Dell OptiPlex 390 (Mid 2011) - Manual (Page 49)

9. Lift the power supply out of the computer.
Installing the Power Supply
1. Place the power supply in the chassis and slide towards the back of the
computer to secure it.
2. Tighten the screws securing the power supply to the back of the computer.
3. Connect the 4-pin power cable to the system board.
4. Connect the 24-pin power cable to the system board.
5. Thread the power cables into the chassis clips.
6. Connect the power cables connected to the hard drives(s) and optical
drive(s).
7. Install the cover .
8. Follow the procedures in After Working Inside Your Computer .
49