On Thu, 21 Jan 2016 10:49:34 -0800, etpm wrote:

I have heard high frequency arcs that sound just like air escaping, at
least to my ears.
Eric

Interesting. What component was failing? There's a 1.5kV transformer on
this board which is encased in opaque grey plastic so if it's something
arcing inside that it won't show up in the dark. I thought perhaps a old
AM radio next to the board might indicate something useful, but I'm
guessing it would be swamped by all the spurious noise these things make
even when they're running right.
Spectrum analyser, perhaps? Bit drastic but I do have one!