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Post subject: Strange Pro Reverb Board
Posted: Fri Jan 25, 2019 9:49 pm
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Roadie
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Joined: Sat Dec 13, 2014 7:34 am
Posts: 245
I just serviced a '66 Pro Reverb that had a couple of plate load resistors that were noisy. I do my usual poking around afterwards with a stick checking for bad solder joints throughout the rest of the amp. When I got near the bundle of filter cap leads (B+) I noticed when I rubbed the edge of the back insulator board it was microphonic kinda sorta, as in picking up the signal of the rubbing stick, no crackle, etc., the sound of the wooden stick moving back and forth. The top board was quiet, the bottom only reacted. (It actually didn't sound to bad if I kept a good rhythm and added some reverb and tremolo) Anyway, I shut it down, drained it, de-soldered a few leads and pulled the backer board, inspected it and cleaned off a little vintage flux. There were a couple of old solder stalactites (this amp had been serviced before) on the top board I clipped off as well. Got everything back together and the issue was gone. I even let it cook for an hour as I tapped and pushed around on the board on occasion (hard at times) and couldn't get a repeat. I know there's a logical reason for this, maybe by tomorrow things will settle back into place.

Here's what gets me - the bottom board reacted, the top board was dead quiet.

UN


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Post subject: Re: Strange Pro Reverb Board
Posted: Fri Jan 25, 2019 10:30 pm
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Rock Star
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Joined: Sat Jul 03, 2010 2:18 pm
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Yes, I've had boards get brittle and become slightly microphonic. Resistors and disc caps can also act as small microphones. In addition, I believe the circuit boards had some measurable electrical quality, like capacitance --- from early days, up through silverface era. So hard, old boards may lend to some microphony. It's all part of the overall "magic" of these vintage amps.

Silicone sealer between the parts & board may help dampen vibrations, in potentially noisy components. As well as dampen vibrations in heavier components, which could lead to board damage (which maybe is what you had). I find this particularly true with thinner, modern circuit boards.

:mrgreen:


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Post subject: Re: Strange Pro Reverb Board
Posted: Sat Jan 26, 2019 8:10 pm
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Professional Musician
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Joined: Thu Aug 07, 2014 3:25 pm
Posts: 1022
If the board had rework and you saw flux residue, it could be leftover paste flux. At plate voltages, that stuff conducts. Clean every trace of it away.


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