Loosen the three Phillips screws securing the rear panel to the iMac.
These screws are captive in the iMac. The center screw will stop turning after about 3.5 turns and the outer screws will stop turning after about 5.5 turns. Do not try to remove these screws from your iMac.
Lift the optical drive near the connector to separate it from the logic board.
Lift the free end of the optical drive just enough to clear the fan cover. Lifting the free end of the optical drive too far may break the plastic positioning pins off the front bezel.
Lift the free end of the optical drive slightly, then pull it away from the edge of the rear case to clear the two plastic positioning pins.
Lift the optical drive out of your iMac.
When reinstalling your optical drive it may be necessary to set the bezel attached to the open end of the drive on the plastic positioning pins molded into the front bezel of the iMac and pull the connector side of the drive toward the edge of the rear case, bending it outward slightly, to properly seat the connector.
Rotate the center Phillips screw on the bottom of the iMac clockwise until the rear panel clamp contacts the edge of the case.
On our specific machine, the threaded portion of the clamp broke off the clamp body causing the center screw to turn without moving the bracket. If your machine is broken like ours, skip to the next step.
While depressing the connector lock, insert the flat end of a heavy duty spudger into the gap between the power supply connector and its socket.
Twist the heavy duty spudger to slightly separate the connector from its socket.
It will be necessary to work from alternate sides until the connector is free.
Your power supply may have an ambient light sensor cable attached from the power supply to the logic board. If so, carefully use a spudger to push out the cable towards you using the sides of the cable alternatively to help you slide it out.
There are several capacitors mounted to the logic board in close proximity to the power supply. Try not to disturb them, as they are delicate and may break off the board.
Grab the power supply from each side and rotate its top edge toward yourself until it clears the logic board.
Lift the power supply out of the midplane.
The power supply requires a large amount of force to remove. You may need to slightly bend the edge of the front case toward yourself for enough clearance to remove the power supply.
Remove the single 8 mm brass Phillips screw securing the inverter to the logic board.
Lift the inverter straight up off the pins on the logic board.
Be sure to lift the inverter straight up off the logic board, as the connector pins are very long and easily bent. If the pins bend during removal, use the connector on the inverter as a guide to bend them straight.
The connectors removed in the next few steps slide onto long pins attached to the logic board. Be sure to only pull them straight up from the logic board to avoid bending the pins.
If you do bend the pins during removal, use the connectors as a guide to bend them back straight.
Use the tip of a spudger to lift the speaker cable connector from its lower edge straight up off the logic board.
Insert the tip of a spudger into the center hole punched into the side of the lower fan connector.
With the spudger still inserted, lift the lower fan connector straight up off the logic board.
The speaker cable connector attaches to the pins closest to the power supply. It is the only connector with four wires. The microphone cable attaches to the pins furthest away from the power supply and is the only connector with a dark orange wire.
Using its attached pull tab, lift the AirPort card slightly and pull it straight away from its socket.
This step does not apply to the ALS model; this bracket is not present. The card attaches with a pair of T6 Torx screws and looks slightly different from this picture.
Depending on your iMac model, the bluetooth board may be blue. Use your fingertip to lift the Bluetooth board from its right edge, disconnecting it from its socket on the logic board.
Be sure to lift the Bluetooth board from its right edge only. Trying to lift it up from any other side will shear the connector off the Bluetooth board.
Use the flat end of a spudger to pry the Bluetooth antenna connector up off the Bluetooth board.
Grab the logic board from both edges and lift it out of the midplane, minding any cables that may get caught.
When reinstalling your logic board, be sure to clean and apply a new layer of thermal paste to the U3 chip (highlighted in red) located on the underside of your logic board as well as its copper heat sink attached to the midplane.