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I picked up a new XFX Radeon HD 7850 Core Edition 1GB to replace a HD 5750. When it's all hooked up the PC won't turn on. When I pop the old card in, everything works fine. I have a 600 watt ultra power supply, which is 100 watts more than the recommended wattage.

Could it be the 8-pin to 6+2-pin connector? The doc says it needs 6-pin PCI-E from the PSU, but all I had was the 8-6+2 pin. Or could the PSU perhaps not have enough juice? I guess it could also be a dead video card. Hopefully not.

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Did you try disconnecting all other drives and such connected to PSU – DaBaer Feb 6 '13 at 18:20
A good way to make sure it's not the card would be to drop it into another computer. It very well could just be DOA. – Shinrai Feb 6 '13 at 18:22
Also make sure you have enough amperage on the +12V rail that the PCI-E external power connectors are on. A 7850 definitely will require more amperage than a 5750, so if you have one of those PSUs with multiple rails, then the current is split among the multiple rails, and the current from any one rail might not be enough to supply the GPU. – allquixotic Feb 6 '13 at 18:27
Thanks for the tips, I'll try disconnecting all the other components and see if it spins up. – af-at-work Feb 6 '13 at 21:58
I suspect the power supply might not have the juice too... would like to rule that out first ;) – af-at-work Feb 6 '13 at 21:58

Replacing the PSU with a larger 750 WATT unit did the trick.

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