Dell P2421D Service Manual page 16

Connect all of the cables to assemble the power board and main board.
S4
Use a Philips-head screwdriver to lock 7 screws for locking the main board and the adapter board
S5
(No.1~7 screw size=D3x6, Torque: 6± 1kgf.cm) (No.9 screw size=M4x6, Torque: 6± 1kgf.cm)
stick 2 pieces of aluminum foil. Use a Philips-head screwdriver to lock 2 screws for locking the mainframe
S6
(No.1~2 Screw size=M3x4, Torque: 3± 0.5kgf.cm)
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