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Custom Sound 2005, Mosfet band amp, made 1988

Volume gradually fades down over 10 minutes after about 1/2 hour as though under some programmed auto gain control. I've only looked over the boards and not powered up with dummy to even isolate to preamp or pa yet. No tranny/diode thermal monitoring of the heatsink and no FET for gain control in the pa or FET in the preamp , all 4558 opamps. No obvious over-heating or solder problems on any of the boards or flaky gain pots. In general terms if no obvious gain killer what to look for , can an opamp internally drop gain over 1/2 hour ? - I don't think I've ever come across that.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N Cook
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Monitor the input to the output section with a scope and wait for it to do its thing.

Reply to
Meat Plow

Check the supply rails to see if any are drooping.

Entirely normal for mosfet designs. They don't suffer from thermal runaway.

Graham

Reply to
Eeyore

If I recall correctly, it`s a pretty basic implementation of the classic Hitachi design.

Ron(UK)

Reply to
Ron(UK)

Dummy loaded and took the heatsink up to 70 degrees C and held that power level for 40 minutes with little variation in preamp output, pa o/p or DC rails so a matter of the usual treatments for the numerous IDC interconnects, sprung IC sockets and 1/4 inputs, despite no response to twizzle stick and reported gradual fade.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N Cook

Does it have a breakjack (insert, effects, etc.)between pre amp and power amp? if so try tapping flexing etc. there, or when the power has faded, try linking them out.

Ron(UK)

Reply to
Ron(UK)

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