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Old 9th September 2015, 12:12 AM
bikerhifinut bikerhifinut is offline
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Location: Penrith, Cumbria
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Default tricky fault diagnosis 88VA

Hi,
I have just spent 2 days trying to find a fault on the 88VA.
I had lost one phase on the Right channel and at first I had that sinking feeling that it was the output transformer because a sharp tap on the transformer cap could bring the phase back temporarily.
However measuring resistances and then voltages seemed to indicate the tx was ok.
Thats when the fun started, I reckoned that the coupling caps may be at the root of it, all voltages checked out and so I began to think the 0.47uF from the 5687 to the non working KT88 had gone O/C.

Couldn't get to the bottom of it, and then I chanced to put the meter probe on the actual lead out wire of the capacitor. Bingo! there was no HT voltage on the "Hot" side. A poke about with the prod and the short wire link on the tag strip from the anode resistor to coupling capacitorwas found to be not fully soldered in, classic dry joint. for years it must have been working due to the mechanical contact alone.
5 minutes later, and all was restored.
All voltages checked and back on the rack. A quick listen and the sound was restored.
I then decided to break out my new matched quad of "Brimar" branded valves that was a bargain purchase at the BVWS wooton basset meet.
I didnt think I'd hear much if any difference but the amps sound seemed to open right out.
Probably just the difference between new valves and ones on their last legs but nice all the same.
Anyway I thought I'd share my tribulations, and i am sure everyones had one of those really frustrating faults that seemed impossible and costly and in the end turned out to be more or less what you suspected to begin with.

Andy
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Old 9th September 2015, 08:50 AM
bob orbell bob orbell is offline
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Default Re: tricky fault diagnosis 88VA

Well don Andy, that is the beauty of valve amplifiers, they are nearly always simple and cheap to fix. BOB
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  #3  
Old 9th September 2015, 12:14 PM
bikerhifinut bikerhifinut is offline
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Default Re: tricky fault diagnosis 88VA

Yes Bob,
And also shows how robust they are. Many transistor power amps would have gone up in expensive smoke whereas all other things being equal, a valve power amp can take a fair amount of abuse.
I had worried about the output transformers because i know that they have both been run inadvertently as open circuit on the secondary and also with the speaker sockets accidentally shorted out, both occurences due to speaker wires becoming undone from their plugs/sockets and not noticed for some time. A friend on another forum thinks that the existence of the zobel network across the secondaries offers a bit of protection due to the AC load it offers?

I wouldnt rely on it though!

Andy.
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