Reported fault: Sound fades but can be reinstated by a big "clang" on the geetar.
I checked for drys around the TDA2050 power op chip but the problem turned out to be a poor contact thru' the headphone jack earth path. This carries the 0V speaker return current and thus effects speaker kill for headphone operation. This is it seems a very common failing of the design.
A re-tension and clean has it working fine but son will convey the message that if wonga is available(unlikely!) dad will fit a new jack, otherewise he can have it back pro bono..THIS time!