On 1 sep, 14:01, poster wrote:
but when you test the power supply *with an older motherboard it
powers up the pc ok and outputs picture to monitor screen
The newer more power hungry mobo works ok though with another power
supply and before it also worked fine with now problematic PS
Replaced output filtering capacitors inside the power supply although
visually they looked ok but still the same problem, any ideas what to
check next please?

There is nothing else to check. You just determined what is wrong: the
original PSU. The fact that the PSU can power an older motherboard,
just proves that the PSU is not powerful enough to power your new
motherboard. ATX PSUs tend to degrade over time... capacitors lose
capacitance, ripple increases... I know that you replaced the
capacitors, but with what? Did you used low esr replacements? What is
the wattage of the PSU? What CPU and motherboard are you using?