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?
Did I mention that an upgraded power supply can be better than repairing a marginal power supply because:
The time it takes to troubleshoot the marginal power supply to component level may mean that you get to keep the old junk because the customer went out and bought a new computer. Any time and money you invest is unrecoverable.
It is more worthwhile to quickly replace the bad power supply or build a whole new machine with new (often sale priced) parts for those who would more appreciate that.
If this is your own machine and you are flat broke/unemployed with time on your hands, you have my sympathy and prayers.
Have you checked the output voltages before and during failure? Most supplies have shut down protection from out of tolerance voltage or current. Even a noisy adjustment pot can trip a supply.
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