I just finished upgrading my HTPC with an A4-5300 & MSI FM2-A75IA-E53 mini ITX motherboard. The computer will power on once about every 20 attempts. Otherwise, I will press the power button and nothing will happen - fans don't spin, LEDs don't turn on, nothing! Old motherboard (MSI E350IS-E45) had no issues.
Eventually, I unplug the power supply & try again 10 minutes later, and the computer will boot right up and work fine. I managed to install Windows 7, etc, this way. But as soon as I power it off, I can't get it to power up again. Worth noting, the power usage (as shown on a kill-a-watt) goes down to 0 watts. If i unplug the power supply and plug back in, power usage goes to .5-1 watt.
I've reseated the power connectors & switch connectors. I also used my multimeter to test the power switch - it's working properly.
Any ideas, or is this just a bad motherboard? Case is Antec ISK300-65. When working, power usage is around 20-40 watts at the wall, with a spike up to 70 during the WinSAT test.
Iwould try a different PSU, even if it's outside the case, just take the one of your desktop that you know that works and try it and see. Help eliminate or confirm it's a PSU. If it's not that, then it's probably the MB. usually the problem you describe happens on old boards with a dying capacitor, I see it a lot. But not much on new boards. Either way, 10 mins and a star head and you can test if it's PSU or the MB.