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FDP Forum / Amp Mods, Repairs, and Projects / Updates

Contributing Member

Whitehorse Canada

I don't get out much
May 1st, 2018 04:15 AM   Edit   Profile  

Hi folks
Here's some remarks about the solutions to my two last amp repair questions.

First one is about the "hum from Hell" post.
That's the 4x10 hot rod amp. Owner connected two amps to the outputs of a stereo pedal. One power cord had no ground pin. Young guy got shocked and both amps died.
There was a cooked 47 ohm resistor, I forget which one it was on the schematic.
Anyway 47 ohms to ground.
I did a quick and dirty replacement without removing the main board.
Switched out, resistor did not burn up but hum from Hell prevailed.
Lifted the main board (which I found quite difficult) and found three burnt traces.
I jumped the traces and the hum was dramatically reduced but still far from acceptable.
At least now I could confirm that the amp passed signal.
What I discovered was...the tone pots worked, everything else did nothing.
At that point I knew I was out of my league and asked a friend for help.
My friend replaced all the chips and transistors.
The amp is now working as expected.

Next update is about my "what has failed to cause an amp to do this" question. 100 ohm resistors from pilot light to case burned up.
I followed Peter's advice and checked the power tubes for shorts.
Peter was right. One power tube showed shorts in four of five short check positions on the short check rotary switch.
Tube tester is a Stark 966...Canadian made, licenced from Hickok. I figure it's mid to early 50s.
Replaced the failed tube and all was good for a few seconds....then, boof, the 100 ohm resistors go up in smoke. The formerly good tube showed the same shorts as the previous failed one.
Solution was replace the power tube sockets and tubes.
I guess the failed tube compromised the socket.

So, thanks so much to everyone who has helped me over the years.

I could always fall back on my primary skill....playing bass, but there's just not enough work to justify the rehearsal.

A friend from London is flying in for a visit and to do some tracking.
I built his amp.
His name is Drew Barron.
Have you heard of him over there?
He's very good.

Contributing Member

Get ready for the

Buena Buena
May 3rd, 2018 04:31 PM   Edit   Profile  

Good info. Thanks Roly!

FDP Forum / Amp Mods, Repairs, and Projects / Updates

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