What could be causing this On/off, on/off, on/off cycle?

As soon as I plug in the power cord, the xbox WANTS to turn on, but only gets power for a split second and shuts off. It will stay in this loop for as long as I keep it plugged in. The fan moves a tiny bit and you can hear a small click from the processor, but then nothing. The power brick turns green for a split second as well and then back to off. This cycle keeps repeating until I unplug the console.

Any Ideas what could be causing this?

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@garymdobis Have you tried a different power brick?

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No I haven't, that makes a lot of sense though. I bought the console from a "friend" and he told me it just needed a power brick. I bought a knock off from Ebay. I have the xbox torn down to just the motherboard as of right now and even with everything removed, when I plug the power brick in, it cycles.

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