HK Actor, chassis only - the shop had removed it from the cabinet for me, as
they always do on these to save workshop space. Reported problem was "powers
up but no sound". At a first look, that did appear to be the case. However,
when the board with the PSU on it was moved ever so slightly, it came on,
but then stayed on until it was next turned off. Once on, no amount of
provocation would make it go off again. Likewise, when it was wrong, no
amount of tapping or hitting would make it work, but as soon as you turned
the chassis over, on it came. I then noticed that when it was wrong,
although the mains switch was lit, giving the impression that it was powered
up, the fan wasn't running. This is powered from its own personal supply
with a single diode and filter cap, straight from one side of the split
secondary winding of the power transformer, so if there is power to the
transformer's primary, then the fan pretty much has to run.

A bit more observation of what happened when it did come on, revealed that a
relay clicked. This is across the inrush thermistor, and again, its coil is
fed from a simple supply derived directly from the transformer secondary. It
then dawned on me what was happening, and a quick tug on the thermistor
proved it. One leg had sheared off inside the hole passing through the PCB,
making the thermistor effectively open circuit, so no supply to the
transformer primary. The switch of course lit up, as the neon is internal to
it. The gap in the broken leg was obviously big enough that the vibration
from tapping, was not enough to close it up, but when the board was flexed,
that was enough, and the leg momentarily made again. This was enough to get
the relay coil supply up, the relay contacts then bridging the thermistor,
so it didn't matter any more that it had gone open circuit again. And once
the relay contacts were shorting the thermistor, no amount of disturbance
would make it die again, until it was actually turned off.

What a pleasant change to get a nice positive fault with reasoning that
works .... :-)

Arfa