"Cursitor Doom" wrote in message
...
On Thu, 21 Jan 2016 15:10:21 +0000, MJC wrote:

Could it be an electrical discharge? Try viewing in the dark (with
precautions against contact!)...

I like your thinking so I tried it. But no sign of any arcing at all.
Anyway it really does sound like more of a hissing than a rasping.



I would look for anything shorted on the secondary side of the supply. Pulls
the switcher frequency down into the audible range. (for those of us who can
still hear anyway...)

Usually the root cause is a regulation problem (bad caps mostly) causing the
monitored voltage to "appear" low, the switcher duty cycle increases, the
actual DC output rails out, usually shorting a zener diode which gives up
it's life to save components downstream.

Example: an 18 volt zener across a 14 volt line shorts because now that line
has shot up to 35 volts.

Common problem in older consumer vcr's, but 'scopes etc often do the same
thing.

My .02 worth...


Mark Z.