Failed output but fan continued.
Any new ideas for that archetypal PbF symptom , of failure with owner
and everything fine with repairer.?
Initially litterally feather-touch twizzling ellicited nothing, plastic
rod twizzling nothing. So now will have temporally "cured" any bad PbF
joint.
So next will be out with the big-guns, hot air and nylon bolt in
engraving tool while listening to output on attenuatored headphones.
Hopefully in speaker line as thermal sw is not in speaker line and owner
reported not hearing hiss on the speaker when it failed, also checked
not a speaker/lead problem.
H&S issue, certainly QC issue, scrap of plastic? under the chassis
ground solder tag complete with another star washer vaguely stuck to
this plastic/paint film , awaiting a minor jolt to loosen , to fall into
the amp. "Made in China" on the cab.

A couple of the active speaker outlet 1/4" socket pins could only resist
0.2N-m of torque appled to a 5mm flat blade screwdriver angled into a
convenient hole in each pin and levering at about 45 degree against the
rear pcb. But of course the pbf solderings looked fine.
But prior to taking that rear apart , hot-air and nylon rod engraver
shaker testing. An area of the preamp showed up to x3 increase in output
level playing low temp ,90 to 100 deg C, hot air over. A "bus strip" of
parallel conductors the length of the preamp pcb showed sensitivity to
severe vibration. So will now have to remove the preamp board and see
what is on the other side of that board. No schematic found.

Text book PbF crap. Another symptom/cure to add to the lengthening PbF
repair manual. The long run of sensitity; one trace ran to a wire jumper
with bad pbf, presumably the engraver vibration ran along the 15 inch
run of copper, with 6 or so 22K off that line. Close look at all those
most basic of components and some more dodgey joints seen, so dealt with
all jumper wires and all XLR and preamp 1/4 in sockets although they
passed limited lever testing. The thermal x3 gain section had a PT2399
reverb, appropriately made by PTC, on the other side of the pcb in a
sprung pin DIL socket , replaced with turned pin socket.

I was just wondering it portland cement mortar would be as reliable as
PbF solder "joints"