I had similar problem with my rig.<br />

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My 4870hd was in full fan speed and does not boot when I power On the CPU. The screen was pitch black and the fan is in 100% full speed until you turn off the power.<br />

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What I did was I ditched the PCI connector (x2) from my power supply and used the x2 4 pin Molex Adapter to x1 PCI connector (x2) for my card and then tadaa.. its working fine again. I lost fans in my case though because I dont have enough 4 Pin Molex Connector from my PSU.<br />

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I think our power supply (or at least the connector from the Mobo) is giving up or has given up.