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?