Custom Sound 2005, Mosfet band amp, made 1988

[N Cook]

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.

 

[Meat Plow]

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.

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

 

[Eeyore]

Volume gradually fades down over 10 minutes after about 1/2 hour as though
under some programmed auto gain control.

Check the supply rails to see if any are drooping.

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

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

Graham

 

[Ron(UK)]

Check the supply rails to see if any are drooping.



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

Graham

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

Ron(UK)

 

[N Cook]

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.

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.

 

[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.

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)