Thanks for your quick reply.

I suspect a capacitance issue based on the problem not occurring (or even
reversing) when I touch a ground on the board (e.g., the audio input
ground). When tapping, there's no specific component that is most sensitive
and, if, instead of tapping, I apply a more constant force to the
components, I cannot reproduce the problem. I have disconnected and
reconnected all the connectors to the main board with no effect. I have
looked at the underside of the board with a magnifying lamp but couldn't
detect any obvious problems.

John.

wrote in message
oups.com...
How does tapping on the board narrow it down to a capacitance issue?

Any mechanical vibration testing that changes the symptom points to a
connection problem. It is possible, though highly unlikely, that the
leads to a capacitor have become partially disconnected internally.

I would start by a good visual going over with a light and magnifying
glass the solder connections on the bottom of the board.