Dell ChengMing 3990 Service Manual page 29

Steps
1. To disconnect the cables:
2. Disconnect the power cables from the system board and unroute them from the routing guides on the chassis
3. Remove the three (#6-32) screws that secure the power-supply unit to the chassis.
4. Press the securing clip and slide the power-supply unit away from the back of the chassis.
5. Lay the computer on the right side
6. Lift the power-supply unit off the chassis
Disassembly and reassembly 29