User`s manual
Each winding has a grounded center tap to per-
mit fullwave rectification using just two diodes (full-
wave bridge rectifiers need four diodes). The direc-
tion of the rectifiers determines the polarity of the
output voltage. Common cathodes are positive, and
common anodes
are negative.
Because of its high-
current require-
ments, the +5-volt
rectifier is usually an
array of parallel
Schottky diodes in a
single package
(Figure 4) that
mounts on a heat
sink. The -5-volt out-
put is often derived
from the
-12-volt rectifier via
an IC regulator (typ-
ically an LM7905
equivalent) rather than from the five-volt
transformer winding. However, I’ve seen it
done both ways.
The output of the rectifiers is filtered first by
an inductor, called a choke, then by a heavy-
duty electrolytic capacitor. In some designs,
the five-volt line is double-filtered to reduce
ripple by cascading two L-section filters on the
output. Invariably, a bleeder resistor is placed
across the output to discharge the capacitors after
power off.
The most common cause of low-voltage failure is
a shorted rectifier. If one blows, so does its compan-
ion, which forces you to replace them as a package
deal. Second on the hit list is a shorted capacitor,
which usually does less overall damage. Most of the
time, the failure is limited to just one output line, but
there’s no guarantee.
The first step is to locate the shorted compo-
nents. For this operation you need access to the bot-
tom side of the printed circuit board. This is the hard
part, because no two supplies are alike. Use your
imagination, and be care-
ful not to damage other
components in the
process. For example,
twisting and turning the
board too many times
can cause attached wires
to break loose.
Now comes the tricky
part, because you have to first locate the affected
parts on the circuit board. Use the road map, “How
To Find Waldo,” to help you in your quest. An ohm-
meter is a good way to probe suspected areas for
shorted devices. Once the area is located, the real
work begins because it’s virtually impossible to tell
the difference between a shorted diode and a shorted
capacitor without removing one or the other. Since
the rectifier is the most likely culprit and the easier to
remove (the electrolytics are glued in place), I’d start
there.
The +5-volt and +12-volt diodes are most likely
nestled inside a transistor case mounted on a heat
sink. The bigger one (Figure 4e) is the +5-volt rectifi-
er, and the smaller one (Figure 4d) is the +12-volt
rectifier. The negative-voltage rectifiers are individual
diodes typically in a DO-41 case.
With the suspect rectifier or diodes in hand, do a
resistance check of the defective voltage output line
again. If the reading is within the normal range, trash
the old part or parts and replace with new. (Helpful
Hint: If the new diodes come in an axial-lead pack-
age, typically DO-41, solder them on the trace side of
the circuit board instead of the component side. It’s a
lot easier.) If the output still shows a short, yank the
electrolytic and check again. If the output is still
shorted, make sure you’re pulling the right teeth.
Exact replacement parts always cost more than
generics, so go with the generic. You can get “univer-
sal” replacements from GE, RCA, and Philips ECG.
Unfortunately, they’re almost as expensive as the
original. For the +5-volt rectifier, I recommend the
MBR series from General Instruments and Motorola
(available from Digi-Key and Allied Electronics,
respectively). The +12-volt rectifier is a dual Schottky
device that’s available from several vendors, and gen-
erally sells for a buck or two. The negative voltage
rectifiers must be fast recovery diodes, like a
1N4933. Replacement electrolytic capacitors are as
close as your local Radio Shack.
When the voltage line has a three-terminal IC
voltage regulator, check the resistance between both
the input and the output (Figure 4) to ground. If only
the output pin is shorted, the output capacitor is bad.
Reprinted from September 1996 Nuts & Volts Magazine. All rights reserved. No duplication permitted without permission from T & L Publications, Inc.
3
Table 2. Output Voltage and Resistance
Nominal Voltage Voltage Range Resistance Wire Color
+5V +4.75V to +5.25V >100 ohms Red
-5V -4.75V to -5.25V >100 ohms White
+12V +9V to +15V >250 ohms Yellow
-12V -9V to -15V >250 ohms Blue
n/a 0V or +5V ~1000 ohms Orange
0V 0V 0 ohms Black
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Figure 6. The high-voltage supply is a simple voltage
doubler circuit.
Figure 7. A cheap VOM is the best way to check tran-
sistors and diodes. Why? Because the test voltage
has to be enough to breach the barrier voltage of a
silicon diode, typically 0.7 volts, and a lot of DVMs
have a probe voltage of 0.3 volts and less.
How
To
Find
Waldo
SOURCES
Allied Electronics
800-433-5700
Digi-Key
800-344-4539
Marshall Electronics
800-877-9839
Newark Electronics
800-344-4539
Radio Shack
800-843-7422
Wyle Laboratories
Electronic
Marketing Group
800-672-3475