oh !
that is very interesting that it ended up being a short..
this suggests after the Flux process, the cleaning cycle was not
thorough, and left some impurities around things that were not flat
leftover chlorides and such tend to catch some moisture eventually
and abet bridging or a migration of the metal, causing a short
(obviously they were not shorted at birth.. and vibration could not
cause then to have moved)
this is a great example of following the cardinal rule..first look CLOSELY
at everything, as you just might SEE what is wrong
now more often we do see "cold" solder joints give way around the
potentiometers and jacks, because they are soldered onto the circuit
board, but then they still go through the Body and often have a nut
spun on them to hold them firmly. This can give a twist to the body
of the Pot or Jack, and stress the solder joint, so if it was weak a
hairline crack develops. This can cause intermittents, noise, or just open
circuits at worst.
the problem FIXING these is.. well there were a series of troublesome Tannoy
Mixers that came out in the 80's.. very popular and inexpensive, but
they had like 30 pots and nuts, so you had to take them ALL off to get
at the board, then you would just re-solder every one of the legs
to the pots, then you had to Oh So Carefully torque all those nuts back ON
without cracking a single solder joint
many Fender amps are similar construction.. working on one is an hour
dis-assembly just to get at everything, then sweating bullets to get
it back together without something else breaking
so this was a good diagnosis and repair.. i hope this remains a
very very isolated incedent of this type of problem
