Hi everyone, I appreciate the responses.
On this occasion, it was on a separate
socket/extension but all the power on stage was from the same ring so not much in the way
of true isolation. I always ask if there is a separate supply for lights/effects but
usually the answer is no!
However, now you mention it the cable in this case
did cross close to the USB cable - almost touching in fact - which is almost certainly not
how I've had it before. I suspect there is a clue here!
The keyboard gets its
power from the USB cable as well incidentally.
I'd be happy to try an RF filter
on the mains connection if it is likely to afford some protection. Even if I'm not using
my smoke machine in future, if my USB connection is susceptible to interfence to rogue
mains fluctuations, I guess it could happen anywhere!
The USB Selective Suspend
suggestion is interestring although I'm using a MacBook here - I wonder if there is a Mac