Remove the following four screws securing the power supply to the outer case:
Two 22.2 mm fine-thread T10 Torx
One 25 mm coarse-thread T10 Torx
One 9 mm coarse-thread T10 Torx
The next few steps bring your hands close to the exposed face of the power supply. To avoid a high voltage shock from the many large capacitors attached to the board, do not touch the face of the power supply.
Carefully lift the power supply out of the outer case and rotate it to expose the cable lock as shown, minding the DC-Out cable still attaching it to the iMac.
Disconnect the DC-In cable by depressing the locking mechanism on the connector while you pull the connector away from its socket on the power supply.
Once the locking mechanism has cleared the socket, pull the DC-In connector away from the power supply.
In the following steps, you will maneuver the logic board out from the outer case. In doing so, be aware of the many cables still attaching the logic board to the outer case.
Be sure the logic board clears the plastic lower optical drive fan mounting pin before lifting it up, as it is easily snapped off.
Now that the lower edge of the board is free from the outer case, rotate the logic board toward yourself to expose its rear face.
The following connector has a tendency to stick in its socket. If so, insert a metal spudger into the gap between the connecter and its socket. Twist the spudger to separate the two components.
Carefully pull the optical drive connector away from its socket on the logic board.
Pull the connector perpendicular to the face of the logic board.
Disconnect the DC-In cable from the logic board by simultaneously depressing the locking lever on the connector while pulling it away from its socket.
Remove the logic board from the outer case.
During reassembly of the logic board, pay attention to the position of the I/O connectors. When the board is back in the case, insert a USB or Thunderbolt cable into one of the connectors to align it perfectly.
Remove two T10 Torx screws securing the audio ports to the outer case.
Lift the audio ports out of the outer case.
A pair of tweezers is helpful to put the ports back in place. First insert the lower tab into the bracket on the outer case, then fasten the screw to secure the jack to the outer case.