Dell OptiPlex 990 (Early 2011) - Manual (Page 49)

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Power Supply
Removing The Power Supply
1. Follow the procedures in Before Working Inside Your Computer .
2. Remove the cover .
3. Remove the PSU thermal sensor .
4. Disconnect the 4–pin power cable from the system board.
5. Unthread the 4–pin power cable from the chassis clips.
6. Disconnect the 24–pin power cable from the system board.
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