasing for the EABC80. As I understand it you have effectively shorted out C43 so I am surprised you are not getting distortion.

As regards the hum, try another EABC80 and check R29.

Cobaltblue · 9th May 2018, 7:14 pm

Agree with UKCOL and Lawrence re R29 it maybe that by shorting C43 you are at least tying the grid to something rather than an open circuit

Cheers

Mike T

PaulR · 9th May 2018, 7:59 pm

Thanks all.

R29 reads over 9M so I will replace it. I don't have anything so large so will have to send off for some. R32 is 780k which is high but within 20%.

I have replaced the EABC80 which improved the FM performance but did not affect the hum.

I have replaced C46 and reformed C47. I did try replacing C47 with a new one in an attempt to remove the hum but it made no difference so I reconnected the original.

I am using the Trader sheet.

I will report back when the new resistor arrives.

Many thanks

PaulR · 12th May 2018, 1:01 pm

I fitted a new resistor today and the hum is improved. It is still there to an extent but it may have always been like that.

I have several FM sets from the same era and they all perform similarly on FM except, surprisingly, for a Sobell 636 WF which is much more sensitive than any of them. It works very well from its own internal dipole even down in the kitchen at ground level whereas the others need some help from a ribbon aerial. I really cannot understand it.

9th May 2018, 12:27 pm	#1
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Cossor 523 hum I have done the necessary to get this set working well on all bands. It is quieter on FM than AM but I think that is to be expected. All that remains is a hum on all bands which is affected by the volume control. It increases at it is turned up to a certain point then decreases to an extent as it reaches full volume. It disappears if I remove the EABC80 and is much improved if I short the wiper connection of the volume pot. The cases of the volume and tone controls are earthed. The leads from the wafer switch to the volume control are long, but they are screened and the screen is earthed.. I have replaced the mains lead but have followed the placing of the original one. Can anyone suggest anything else I can try? Thank you __________________ Paul

9th May 2018, 6:44 pm	#2
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: Cossor 523 hum Just having another look at the set I removed C43 and made a direct connection from the volume pot wiper. The set seems much better without it, but does it perform any useful function? __________________ Paul

9th May 2018, 7:05 pm	#3
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Cossor 523 hum C43 stops the detectors -ve voltage from reaching the grid, with a suitable signal that voltage could be enough to cut off the valve, has R29 been replaced or checked? Lawrence.

9th May 2018, 7:05 pm	#4
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,879	Re: Cossor 523 hum C43 is a DC blocker The set develops the DC component from the detector accross the volume control. By shorting it out you are changing the DC conditions on the triode section of V4. Has C46 and C47 been replaced or are known to be good? Check R32 its a high value and liable to go higher. Also have you checked the EABC80 by substitution? Cheers Mike T Crossed with Lawrence __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk Last edited by Cobaltblue; 9th May 2018 at 7:06 pm. Reason: Crossed with Lawrence

9th May 2018, 7:08 pm	#5
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Cossor 523 hum Which service information are you using? C43 is different in Manufactures and Trader sheets? __________________ Frank

9th May 2018, 7:09 pm	#6
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Cossor 523 hum Hi Paul, Assuming you are using the Trader Sheet - C43 (0.01uF) along with R29 (6.8M) provide grid biasing for the EABC80. As I understand it you have effectively shorted out C43 so I am surprised you are not getting distortion. As regards the hum, try another EABC80 and check R29.

9th May 2018, 7:14 pm	#7
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,879	Re: Cossor 523 hum Agree with UKCOL and Lawrence re R29 it maybe that by shorting C43 you are at least tying the grid to something rather than an open circuit Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

9th May 2018, 7:59 pm	#8
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: Cossor 523 hum Thanks all. R29 reads over 9M so I will replace it. I don't have anything so large so will have to send off for some. R32 is 780k which is high but within 20%. I have replaced the EABC80 which improved the FM performance but did not affect the hum. I have replaced C46 and reformed C47. I did try replacing C47 with a new one in an attempt to remove the hum but it made no difference so I reconnected the original. I am using the Trader sheet. I will report back when the new resistor arrives. Many thanks __________________ Paul

12th May 2018, 1:01 pm	#9
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: Cossor 523 hum I fitted a new resistor today and the hum is improved. It is still there to an extent but it may have always been like that. I have several FM sets from the same era and they all perform similarly on FM except, surprisingly, for a Sobell 636 WF which is much more sensitive than any of them. It works very well from its own internal dipole even down in the kitchen at ground level whereas the others need some help from a ribbon aerial. I really cannot understand it. __________________ Paul