dave wrote:

Yes is certainly sounds (no pun etc) like a bad
connection/plug/socket/connector somewhere. Can you get at the
insides of it and have a good look at such things?

It's got a moulded mains plug on the power lead and a kettle lead type
connector going into the keyboard, neither of which look dodgy. I haven't
investigated further because the thought of taking the monster apart scares
me to death :-)

I'll have a look later to see if the mains input part of the keyboard comes
apart seperately with a couple of thoughtfully placed screws. Including,
perhaps, a slightly corroded fuse holder? Yeah right.

Thanks all, for the thoughts.

Si