It’s 8:00 a.m., the neighbor’s dog barked all
night, your coffee tastes like weak tea, and the phone
message light blinks frantically. Full of resolve, you
flip on your PC’s power switch, and ... presto —- nothing! No lights, no beep, no fan, nada. Suddenly you
realize, it’s gonna be a really bad hair day.
While there’s nothing I can do about the early
hour or the coffee, I can probably help you get your
PC back on its feet. The most common case of
“Sudden PC Death Syndrome” is a defective power
supply. The problem can come from many sources,
like heat, power surges, and old age. While it’s easy
enough to replace a power supply by swapping the
old for new, it’s not always practical.
A case in point: I have an AST 486SX that died
when a truck plowed into the corner power pole and
caused a two-hour black out. When the power came
back on, my PC didn’t. A quick check showed the
cause was a fried power supply. Unfortunately, a call
to AST revealed, to my horror, that a replacement
power supply costs $150.00. Moreover, because of its
unique case design, there’s no generic substitute.
Fortunately, it’s not difficult to fix PC power supplies. While they may look different on the outside,
most PC power supplies use the same electronics on
the inside. In this article, I’ll show you how easy it is
to fix a dead power supply.
The Basics
The power supply is a large metal box, mounted
inside the PC that provides power to the motherboard and various peripherals. It’s easily identified by
a warning sticker on the case that reads “CAUTION!
Hazardous Area” (or a similar high-voltage warning).
On the back of the power supply is an AC connector that plugs the PC into the wall. Often there’s
another AC connector that’s used by some monitors.
Most power supplies also have a voltage selector
switch that lets it work with 110V or 220V power
sources.
A typical PC power supply provides four DC output voltages: +5, +12, -5, and -12 volts. These voltages are available through four different types of connectors (Figure 1; 1-4). The color of the wire identifies the voltage and its use (Table 1).
Getting Started
Alot of power supply failures are actually simple
problems that are easy to fix. Obviously, the
place to start is at the beginning -- in other words, are
you getting power from the wall to the PC? As stupid
as it sounds, the first thing to do is look under your
desk and see if the PC is plugged into the wall. If it is,
move the plug to a different socket (they go bad, too,
you know).
That done, pull the
power cord from the
back of your PC and
see if the power is getting that far. You can
do this using a VOM or
a simple neon lamp
circuit tester, like part
number 22-102 from
Radio Shack.
If there’s no power,
and you’re plugged
into a power strip or
surge protector, the
strip is probably the
culprit. To test it, simply remove the PC’s
plug from the strip and
plug it into a wall socket. If the PC starts working, the
problem is in the strip. Generally, the problem is a
blown fuse or a tripped circuit breaker. You’ll find
both at the cord end of the strip. The last item you
should test before popping the hood is the power
cord itself; replacing it with another cord is the fastest
and safest method.
Under The Hood
Still nothing? Now it’s time to remove the cover.
Most covers are attached by five or six screws on
the back. Before going any further, carefully read the
instructions in the section called “Safety First.”
The next logical place to look is at the power
switch. Unfortunately, this may not be possible at this
stage of the game. Many power supplies have a builtin power switch which isn’t accessible until you disReprinted from September 1996 Nuts & Volts Magazine. All rights reserved. No duplication permitted without permission from T & L Publications, Inc. 1
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