on boot power to fans but not hd

im fairly new to macs, though i have extensive experience fixing pcs. i have an imac 27 that when i boot it up i dont head anything going on inside. i opened it up, and removed the display panel and i see that the two fans spin sporatically, approximately 3 seconds at a time, but theres no activity on the logic board (im assuming theres LEDs that would light up) and no power to the hard disk. i tried replacing the power supply as that usually is the cause of such issues but no luck. any suggestions would be greatly appreciated

thanks!