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POWER SUPPLY REPAIR

Copyright November, 1994 by Randy Fromm

We live in a "disposable society."  Disposable razors, lighters, pens and
a myriad of other items are a part of our daily life.  Many operators
consider power supplies to be disposable as well.  Their rumored
inability to be repaired and relatively low cost create the impression of
disposability.

This is not the case, however.  I repair lots of power supplies and it's
really quite easy and practical.  In fact, a repaired supply may end up
having a longer life expectancy than a new one, as you'll see.


Switching Power Supply Operation

Modern power supplies are known as "switching regulator power supplies."
In most switching supplies, the 110 volt AC input is first rectified by
two diodes and filtered by a pair of capacitors.  This creates two high-
voltage sources; one positive and the other negative.

A pair of transistors is then used to switch these high voltage supplies
across the primary winding of a transformer.  This switching action is
very fast.  A typical switching speed is around 40,000 cycles per second
or 40 kilohertz.  An integrated circuit is commonly used to control the
transistors.  This IC not only controls the speed at which the
transistors are switched, but also controls the amount of time that each
transistor is energized.  The output voltage of the power supply is
determined by the "on" time of the transistors.  If the transistors are
keep on for a longer period of time, the output voltage of the supply
will rise, while shorter times lower the output voltage.  This is known
as "pulse-width modulation."

The output of the transformer (which is now alternating current) is then
rectified by special high-speed diodes to change it back to direct
current.  This output is not pure DC however, and requires extensive
filtering to remove the high-frequency "noise" that is generated by the
rapid switching action of the transistors.  Filtering is accomplished by
using a combination of coils (also known as "chokes") and capacitors.

The output voltage of the power supply is regulated by feeding some of
the output back to the integrated circuit that controls the switching
transistors.  If the output voltage is too low, the IC allows the
transistors to remain energized for a longer period of time, raising the
voltage.  An output voltage that is too high signals the IC to cut back
on the transistors, lowering the output voltage.


Power Supply Failures

I have found that there are only a small handful of components that fail
in switching regulator power supplies.  The most common failure is the
switching transistors themselves.  The transistors short-circuit, causing
massive amounts of current to be drawn across the transformer and blowing
the fuse.

Transistor failure is often caused by bad capacitors.  It is extremely
common to find output filter capacitors that are swollen or leaking.  Any
capacitor that appears to be bad should be replaced.  To prevent a
recurrence of this all-to-common failure, output filter capacitors should
be replaced with special "low ESR" (Equivalent Series Resistance)
capacitors.  These capacitors are specifically designed to handle the
rigors of filtering in a switching supply.  Most power supply
manufacturers do not install low ESR capacitors as original equipment
because they are somewhat more expensive that conventional capacitors.
However, it is well worth the money to use them as replacement components
as they will greatly extend the life of the power supply in the field.
When I work on a power supply, I replace all the output filter capacitors
with low ESR caps regardless of whether they appear to be good or bad.
Since a service call costs far more than the capacitors, it's a prudent
thing to do.

Diode failure is another common problem.  There are quite a few diodes
in a switching supply and failure of any one of them will cause the
supply to blow the fuse or shut down.  The most common diode failures are
shorted +12 volt or -5 volt output rectifiers.  Failure of these diodes
will not blow the fuse.  The supply simply detects the short and shuts
itself down.  Some of these failures may be caused by using the +12 or -5
volt outputs to power coin door lamps.  The -5 volt output is not over-
current protected in all power supplies.  A shorted lamp socket may blow
the diode by drawing too much current from the supply.  The +12 volt
diodes may be blown if 6 volt bulbs are inadvertently used instead of 12
volt bulbs.  The high-voltage input diodes may also short-circuit.  This
is often accompanied by shorted switching transistors and will blow the
fuse.


