Dell OptiPlex 990 (Early 2011) - Manual (Page 38)

10. Slide the power supply inward and remove the power supply.
Installing The Power Supply
1. Place the power supply in the chassis and slide it outward to secure it.
2. Tighten screws to secure the power supply to the chassis.
3. Connect the cables to the system board.
4. Install the heat sink .
5. Install the intrusion switch .
6. Install the drive cage .
7. Install the front bezel .
8. Install the cover .
9. Follow the procedures in After Working Inside Your Computer .
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