Incase the poster a couple of weeks ago is still here.
I mentioned in that thread a phone call from someone , who has now brought
his in . He leant it to a "girlie , so of course she has no idea what she
did wrong". I had previously covered that Yamaha booby trap of chromed steel
handle with nylon spiral wrap and covered the boobytrap metal elbows of the
short speaker lead for integral speaker, with heat shrink - so what did she
likely do ? All she did was use a pair of extension speakers plugged into
those plastic outlet sockets.

I had previously repaired this, nice to know bulk standard IRF740 instead of
near unobtanium ST P14NF12FP 120V 14A , and awkward 6x3mm 0.056R 1W SMD
R/fuse replaced with with mica protected 2x 15mm long pieces of 8R/metre
resistance wire are functional replacements (perhaps 2 years of weekly gigs
plus rehearsals). That amp is fine , the other one blown this time. Cold
testing the original powerfets seem ok, one of those R3134 SMD .056R o/c
with a pock mark blow hole and a load of mysterious grey plastic and cotton
wool laying about. Exploded C2121 22uF/50V SMG (M)85 deg C electrolytic.
Have to explore a bit further around that R and C for possible other
colateral damage, before powering up with replacements

I took another opportunity to search these pa boards for the difference , ie
an antiphase pinning or device. Nothing found again - anyone know the secret
of the difference between boards 6479e the inner amp, and 6479b the outer
one ,or even whether you can safely swap them around . As long as one of
each designation