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My Xbox 360 won't power on. I cracked open the case (beyond warranty) and started testing voltage points with a multimeter. No point was registering anything. So I tested the power supply, and it only registered 5V, but it should be 12V.

To narrow down the problem, and be sure, would the power supply start supplying the full 12V only when plugged into a working Xbox (like a failsafe)?

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The Power Supply does provide 5 volts to the xbox through what is known as +5VSB (VSB means Volts StandBy) this in turn is then shorted to POWER_ENABLE to turn on the power supply and activate the three 12 volt rails. This short only happens when you press the power button on the Xbox and it turns on. The short is then removed when the console no longer requires power.

The +5VSB is also used to power and charge anything connected to the USB ports on the console when it isn't on.

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As a clarification, Power_Enable is pulled to 5v high, the entire time the console is fully on. – Passerby Apr 12 '13 at 6:16

I highly doubt it. That would be a strange design, with no practical benefit. Sounds like your power supply is junk.

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Thanks, Matt. I figured that, but I don't know enough about electronics to know if that would be unusual. Looks like I'll be putting in an order for a replacement. – user1097108 Sep 5 '12 at 5:31
While old, that is incorrect. The Xbox 360 Powersupply does not enable 12v power output unless there is 5v (or so) on the Power_Enable pin, as the other answer has stated. It is for power conservation. No need to enable a 12v 16 AMP power supply unless needed. – Passerby Apr 12 '13 at 6:15

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