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  #21  
Old 27th September 2019, 11:17 PM
A.N. Beal A.N. Beal is offline
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Join Date: Feb 2006
Location: Leeds, West Yorkshire, UK
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Default Re: KT88 Intermittent problem

After all this time, some more news.

After replacing the volume pot the problem more or less went away for a while but it then gradually returned. When it got too annoying I opened the amplifier again, did some serious poking of the right channel output board with a plastic chopstick (my preferred dry joint finding tool), fixed some joints. Once more it was better but gradually the problem returned again. I found that when the right channel volume surged this could be cured either by turning the volume up and down - but it could also be cured by switching the amplifier off and on again, so it definitely wasn't a volume control problem.

Eventually the right channel volume surges became frequent enough to be annoying, it was time to have another go. Logic pointed the finger of suspicion at the feedback wiring, as disconnecting feedback (or shorting it out) increases gain. I had previously checked the 'circuit board' end of the feedback wiring, so this time I concentrated on the 'speaker terminal' end. I resoldered the relevant joints and also carefully pulled back the end of the screen at the end of the feedback cable and made certain there were no stray wires that might make contact where they should not.

Result? I suspect I've finally pinned down the cause of the problem, as it's been perfect ever since. I thought it was worth reporting this in case anyone else ever has a similar problem.
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  #22  
Old 29th September 2019, 12:30 PM
Richard Richard is offline
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Join Date: Dec 2005
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Default Re: KT88 Intermittent problem

Hi Alasdair, crikey, 3 years since this thread started! Well done fingers crossed you've nailed it. These things can be a devil and often turn out to be a bad joint. I fixed one once which was doing very similar with low power on one channel. I flexed the pcb with a plastic probe whilst it was up and running and found an area which affected the sound. I re-soldered all the joints in that area, never found the exact one, but it was fixed
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