Marshall JCM900 4100 w/5881’s volume drop

[evil79]

Did work on a 4100 JCM900 w/5881’s. Cap job, replaced tubes, replaced a few burnt resistors, resoldered almost all solder joints, cleaned jacks and sockets, and retentioned tube sockets. But it’s still putting out about half power intermittently. It sounds great right when turned on and then drops volume. It comes back sporadically, but mostly it’s low power. I’ve chopsticked everywhere and tried everything I can to provoke it and showcase the problem. No luck. I’ve bridged the fx send and return with a cable and that seems to slightly help. I’m running out of ideas. Anybody have this problem before and successfully fix it? Please help!! I can post pics and schematics. Thanks

[evil79]

Here’s some pics

[evil79]

Here’s the schematic
http://www.ampwares.com/schematics/m..._100w_4100.pdf

[Leo_Gnardo]

Speaker impedance switch - try working it back n forth a dozen times or so, might be a bit of corrosion there. Shot 'o cleaner spray might help there too. Worth a try. If no joy... on to the next suspect.

[Jon Snell]

I was going to suggest the impedance switch or the Power Select switch. I have had both giving issues.

[J M Fahey]

Feed a constant signal, say 100mV 1kHz or 440Hz, scope output, what happens when volume drops?

Move scope probe towards front, until you identify the stage where signal drops on its own.
Thatīs what scopes were invented for.

IF problem happens at power tubes but not earlier (not even PI) then those switches suggested above are highly suspect, since they affect only power tubes.