Dell XPS 8500 (Early 2012) - Manual (Page 112)

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3
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1 screws (4) 2 power supply
3 power supply clamps (2)
Replacing the Power-Supply Unit
1 Slide the power supply towards the back of the chassis.
2 Align the screw holes on the power supply with the screw holes on the
chassis.
Power-Supply Unit 112