If it's not restarted after a power down and there's no 'ticking' sound
from it then it's likely to be a problem with the bootstrap supply,
switchers will run for years once they're running but while they're running
the small capacitor (of the order of 47uF) and the resistor that feeds it
(often 220K-470K but can be outside that range) will dry up or go high in
value and as soon as you power it down and try to restart, it fails because
that little circuit doesn't provide the supply needed to kick the chip into
life.

First order of business, check the rails on the secondary for shorts
(tantalums have been mentioned by someone else) and then, after making sure
the main reservoir capacitor is discharged, check and high value resistors
and low voltage/value capacitors on the primary.

The data sheet for SMPSU control chips like the UC3844 usually have a
reference circuit which is often copied by SMPSU manufacturers with only
minor component changes.

On 6 March 2018 at 04:36, Mark Sims <hol...@hotmail.com> wrote:

> First check for power / ground shorts on the power supply outputs with an
> ohm meter.   There is a good chance of a shorted tantalum cap somewhere in
> the system shutting down the supply.  If the power supply is connected to
> the system via a connector,  disconnect it from the system and see if the
> supply puts out anything.
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-- 
Clint. M0UAW, 2E0EOA, M6KOX

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