Volume coming and going in one channel of a Decca SRG500 - suggestions? - UK Vintage Radio Repair and Restoration Discussion Forum

RojDW48 · 7th Jul 2017, 9:50 pm

This has just started happening on both radio and disc inputs. I tried changing the EL84, which helped for a bit - then the problem came back. Any pointers to caps with a criminal record etc. very welcome!

Station X · 7th Jul 2017, 9:55 pm

Unless C43 is something like a Mullard Mustard it should have been changed.

If C48 is leaking it will change the bias on the pentode's control grid.

A few voltage checks will tell you what's happening.

EDIT. That circuit doesn't look right to me, but I'm sure someone will be able to explain how it works.

ms660 · 7th Jul 2017, 10:09 pm

There's a horizontal resistor missing.

EDIT: Correction...an HT feed line missing

Lawrence.

Station X · 7th Jul 2017, 10:37 pm

Yup!

It's shown on the Trader Sheet.

RojDW48 · 8th Jul 2017, 12:03 am

Many thanks gentlemen - those are the very fellows I suspected. There are other oddities in this circuit diagram (the trader sheet). The output valves (V5/6) are ascribed EL84 - but in the plan view (very helpful for an amateur like me, but in this case - a bit confusing) V5 is identified as an EL85!

7th Jul 2017, 9:50 pm	#1
RojDW48 Nonode Join Date: Jun 2014 Location: Bristol, UK. Posts: 2,074	Volume coming and going in one channel of a Decca SRG500 - suggestions? This has just started happening on both radio and disc inputs. I tried changing the EL84, which helped for a bit - then the problem came back. Any pointers to caps with a criminal record etc. very welcome! Attached Thumbnails __________________ '....don't go mistaking Paradise for that home across the road!' (Bob Dylan)

7th Jul 2017, 9:55 pm	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Volume coming and going in one channel of a Decca SRG500 - suggestions? Unless C43 is something like a Mullard Mustard it should have been changed. If C48 is leaking it will change the bias on the pentode's control grid. A few voltage checks will tell you what's happening. EDIT. That circuit doesn't look right to me, but I'm sure someone will be able to explain how it works. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

7th Jul 2017, 10:09 pm	#3
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Volume coming and going in one channel of a Decca SRG500 - suggestions? There's a horizontal resistor missing. EDIT: Correction...an HT feed line missing Lawrence. Last edited by ms660; 7th Jul 2017 at 10:31 pm.

7th Jul 2017, 10:37 pm	#4
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Volume coming and going in one channel of a Decca SRG500 - suggestions? Yup! It's shown on the Trader Sheet. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

8th Jul 2017, 12:03 am	#5
RojDW48 Nonode Join Date: Jun 2014 Location: Bristol, UK. Posts: 2,074	Re: Volume coming and going in one channel of a Decca SRG500 - suggestions? Many thanks gentlemen - those are the very fellows I suspected. There are other oddities in this circuit diagram (the trader sheet). The output valves (V5/6) are ascribed EL84 - but in the plan view (very helpful for an amateur like me, but in this case - a bit confusing) V5 is identified as an EL85! __________________ '....don't go mistaking Paradise for that home across the road!' (Bob Dylan)