Replacing A Power Supply  

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 
http://www1.amd.com/athlon/power 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 http://www.pcpowerandcooling.com. 

How to replace it 

Replacing the power supply can be relatively easy and safe if done 
correctly. First shut down your machine, unplug the power cord, and take 
the cover off the case. Disconnect all the power cords from the drives and 
the motherboard.  

On the left you see a typical connector used for your hard drives and CD-
ROM drives. This is often called a Molex connector. To remove them, simply 
pull them out. But don't pull at the wires, grab the white plastic part of 
the plug and wiggle it back and forth while you pull. These can be tough to 
remove sometimes. 

Here you see a typical connector for your floppy drive. It is commonly 
called a mini-plug. It also can be easily removed by simply grabbing the 
white plastic part and pulling it off. When you pull it off, make a mental 
note which way it was on for later. 

This is a typical ATX style motherboard power connector. You notice that on 
one side it has a little clip that secures it in the socket on the 
motherboard. Make sure you push this clip when removing it so you don't 
yank the entire socket off the motherboard. This article is only about 
replacing your power supply, not your motherboard ... Note: The older AT 
style connectors actually consist of two identical looking plugs 6-pin 
plugs, usually labeled P8 and P9. They plug in right next to each other. 
Make a mental note that the two black wires go next to each other in the 
center when you reconnect them. 

Now that you've disconnected all the cables from the power supply you can 
remove the power supply itself. Look at the back of the case. There are 
four screws around the power supply, on the outside of the cutout for the 
power supply. Remove the screws, but be sure to grab the power supply with 
one hand and hold it in place when removing the last screw so it doesn't 
drop into the case and damages any components. Carefully lift it out. 

That's it. You just successfully removed the bad power supply. Wasn't that 
hard, was it? And now that you're familiar with it and how it fits into the 
case, installing the new power supply will be even easier. 

Installing the new power supply 

Before you install the new power supply, take a look at the back of it. 
There should be a red switch right underneath the plug for the power chord. 
Be sure that this switch is set to the correct voltage. For example, if you 
live in the U.S. it should be set to 115V. Most of the time they are set 
correctly by default, but you don't want to take any chances, so take the 
extra second and check to be safe. 

Now carefully insert the power supply into the case and position it in the 
proper position. You'll notice that the cutout for it is oddly shaped and 
that it only fits a certain way. Screw it in securely. Then look at the 
bundle of cables coming from the power supply. Figure out which string has 
what type of connectors on it. Identify the connectors and figure out the 
best way to use them. 

First connect the power connector for the motherboard. If you look closer 
at the bottom of the connector you'll see that some of the 20 pins have 
different shapes. This prevents you from inserting it the wrong way. In 
addition, you'll notice the clip on one side. The socket on the motherboard 
that receives this plug has a notch on one side where the clip goes. 
Another easy way to match it up the right way. 

Similar case with the power connectors for the drives. They both are 
uniquely shaped and only fit one way. The bigger one is for the hard 
drives, CD-ROM drives etc. and the smaller one is for the floppy drive. But 
since you had to unplug the old power supply connectors you'll probably 
remember how they fit anyway. Go ahead and connect the appropriate power 
connectors to all the drives in your computer. 

As a last step take all the cables dangling around in your case, gather 
them in an orderly fashion and carefully tie them up with plastic zip ties. 
This will make working inside the case much easier in the future and 
optimize airflow as well. Now double-check all your connections, make sure 
you didn't forget to hook up any of your drives, make sure the power supply 
is securely mounted, that no cables are dangling near any fans, that the 
power supply is set to the correct voltage, etc. 

AT-Style Power Supplies And Power Switches 

If you're installing an AT-style power supply, you'll notice that there is an 
extra cable that we haven't mentioned yet. It is for the power switch. One of 
the differences between AT and ATX is that with AT-style cases the power 
button connects directly to the power supply to turn the power on and off. On 
ATX-style cases the power button connects to the motherboard instead. 

The replacement AT-style power supply probably came with not only the cable 
to the power switch, but also had a new power switch attached to it. This 
will leave you with two choices: 

1) Use the new switch - This means you have to remove the old switch and 
install the new one instead. While this is theoretically possible, it is a 
major pain in the butt, because old AT-style cases are usually a pain to work 
with. And replacing the power switch is no exception. 

2) Use the old switch - This means you leave the old switch in the case. If 
you look at the switch from inside the case, you'll notice that it has four 
different colored wires that run to the power supply. Carefully write down 
what color wire connects to what pin, then simply pull the wires off each 
pin. After installing the new power supply, remove the new switch it came 
with and plug the four wires onto the pins of the original switch following 
the color coding you wrote down earlier. 

Our recommendation: Go with the second option. Keep the old switch unless you 
have reason to suspect it's bad and needs replacing. Using the old switch is 
a lot easier, and doesn't require removal of the front bezel, trying to screw 
tiny screws into even tinier plastic holes at impossible angles. 

All set? Then close 'er up, plug in the power chord, push the power button 
and enjoy the quiet hum of your new power supply providing your computer 
with the juice of life! 


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