Found another nasty under the ps board but don't think it is the main
problem.
This pair of wires to the speaker switch had one of the wires squashed
between a large W/W ceramic cased dropper under the ps and the chassis.
Melted through but as it cuts the earth to the speaker presumably no problem
as such and wire not broken. But there is smoke blackening or something
grimy in that area but it could be a small electrolytic parallel to the fan
cooked as it is directly over the large droppers and leaked electrolyte but
all rather nasty. The aluminium of the chassis directly under this
particular dropper has a strange flecked grey corrosion or something that
probably more likely due to electrolyte rather than vapours off the charred
PVC insulation. I don't see why this insulation has charred rather than just
melted as the owner never switched this speaker switch, I assume somewhere
maybe ages ago it was open at some point

Anyone know the function of the big triac on the mains, crowbar operation to
blow the mains fuse if too much mains power drawn ?, I've not traced any
triac associated line back into the amp that could trigger it as more active
amp protection.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/