In message .net,
DaveC writes
Now when first powered on after an overnight rest, it will shut off
immediately (stays on for maybe 5 seconds). After power cycling the main
switch (can't be done via remote -- must use the switch on the back), it will
run all day without problem.
You've missed a capacitor or an open circuit resistor in the PSU
primary, specifically in the startup supply that feed an SMPS controller
chip before the main PSU starts.

Check the high value resistors and the low value capacitors (usually
under 100uF) in the primary circuits.

If you feel happy working on energised mains fed equipment then while
the PSU is in the faulting state, check for voltages on the outputs (if
you know the voltages expected then a bunch of LEDs with appropriate
series resistors can be very useful for missing voltage checks)

Power cycle it and then check again.

Bets are that it's not one single PSU, but a couple or more possibly on
the same board. Probably one of them isn't starting up correctly after
being off for a few hours and/or cooling down.

All PS voltages are OK (factory sticker inside lists them); all are a few
tenths high (ie, 5v = 5.04).
That's within 1% and the accuracy of your meter comes into play as well
at that sort of margin, I'd expect the voltages to be +/- a couple of
percent minimum, possibly up to 10% and still be OK so I don't think
there's anything to be concerned about there.

What next step would you take in troubleshooting this PS?
Go back and check the rest of the components one at a time.

Thanks.

--
Clint Sharp