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Gareth Magennis Gareth Magennis is offline
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Default Interesting little HK fault today ...


"Arfa Daily" wrote in message
...
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



Dry joints on these thermistors are surprisingly common. I think it may be
because the higher powered ones are kind of tall and weighty and have no
support other than their leads.
Powered speakers are a favourite for this, possibly because they get a lot
more mechanical shocks in transport than your average punter would give to,
say, his desk or FX rack, and also because they get a LOT of vibration in
use.

I always wiggle them whenever I see them, and if the PCB comes out I will
always resolder them (along with the usual suspects, e.g. zener diodes,
power resistors that get hot etc).


Cheers,


Gareth.