Keep those electrons flowing by replacing your broken power supply!
Lay the iMac display-side down on a flat surface.
Loosen the three Phillips screws securing the rear panel to the iMac.
Lift the rear panel slightly near the bottom of the iMac.
Pull the rear panel toward yourself and remove it from the iMac.
Remove the three Phillips screws securing the power supply to the chassis.
Rotate the center Phillips screw on the bottom of the iMac clockwise until the rear panel clamp contacts the edge of the case.
While depressing the connector lock, use a heavy duty spudger to pry the power supply connector away from its socket on the logic board.
Lift the power supply slightly from its left edge, then grab each side and rotate the power supply toward yourself.