Reported as problem with guitar input socket, owner tested with 2 leads
and 2 guitars, and amp problem. But of course no problem with me, as
received and now with the thing dismantled , including the Tx , to
reconfigure under power and rattle and bend .
Yes PbF, but i/p socket solder points look ok under x30, and trying to
force signal contact pin out of the pcb with circlip pliers acting
between pcb and protruding lump at axial end of the socket, just flexes
the pcb.
Anything other than wiring in a replacement conventional socket and
bypassing a tiny via, in the signal line, and crossing-fingers against a
bounce? Plenty of tip contact clearance in open position.
The phosphor-bronze pcb grounding contacts do not constitute a ground
line through the amp casing.

This amp seems to use a control line from the static ground contact of
the input socket, via a couple of 4.7K and 5V to pin 90 of the 100 pin
DSP chip. So normal operation is non-grounded of this static contact,
hence dummy plug required , at the guitar input, to use the external
signal input. I cannot see how that line could go LOW in guitar use and
inhibit the amp.

There may be a generic problem with these amps. I dug out a scapped
board of one of these amps. That had the opposite problem, never managed
to get that amp working. When I looked at it again, the static sleeve
contact was tied to ground, with a dummy plug in place. Scraped around
the insulating ring of ground plane around that pad and the grounding
disappeared.
Without a schematic, that is what you mould normally expect, the static
sleeve contact being at ground. This insulating ring space is normal
dimension, unlikely to induce a tin-whisker, what was bridging it.?
This one , that I cannot make fail, I've messed about so much that I
would have destroyed any bridge. Any ideas what was going on in the way
of a shorting bridge?

Is 100 +/-20 micron bridgeable by a tin-whisker? This minimum insulation
gap (under x30 visibly compared with a .08mm diameter wire) is between
the pad for that control pin and one of 4 grounded pads for a screening
can, not used in this amp, but was used on the scrapped amp. The
remainder of the ring insulation is about .3 mm. Could a flux or flux
remover associated with PbF induce a whisker over 100 micron ?

Next time I come across on of these, I'll make a x30 inspection of this
guard band to see if I can spot any whiskers growing across, maybe under
the lacquer. Just a mess of powder on both these boards now, ex lacquer,
down to pcb polyester. Tried out the old board and still dead.