User`s manual

If only the input pin is shorted, the rectifiers are bad.
If both are shorted, the chance is both the diodes and
the IC are shorted. To verify this theory, remove the
IC and check the resistance again. If it reads okay,
replace the semiconductors. The re-placement for the
-5-volt IC voltage regulator is an LM7905.
High-Voltage Repair
I
f the new fuse blows when you apply power, there’s
a problem in the high-voltage section. We know this
because the low-voltage section has an automatic
shutdown circuit that reacts a lot faster than the fuse;
that is, a low-voltage problem disables the power sup-
ply long before the fuse has time to blow. That does-
n’t necessarily mean the low-voltage outputs are
okay, because failure of the -12-volt line can cause
cascading damage that goes all the way back to the
high-voltage section.
The high-voltage section is divided into two
parts: the high-voltage power supply and the switch-
ing circuit. Most high-voltage failures occur in the
switching circuit.
WARNING: COMPLETELY DISCHARGE
THE INPUT CAPACITORS BEFORE WORKING
IN THE HIGH-VOLTAGE SECTION!
If the fuse has a “mirrored” look to it, you can
bet the farm that at least one of the two switching
transistors is shorted (Figure 5). Typically they perish
as a couple. These transistors are mounted on the
heat sink(s) closest to the two largest electrolytic
capacitors (see “How To Find Waldo”). With the red
probe of a VOM on the collector of the first transistor,
check the collector-to-emitter resistance, then the
collector-to-base resistance (Figure 4). If a short is
found, replace both the transistor and the damper
diode that’s across its emitter-collector. I normally
use a Motorola MJE13009 for the power transistor
and a 1N4937 for the damper diode.
You should also replace the low-value resistor
that’s in series with the transistor’s base. This resis-
tor is often used as a fusible link that goes puff
when the switcher fails. Its purpose is to protect
other components in the chain from harm. If the
resistor is burned beyond recognition, you can
replace it with any 1/4-watt resistor with a value of
1 to 10 ohms (the exact value isn’t important).
Sometimes, though, even the fusible isn’t fast
enough to prevent damage. So before installing the
new parts, it’s wise to check out the pulse shaper
network (typically a resistor-diode-capacitor combi-
nation) associated with the base circuit, too. A
quick way to test all three components at once is
to treat the network like a single diode, checking it
as a whole for shorts and opens (Figure 7). Now
repeat the procedure for the second switching tran-
sistor.
The high-voltage supply is a simple voltage
doubler with an output of about 300 volts (Figure
6). While this section rarely fails on its own, a
shorted switching transistor can wipe out the
bridge rectifier in an instant. Check the AC input
for shorts, and replace the entire bridge if a short is
found. Bridges can be either discrete diodes or a
large, rectangular module, and you can find suit-
able replacements from Radio Shack. There’s prob-
ably a one-ohm resister in line with the AC input
that needs to be checked, too. On the outside
chance that one of the doubler capacitors is short-
ed, do a resistance check of each.
When powered from a 220-volt AC power source,
the capacitors serve as voltage dividers to provide
an artificial ground. Consequently, the capacitance
and ESR (equivalent series resistance) values of
the capacitors are critical when operating from a
220-volt line and have to be evenly matched, other-
wise the switching voltages will be uneven. As elec-
trolytics age, both the capacitance and ESR
changes. If the mismatch is too great, one voltage
could exceed the limits of the switching transistor,
which can start parts a-poppin’. You can check the
voltage balance with a VOM. Always replace both
capacitors, not just one, and use a good grade
capacitor, like the Panasonic TSU series.
It’s Showtime
I
f you’ve made it this far, you probably have a work-
ing power supply. But before you apply power, let’s
make sure we’ve covered everything.
-- You did a final resistance check on the output
voltage lines, and all are within the specifications of
Table 2, right?
-- You checked the resistance across the AC
input (with the power switch on) and it measures 1
megohm or better, check?
-- You checked the fuse.
-- Any broken wires or burned parts?
Good! Then it’s showtime. Re-assemble the
power supply. Plug the dummy load into one of the
disk drive connectors. Apply power.
If both lights light, congratulations! You’ve got
yourself a working power supply, because the power
supply itself needs the -5- and -12-volt lines to oper-
ate. Consequently, you don’t have to test them,
unless you’re as curious as I would be. Now all you
have to do is put everything back together and enjoy
a more peaceful day —- except for the coffee. Here I
suggest ... NV
Reprinted from September 1996 Nuts & Volts Magazine. All rights reserved. No duplication permitted without permission from T & L Publications, Inc. 4
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N
ine times out of ten, the troubleshooting techniques presented in this article will solve your PC power supply
problems. But what if the power supply still doesn’t work? There can be lots of reasons, ranging from a
faulty transformer (good luck finding a replacement) to a bad solder connection. In most situations, I’d cut
my losses and find a substitute power supply or try to salvage the motherboard for use in another system.
But if you’re really dead set on reviving the system, there is one more stage we haven’t discussed —- the PWM (pulse-
width modulator). But put your seat belt on, ’cause this is gonna be short and fast. It’s not for everybody.
The PWM (Figure 8) is what drives the switching transistors, and when it doesn’t work, nothing works. Consider
it the brains of the power supply. The PWM is generally a single IC chip, most likely a Motorola TL 494. But before
you replace the chip, let’s see if it’s working or not. For this you’ll need an oscilloscope and a power supply.
The simplest way to test the PWM chip is to grab a disk drive connector and pump +12 volts into its yellow wire
from an independent power supply. This can be done using another PC power supply or any other DC source (batter-
ies work, too). Once power is applied to the PWM chip, observe the output waveforms on pins 8 and 11. Both out-
puts should be active squarewaves. If at first you don’t succeed, ground pin 4 and try again. If the scope still shows
nothing, replace the LT 494 chip. If the scope shows waveforms, the most likely culprit is the LM339 comparator.
The LM339 is cheap, about a buck, and readily available, so it’s worth a shot.
My method of replacing an IC is to clip the leads as close as possible to the body of the IC, leaving 14 or so
metal pegs standing upright from the main board. Paying attention to direction, slip the replacement IC alongside
the pegs and solder the new component in place.
If by now the power supply still doesn’t work —- chuck it.
Still Don’t Work, Huh?
Figure 8.
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