Testing and Repair

All testing is done with the power off.  Start by testing the pair of
switching transistors.  These will be mounted on a heatsink that helps
them run cooler.  Test them by using an ohmeter or a digital multimeter
set to the diode test range.  Check each transistor for a short between
emitter and collector.  Replace any transistors you find to be bad.
Although some technicians claim that you should replace them both even if
just one is bad, I have not found this to be necessary.

By the way, these transistors will always seem to test shorted between
base and emitter when tested "in-circuit."  I generally don't bother
testing the base-emitter junction of the transistors.  When the switching
transistors fail, they always short between emitter and collector.  If
you're in doubt, pull the transistors out of circuit to test them.  If
the transistors are shorted, the fuse will have blown.  Be sure to test
the high-voltage diodes as well.  The high-voltage diodes are usually
part of a bridge rectifier, although they may be individual diodes.

Next, test the output rectifiers.  There are three pairs of diodes to test.
  One pair is for the -5 volt output.  These will be fairly small;
approximately the same size as the ubiquitous 1N4004 with which we are
all familiar.  The +12 volt diodes are usually somewhat larger.  The two
+5 volt output diodes are housed together in a "dual-diode" package that
looks very much like a transistor.  Like the switching transistors, this
diode package is mounted on a heatsink.  It will generally have the diode
schematic symbols printed on it.  This diode will usually not test
properly in-circuit.  Testing can be simplified by unsoldering it with a
"solder sucker" instead of removing it completely from the printed
circuit board.  I have seen very few failures of the +5 volt output
diodes.  All diodes must be replaced with high-speed diodes or the power
supply will generate excessive noise.

Follow these tests by replacing all the output capacitors with low ESR
caps and fire up the power supply.  The supply should be tested under
load.  Use a 1 ohm, 50 watt resistor or equivalent as a "dummy load",
connected between the +5 volt output and ground (DC COM).  This will draw
5 amps from the supply, which is adequate for test purposes.  If the
supply is still inoperative, the integrated circuit may be bad.  Test the
IC by removing it from the printed circuit board and installing it in a
power supply that you know to be good.  I have a spare  power supply with
a socket in it that I use exclusively to test integrated circuits.  Just
about all the supplies use the same IC; a type 494.  Equivalent
integrated circuits are: TL494CN, uA494, uPC494C, IR3MO2, and MB3759.
The over-the-counter replacement for these is ECG1729.

Obtaining Replacement Components

One of the main arguments for tossing bad power supplies in the trash
has been that the cost of the replacement components is almost equal to
the cost of a new supply.  That's just not true.  The switching
transistors are available for around $.90 each from J.C. Electronics, Inc.
  1428 Broad Street  Hillside, NJ  07205  (201) 705-3400  (800) 828-4560.


Capacitors can be obtained from TTI.  TTI is the nations largest
distributor of passive components such as resistors and capacitors.  The
capacitors you want to order are made by Nichicon.  Order 3300 microfarad
at 16 volts (part number UVX1C332M) and 1000 microfarad at 25 volts (part
number UVX1E102M.)  These will be suitable as replacements for output
filter capacitors in virtually all makes and models of power supplies.
Remember, you can always substitute a capacitor of higher voltage when
replacing filter capacitors.  E.G.  A 1000 microfarad, 16 volt capacitor
can be replaced with a 1000 microfarad, 25 volt.   TTI has 20 offices
nationwide.  Contact them at (800) CALL TTI.

Minus 5 Volt Output Too High

Most switching regulator power supplies have three DC outputs. One is the
main +5 volt DC output that powers the computer system. The others are
the +12 and -5 volt outputs. These DC outputs are often used to power the
sound generating system and the audio amplifier itself. When you're
testing a power supply, it's important to check all three of the outputs.
This is especially true when you have a game that basically works okay
but has distorted or missing audio.

When a switching regulator power supply fails, all three outputs will
usually drop to zero volts (see Play Meter, June, 1989, page 107.)
Sometimes, however, the output voltage may rise. If you find that the +5
VDC and +12 VDC outputs are normal but the -5 VDC output is too high
(more than -6 VDC), try replacing the -5 output filter choke.

