I’m experience a problem i never see before. I have an aoc tv lcd 26inc which is dead but after a minute with supply On then the standby light comes on and after you press power the light go off and nothing happen. By looking at psu voltages without the connecter in mainboard just by shorting the ps On voltages remain ok 24v 12v and 5v seems psu is ok, the strange is that when I put the connecter in the mainboad the 5v become 4v so no standby light On. The 12v and 24V were good. After a minute what happens is that the stand by light comes On and 5 volts are back but then no 12volts and no 24v im very stuck thanks.
In your situation it can be the problem in your power supply ( no regulation) or something shorted in the mainboard that causes power shutdown. If you have an external power supply than comes with the the voltage/current setting you can actually pump in the 5,12 and 24 volt to the mainboard and see if the supply line will draw lots of current. If less or normal current draw this means the dc voltages line components are good and probably the power supply can’t handle the power draw from the mainboard and shutdown. In this case you may have to check the power supply feedback circuit because without connecting to mainboard it works fine but with mainboard the output power drop. It can be an open resistor, resistor increse in resistance, bad TL431 IC or even bad Optoisolator IC. You may have to direct replace the TL431 and Optoisolator IC and retest because these two ICs can breakdown when under load.
Hope this helps.