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.

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 <holrum@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. > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > -- Clint. M0UAW, 2E0EOA, M6KOX *No trees were harmed in the sending of this mail. However, a large number of electrons were greatly inconvenienced.*