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It sounds like you may have fried your motherboard if you heard a loud pop when you turned it on. Pull all the cards and memory off the motherboard and turn it on. If you do not hear any beeps then, the motherboard is broken.
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Did you re-wire the 20 pin connector? or just connect it upside down?
is this the psu from PC Power and Cooling? If so you shouldn't have had to re-wire it
HAs the problem been fixed?
what did dell say?