It's easy to locate the -5 volt filter choke, even without a schematic
diagram. Just follow the trace on the printed circuit board back from the
-5 VDC output of the power supply. You will eventually come to a
component that may look something like a capacitor but will be clearly
labeled "L" on the board and will generally be accompanied by the
schematic symbol for a coil as well. The coil is wound on a ferrite coil
and is covered with a plastic sleeve that has been heat-shrinked over it.
Examine the coil. If the heat-shrinked cover has been melted or is
missing entirely, the coil may be bad.

This is not a component that you can boogie down to Radio Shack for a
replacement. There are a couple of options for obtaining a replacement
coil. The preferred method is to take the coil off a junk power supply.
Alternately, you can pull the burned wire off the ferrite core and rewind
the choke yourself using the appropriate gauge wire. There aren't that
many turns of wire on it that you can't rewind a new coil in five minutes.



Switching Regulator Power Supplies ("Peter Chou" Style)
Output Capacitor Replacements

I have received a number of calls and letters from operators and
technicians that are having trouble obtaining replacement capacitors for
switching regulator power supplies. I recommend using Nichicon brand
capacitors. I have been using them for almost two years and to date I
have not seen a repeat capacitor failure.

Nichicon brand capacitors can be obtained from a company called TTI.
They have a score of offices across the United States of America, all of
which can be reached by calling (800) CALL TTI. I recommend that you
order just two different capacitors for use as replacements for the
output filter capacitors in the "Peter Chou" style power supply. It helps
a great deal when you have the part numbers. For the +5 VDC output, use
3300 microfarad, 16 VDC capacitors. The Nichicon part number is UVX1C332M.
 Each power supply requires two of these.

To make ordering and stocking easier, I use the same capacitor for both
the +12 VDC and the -5 VDC outputs. It's a 1000 microfarad, 25 volt
capacitor. The Nichicon part number is UVX1E102M. Although some power
supplies use a 2200 microfarad capacitor for the +12 VDC output, I have
found the 1000 microfarad to be perfectly satisfactory. Most power
supplies use one capacitor each for the +12 VDC and -5 VDC outputs so
order the same number of 1000 microfarad capacitors as you do the 3300
microfarad capacitors. When you replace the output filter capacitors,
it's a good idea to change them all at once.

Switching Regulator Power Supplies ("Peter Chou" Style)
Output Diode Replacements

Output diodes are a common failure item in the switching regulator power
supply. I would say that around twenty-five to thirty percent of them
have bad output diodes.

High Speed Diodes

There are three pairs of output diodes; one pair for each of the outputs:
+5 VDC, +12 VDC, and -5 VDC. These are not ordinary diodes. They are
special, high-speed, "fast-recovery" diodes. High speed diodes are made
to handle the very fast switching action (around 40 thousand cycles per
second) of the power supply.

I have rarely replaced the +5 volt diode assembly in a switching
regulator power supply. The +12 and -5 volt output diodes are the most
common failures. It is normal for these diodes to test bad when checking
them "in-circuit." There is usually a low ohm resistor (normally around
100 ohms) across the output of the power supply that causes a very low
reading when checking the +12 or -5 volt output diodes. Most people
unsolder and remove one end of each diode to test it but you can usually
bypass this step. When these diodes fail they will generally short
completely. Instead of reading around 100 ohms, you will get a reading of
around zero ohms; a dead short!

Substitute Diodes

The +12 volt output diodes will usually carry an original part number
like PXPR302 or FR302. These are 3 amp diodes. The -5 volt output diodes
will often be type PXPR1502 or similar. Good engineering practice
dictates that high speed, "fast-recovery" diodes be used in this circuit.
I have found normal diodes will fail prematurely and as such are
unacceptable as substitutions.

That's all there is to it.  The more you work on repairing power supplies,
the easier it gets.  When you consider that many power supply repairs are
effected with the replacement of a single diode, you can see that they
are anything but disposable!

-
  RANDY FROMM  YMNP18A@prodigy.com


 


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