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Lifehacker's Rick Broida show you how to replace a dead power supply in a PC, he writes -

"Power supplies die. You never know when it's going to happen, but it's always at the worst possible time. You hit the power button and your PC just sits there. Idle. Quiet. Dead. At least hard drives have the courtesy to make that wrenching death-rattle when the end is near. But not power supplies: They just give up the ghost. Poof.

Here's how to extract and replace a power supply and, in the process, resurrect your fallen PC." - Link.

Related (other things you can do with a PC power supply):

• A PC solderless power supply for experimenters - Link.
• ATX Power supply to bench unit mod - Link.
• HOW TO - Make a microcontroller based DC power supply - Link.
• DC Power Station for a lot of Cell Phones? - Link.

More:

• Building The Perfect PC 2nd Edition - Regardless of your technical experience, Building the Perfect PC will guide you through the entire process of building or upgrading your own computer. You'll use the latest top-quality components, including Intel's Core 2 Duo and AMD's Athlon X2 CPUs. And you'll know exactly what's under the hood and how to fix or upgrade your PC, should that become necessary. Not only is the process fun, but the result is often less expensive and always better quality and far more satisfying than anything you could buy off the shelf - Link.