User`s manual

mantle the unit. If you have a tower computer case,
though, the switch is located on the front panel, and
connected to the power supply via four wires. All you
have to do is unplug the wires from the switch -- with
the computer unplugged from the wall, of course --
and test the switch with an ohmmeter. If you want to
do a hot test of the switch (that is, bypass the
switch), you can short the power wires together using
two insulated jumper wires and plug the computer
back into the wall. Just be careful that the jumpers
don’t touch anything.
Let’s now look at the DC voltages. (If you
removed the AC wires from the front-panel power
switch, replace them first.) With the main switch off,
locate a free power connector (the 5-1/4 inch version,
Figure 1d, is preferred) or unplug a floppy drive to
free up one. Don’t unplug the hard disk; you’ll need it
for the entire duration of this test. Power up the PC,
and measure the +5-volt (red) and +12-volt (yellow)
lines using a VOM (black is ground). Make sure they
fall within the voltage range specified in Table 2.
If they are out of range, power off the system
and disconnect the mechanical drives one at a time,
beginning with the
floppies. Measure the
+5- and +12-volt lines
at each step. This will
tell you whether or not
the power problem is
specific to a device.
Don’t forget to power
off the system each
time you disconnect a
device. With the hard disk(s)
still connected, remove plugs
P8 and P9 (Figure 1) from the
motherboard.
Finally, it’s time to deal
with the unlikely possibility of
a shorted hard disk. If you
have more than one hard
disk, start shedding them one
at a time. When you’re down
to your last hard disk, unplug
it and connect its power plug
to the dummy load shown in
Figure 2. (I don’t recommend
running a PC power supply
without a load.) If the power
supply is still dead, it’s off to
the drawing board.
The Drawing Board
N
ow that we’ve done all that we can do with the
power supply inside the cabinet, it’s time to
remove the unit and place it on the workbench. Since
we’ve already disconnected all the power connectors,
it’s a simple matter of removing the mounting screws
and sliding the power supply out of the cabinet,
right? Well, hopefully.
Unless you have a tall tower, you’ll probably run
into obstacles, like adapter boards, disk drive signal
cables, and support brackets. If you’re
lucky enough to have a detailed user’s
manual, it shows you the procedure.
Otherwise you’re on your own. In either
case, make notes of where everything is,
how they’re connected, and keep the
screws with the items they came from.
WARNING: MAKE SURE THE PC IS
DISCONNECTED FROM THE WALL
BEFORE STARTING DISASSEMBLY!
If the supply was powered from the AC line with-
in the last few minutes, the large electrolytic capaci-
tors in the high-voltage section will most likely still
have a charge in them that could give you a shock-
ing awakening. If so, let the power supply rest for a
while before you crack the case.
Each case has its own method of construction,
but generally two sides of the enclosure are what pro-
tect the inside electronics. Remove the cover screws,
taking care to watch out for attached leads, switches,
and sharp edges. If you have to disconnect any leads
(typically fan wires) or mechanical parts, note care-
fully how they go back together.
Give the electronics a good looking over, paying
attention to any scorched or burned parts that may
point to a failure. If you have a built-in power switch,
now is the time to check it. Next, check the fuse. Is it
blown? If in doubt, use the VOM to test for continuity
(use the X100 range). If the fuse is blown, replace it
with one of the same type and rating before going
any further. It’s possible the trouble is the result of
metal fatigue or mechanical failure of the fuse itself.
To see if this solved the problem, connect the
dummy load to one of the drive connectors and
apply power.
If nothing happens, remove the dummy load
and proceed to the resistance checks procedure. If
the fuse blows with an explosion, go to the high-volt-
age repair section.
Resistance Checks
R
eferring to Table 2, perform a resistance mea-
surement test. Keep the VOM’s polarity correct,
that is red to ground when testing a negative source,
and wait for the filter capacitors to charge before tak-
ing a reading. The resistance values listed in Table 2
are only representative (the figures were gathered
from actual measurements of several power supplies
using a cheap VOM), so don’t worry if your values
are different from those listed.
However, if a resistance value is abnormally
high or low, you have a problem. As a rule of thumb,
a reading of 50 ohms or higher on the 5-volt and 12-
volt lines means the output is probably okay. A resis-
tance value of 40 ohms or less indicates a short, gen-
erally in the rectifier diodes. The five-volt line is the
most prone to failure because it carries the heaviest
load (typically 20 amps). An extraordinarily high
resistance reading indicates an open, probably a
zapped board trace or a burned resistor. Both condi-
tions are often harbingers of problems in the high-
voltage section, but not necessarily. It depends on
how fast the shutdown circuit reacted. But before we
face that possibility, we first need to find the extent of
the low-voltage damage.
Low-Voltage Repair
T
he low-voltage section of the power supply is a
very simple rectifier, L-section filter design (Figure
3). Key to the success of this design is a multiple sec-
ondary power transformer. There is a 5-volt winding
and a 12-volt winding. In high-power supplies (250
watts and larger), there are usually two five-volt wind-
ings that are paralleled for higher output current —-
yet treated as a single winding.
Reprinted from September 1996 Nuts & Volts Magazine. All rights reserved. No duplication permitted without permission from T & L Publications, Inc. 2
Table 1. Power Supply Color Codes
Wire Color Voltage Use
Red +5V Motherboard, adapter cards, disk drives
White -5V Logic circuits (rarely used in modern PCs)
Yellow +12V Disk drive motors, RS-232 serial port, fans,
adapter cards
Blue -12V RS-232 serial port, fans
Orange n/a Power OK signal
Black 0V Ground (GND)
L:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:J
Figure 5. The switcher section is the
most common to fail. The power transis-
tors have to have a breakdown voltage
of 600 volts or more, and the damper
diodes have to be fast recovery (a
1N4005 won’t work).
Safety First!
Would you put a hairpin in an AC outlet socket? Not
hardly! So why would you consider putting your finger in
a power supply that is clearly labeled CAUTION!? Always
unplug your PC before going under the hood. Once there,
pay attention to my WARNING! signs. I’ve done my best
to make the troubleshooting processing as shock free as
possible, but power has to be provided at various stages
of the game. Be alert, don’t be stupid, and if you don’t
know what to do next, stop now!
Figure 4.
You can gen-
erally identi-
fy the semi-
conductors
by their
shapes.
From left to
right, the
first three
are diodes,
+12V rectifi-
er, +5V recti-
fier, and
switching
transistor.