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PC911 > How-To > Hardware > Replacing A Power Supply

- Alex -

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The power supply is one of the most important, but also most ignored pieces of a computer. You plug it into the outlet and turn it on, what's the big deal, right? Wrong! The power supply has to work hard to provide a constant and stable level of electricity to the devices in your computer without fluctuations. It has to be strong enough to feed all the devices in your machine, and in some cases it has to be approved to work with certain parts of your PC such as an Athlon CPU.

What if it fails?

A power supply doesn't last forever. Sooner or later they'll fail. They can last all the way from a few months to many years, it all depends on the quality, how hard it has to work, and what conditions it is exposed to (temperature changes, bad electricity, dirt, etc.).

The component inside a power supply that is prone to fail first is the fan. It usually starts with a grinding or high-pitched noise that initially disappears a few minutes after you turn the PC on, but soon gets worse. Once the fan is dead, the hot air is not being properly exhausted from the power supply which causes it to overheat and accelerates its demise. In addition, often the power supply fan also exhausts hot air from the inside of the computer, and if the fan fails, you lose an important part of cooling.

Warning: Don't try to replace the power supply fan yourself unless you know what you're doing! It requires some soldering and should only be done by somebody who is familiar and comfortable with such a procedure. I rather recommend replacing the whole unit with a better quality one.

When the actual power supply fails, it can exhibit a number of symptoms. You could experience crashes, data corruption, or hardware failure. Another thing that could happen is that when you turn on your PC, the lights and fans come on, but it doesn't boot, because the BIOS cannot verify a sufficient and consistent power flow is established before it continues the Power On Self Test (POST) and the boot process. Or the PC does not boot at all if the power supply is completely dead and nothing happens at all when you push the power button.

What to buy

Either way, it's time to replace the bad power supply. Before you go shopping for a new one, find out what type of power supply is in your machine. There are two main styles, AT and ATX. The older AT style is usually found in first generation Pentium and older AMD machines. If your machine is less than 3 years old, it most likely has an ATX style power supply. The main difference between the two is in how they connect to the motherboard. The AT style has two identical looking 6-wire connectors, usually labeled P8 and P9 that plug right next to each other into a special connector on the motherboard. The ATX style has one 20-wire connector. But the easiest way to find out what you need is to take the cover off your computer and look at the power supply. It usually has a sticker on it that documents the type and power. For example, it might say something like AT-250W, indicating it's an AT style 250 Watt, or ATX-300W, indicating it's an ATX style 300 Watt power supply.

As computers get faster, have more room for expansion, and hardware requiring more power, it's important to buy a strong, good quality power supply. A 300W should be the minimum these days. If you plan to replace it in an AMD Athlon system, you should first consult the AMD web site at to find out what model is approved. Don't get the cheapest one in the store. This is an important investment, so buy quality. A good resource for high-quality power supplies is PC Power & Cooling at

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