Jim wrote:

Hi all,

It's a standard computer power supply - electrically ATX type, but in
a size for a desktop (not tower) - which makes a cheap source
difficult. Size is WxLxH = 120x155x85 mm.

The brand name is Pro-V Power, model MPT-251 (and MPT-301). It is made
by Marcon.

The MPT-251 failed, so I replaced with MPT-301 which worked for 6
months followed by identical failure.
I am NOT overloading it.

Maybe not according to the manufacturer's joke rating plate


I feel almost CERTAIN the problem is dried out electrolytic capacitors

That comes with time, heat, poor design and under-rated or fake
components.


(probably just one) - but there are so many and it is tight in there.

The next replacement I acquired has worked for over a year, but is
horrible - noisy, also fan bearing noise at start-up, also horrific
radio interference.

The symptoms are - when attempting to switch on, the CPU fan runs for
a couple of seconds, then nothing.

If anybody could help with a SCHEMATIC or tips, that would be great.

Sounds to me like you're buying cheap junk supplies. Try a decent brand
name.

Graham