I just swapped power supplies... no joy.
I gather the motherboard tells the power supply what to do, so maybe it's more serious than I thought.
With the power supply turned on there is power to the USB ports (my card reader LEDs light up) but nothing else.
When I've just turned the switch on at the back of the power supply, and then press the power button on the PC, the power LED flashes, and I get a pulse through the speakers. Otherwise, there's no action.
I've had a look at the motherboard for burns but none evident, and I've checked connections and blown it all out in case some dust was bridging some tracks on the motherboard...
Running out of ideas here...

so I'll appreciate any ideas others may have...
Might be a new motherboard is required?
Al.