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(That's a 10 percent voltage lossl)
Further, this voltage drop produces
heat which can destroy the edge
connector in your game.
Some notorious offenders are
Atari's Tank and Jet Fighter and
Electra's Avenger. If you operate
these games, keep your eye on the
state of your edge connector. If it's
dirty, clean it or tin it with a small
amount of solder. (Having learned
from past experiences, manufactur-
ers are doubling up on contact area
for high current flows and this
doesn't seem to be a problem on
newer games.)
Poor power connections can
show up as bad video (distorted or
broken-up picture) or, more often,
no video at all. A quick voltage
check on the P.C.B. will let you
know if your power is getting to the
board okay. If not, check backward
from the board and see where
you've lost it (bad connector,
transformer, etc.). In most Atari
games, the power supplies are
integral with the P.C. B. and only
require various A. C. inputs to power
them. Midway, Exidy and others
use an outboard power supply and
run D.C. to the board.
Both
systems seem to work equally well,
the major difference being in
manufacturing costs. It's cheaper
and easier to make a single board
than to gear up for both logic and
power supply boards.
For a closer examination of
regulated power supplies, take a
look at the block drawing, Figure 1.
[Note: This discussion of power
supply design will, of necessity, be
brief. Many excellent texts now
exist that can teach you all that
you've wanted to know about
power supplies, but didn't know
whom to askl) Since this is a repair
oriented feature, let's concentrate
on a power supply's weak points as
we look at its electronic structure.
Block #1 is the transformer. The
transformer's function is to reduce
the line voltage ( 117 volts A. C.) to a
lower voltage for the rest of the
power supply circuit. This is a very
low failure item, and a bad
transformer can often be diagnosed
by that wonderful smokey bacon
smell, so familiar to pinball mechan-
ics as a burned coil. To check the
transformer, compare the two ends
of the secondary winding to the
center tap. They must be exactly
equal. If one side is a volt or more
off in comparison to the other, you
probably have shorted windings.
This can cause faint to moderate
hum bars- even though the rest of
the power supply circuit checks out
100 percent okay.
Block #2, the rectifier, is a
one-way gate for electron flow and
changes the alternating current
(A. C.) to direct current (D.C.). The
rectifier diodes are a high failure
item.
Their failure, however, is
often a function of a component
failure further down the line. Since
the diodes are rated at a certain
maximum current, an excessive load
can easily "pop " them.
Before
repairing blown diodes, check the
rest of the circuit for shorts.
Block #3, the filter capacitor, is
like a short-term battery that
charges and discharges to provide a
constant but unregulated voltage to
the voltage regulator. While the
filter capacitor is not a major failure
item, it's a good idea to have a few
around. When this baby fails, your
monitor becomes hum bar city.
Look for a white deposit or a bulge
on the positive end of the can.
Substitution is the best method of
checking a filter capacitor.
Block #4 is an integrated voltage
regulator. The case is usually the
reference input and, as such, can be
grounded for simplified power sup-
ply design. The voltage regulator
takes an unregulated input and spits
it out at a preset voltage above its
reference input. (In this case, the
reference is ground or 0 volts and
4
2
TRANSFORMER
RECTIFIER
the output is 5 volts.) The voltage
regulator has thermal shutdown
capabilities and will turn itself off if it
gets too hot. If you find the input to
the regulator is good, but the output
is low, pull out the regulator and
replace it. If the output voltage is
still low, you've got a short on the
board. Feel around for a hot chip or
cut power busses until the voltage
comes up. The LM309 (1 amp),
LM323K (3 amp), and 78H05 (5amp)
voltage regulators are necessary
replacement items for anyone who
plans on performing any P.C.B.
repair.
Because of space limita-
tions, on-board power supplies
cannot be adequately heat-sinked
(heat-sunk?) and this causes the
regulator to run extremely hot.
While this high temperature is
within operational limits, I don't
imagi ne the heat is doing the device
any good. This is a common failure
item. Keep a few of each type on
hand.
I cannot adequately stress the
importance of research. There are a
myriad of publications to teach you
about basic electronics.
One
excellent manual on power supply
design is the "Voltage Regulator
Handbook" that is published by
National Semiconductor. This text
can often be obtained free of charge
from your local National Semicon-
ductor distributor. (Just convince
him that you'll buy lots of regulators
from him!) The public libraries are
full of books on all facets of
electronics.
The technical staff at Play Meter
will continue to bring you informa-
tive service material. But we need
your response and, most of all, your
questions. Please drop us a line and
let us know what's bugging you
about your games.
Write to:
The Technical Editor
Play Meter
Box 24170
New Orleans, LA 70184
FILTER
REGULATOR
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