theres a better way to find out if its the psu or mobo ,do as follows
1) set the psu on/off switch to off or unplug the power cord
2) unplug the main atx connector from the mobo
3) use some wire thats stripped in both ends and put it between the pin with the green wire to one of the blacks
4) set the switch to on or put back the power cord , and then youll know.. if its good it will power up