Dell P2421DC Service Manual page 18

Use a Philips-head screwdriver to lock 3 screws for locking the key board and the middle frame.
S2
(No.1~3 screw size=M2X2.5, Torque: 6± 1kgf.cm)
Use a Philips-head screwdriver to lock 3 screws for locking the bezel and the middle frame.
S3
(No.1~3 screw size=M2X2.5, Torque: 6± 1kgf.cm)
Connect all of the cables between the power board and main board.
S4
stick the mylar by hand and Use a Philips-head screwdriver to lock 9 screws for locking the main board and the adapter
S5
board
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