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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 21st Aug 2010, 9:10 am   #1
awc's Avatar
Join Date: Jul 2005
Location: Somerset, UK.
Posts: 334
Default Curing unwanted oscillation

As you may have seen from the Vintage Radio (domestic) section, I sought help in curing a severe case of unwanted oscillation in my recently restored Pye QAC2.

The problem was helped by wrapping copper wire around the bottom of V2 (Every Ready A50P) but the oscillation was still bad. I prodded and poked about in the wiring and this did reduce the problem but did not remove it.

On turning the chassis right way up and approaching the top cap grid of the output bottle (Pen4DD) the oscillation was reduced further and by grasping the lead to the top cap, further still. The set now worked ok at lower volumes but came on again as the volume control was advanced.

As the top cap lead wasn't screened, I replaced it with a bit of audio co-axial which as it's black doesn't look much out of place. This was earthed only at the chassis end. This cured the problem completely and the set now performs fautlessly.

Worth giving a go if your set exhibits similar problems
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Old 21st Aug 2010, 2:01 pm   #2
merlinmaxwell's Avatar
Join Date: Jul 2008
Location: Maidenhead, Berkshire, UK.
Posts: 10,952
Default Re: Curing unwanted oscillation

Nice detective work, some grid top sockets also have a stopper resistor of a few k there to stop parasitic oscillations.
Cats have staff, it's dogs that have owners.
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Old 22nd Aug 2010, 4:21 pm   #3
ColinB's Avatar
Join Date: Apr 2003
Location: Newton Abbot, Devon, UK.
Posts: 699
Default Re: Curing unwanted oscillation

Yes, it could make good sense. I'll try it with a set I've got here with this problem.
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