Wartime Civilian Radio decoupling problem - UK Vintage Radio Repair and Restoration Discussion Forum

coopzone · 29th Nov 2018, 5:25 pm

Hello,

The problem is decoupling feedback (I think) that is to say when you tune a station in you get a beat frequency that nulls out in the centre of the station. The normal suspects have been replaced (waxies) but I noticed that when I disconnect C7 (Trader sheet 688) V2 CG decoupling the problem stops! But the signal is still noisy but almost perfect. This is the exact opposite of what you may expect!

So far checked all resistors in the AGC feedback circuit - they are OK
All waxies replaced.
All electrolytes replaced (already done by someone else)
I've swapped out the V2 and swapped out V1 from a working set (did not really think this would make a difference but...)
The wextector has been replaced (again already done) by an OA90 or similar.

The AGC voltage with C7 disconnected hovers around -0.9, then if you connect C7 it goes positive to about 1.8v. If I short the AGC line to ground the radio motorboats.

Anyone any clues?

Derek

cathoderay57 · 29th Nov 2018, 5:31 pm

Are all the electrolytic smoothing capacitors fully serviceable? Is the screening of the IF amplifier valve intact and if there is a screening can is it fitted? What voltages are present on the anodes and screens compared to the service data? Cheers, Jerry

PJL · 29th Nov 2018, 6:07 pm

It sounds like IF instability. Removing C7 is reducing the loop gain so it remains stable. This kind of fault is difficult to track down as voltage measurements will be meaningless when it is unstable but worth taking with C7 disconnected.

I think this set uses the equivalent of an ECH35/EF39 combination so the first place to look would be failure of the metalisation.

coopzone · 29th Nov 2018, 6:33 pm

I've replaced the electrolytic's (previous replacements where the wrong value), so they are now good. HT voltage 210v Anode and screen on valves are ok, a little low on the anode of the OSC V1 but it's about the same as the other example of this radio I have.

Screen on valve looks in good condition, but this valve has been swapped out with another just to double check. No screen can fitted.

coopzone · 29th Nov 2018, 7:19 pm

I think i found the problem, it's really odd. The last IF can was corroded around the base and was not making contact with the chassis - even the brass screws holding in in place did not ground it. Someone had painted the threads at some point no doubt to prevent the loosening!

I took the can of the IF transformer cleaned the bottom and the chassis replaced it and bingo - now works a treat. Just listening to 5-live on it.

Derek

cathoderay57 · 29th Nov 2018, 10:54 pm

Well done Derek. I recently had similar trouble with an HMV 651. Wiggling the screening can around the IF valve produced heterodyning. Valve was OK, cleaned pin sockets, still had the problem. The issue was similar to yours insofar as the joint between the valve screening can receptacle and chassis had corroded. An earth bonding lead did the trick as I didn't want to drill out the rivets from the valve holder. Cheers, Jerry

ms660 · 29th Nov 2018, 11:27 pm

The IF stage is a tuned plate tuned grid oscillator in waiting, it don't take much.

Lawrence.

29th Nov 2018, 5:25 pm	#1
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Wartime Civilian Radio decoupling problem Hello, The problem is decoupling feedback (I think) that is to say when you tune a station in you get a beat frequency that nulls out in the centre of the station. The normal suspects have been replaced (waxies) but I noticed that when I disconnect C7 (Trader sheet 688) V2 CG decoupling the problem stops! But the signal is still noisy but almost perfect. This is the exact opposite of what you may expect! So far checked all resistors in the AGC feedback circuit - they are OK All waxies replaced. All electrolytes replaced (already done by someone else) I've swapped out the V2 and swapped out V1 from a working set (did not really think this would make a difference but...) The wextector has been replaced (again already done) by an OA90 or similar. The AGC voltage with C7 disconnected hovers around -0.9, then if you connect C7 it goes positive to about 1.8v. If I short the AGC line to ground the radio motorboats. Anyone any clues? Derek

29th Nov 2018, 5:31 pm	#2
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,388	Re: Wartime Civilian Radio decoupling problem Are all the electrolytic smoothing capacitors fully serviceable? Is the screening of the IF amplifier valve intact and if there is a screening can is it fitted? What voltages are present on the anodes and screens compared to the service data? Cheers, Jerry

29th Nov 2018, 6:07 pm	#3
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Wartime Civilian Radio decoupling problem It sounds like IF instability. Removing C7 is reducing the loop gain so it remains stable. This kind of fault is difficult to track down as voltage measurements will be meaningless when it is unstable but worth taking with C7 disconnected. I think this set uses the equivalent of an ECH35/EF39 combination so the first place to look would be failure of the metalisation.

29th Nov 2018, 6:33 pm	#4
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Re: Wartime Civilian Radio decoupling problem I've replaced the electrolytic's (previous replacements where the wrong value), so they are now good. HT voltage 210v Anode and screen on valves are ok, a little low on the anode of the OSC V1 but it's about the same as the other example of this radio I have. Screen on valve looks in good condition, but this valve has been swapped out with another just to double check. No screen can fitted.

29th Nov 2018, 7:19 pm	#5
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Re: Wartime Civilian Radio decoupling problem I think i found the problem, it's really odd. The last IF can was corroded around the base and was not making contact with the chassis - even the brass screws holding in in place did not ground it. Someone had painted the threads at some point no doubt to prevent the loosening! I took the can of the IF transformer cleaned the bottom and the chassis replaced it and bingo - now works a treat. Just listening to 5-live on it. Derek

29th Nov 2018, 10:54 pm	#6
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,388	Re: Wartime Civilian Radio decoupling problem Well done Derek. I recently had similar trouble with an HMV 651. Wiggling the screening can around the IF valve produced heterodyning. Valve was OK, cleaned pin sockets, still had the problem. The issue was similar to yours insofar as the joint between the valve screening can receptacle and chassis had corroded. An earth bonding lead did the trick as I didn't want to drill out the rivets from the valve holder. Cheers, Jerry

29th Nov 2018, 11:27 pm	#7
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Wartime Civilian Radio decoupling problem The IF stage is a tuned plate tuned grid oscillator in waiting, it don't take much. Lawrence.