Star Tech Journal

7
STAR*TECH JOURNAL/NOVEMBER 1982
Sometimes location owners ( especially in bars)
replace light switches with dimmer rheostats, and
the game is sometimes on the same line. Obviously,
the voltage available to the game is going to drop
dramatically when the dimmer is turned.
In any case, the way to check for correct line
voltage is with your VOM. Set the VOM to 250vac
and stick the probes in the wall receptacle. If it's
OK here, check the transformer primary connec-
tions. If you do not get 11 ?vac, examine the solder
joints on the transformer, fuse block, and interlock
switch. If you do get 11 ?vac, the problem must be
either in the transformer, harness connections, or
in the PCB power supply.
If you suspect the transformer, check its
secondaries with the VOM set to 50vac and
correlate the readings with the legend on the side of
the transformer. The transformer must also be
correctly grounded, so check the ground potential
as well, especially if there is a hum bar rolling up or
down the TV screen.
Harness Problems
Other harness problems include blowing fuses and
malfunctioning controls. The repeating blown-fuse
problem can sometimes be quite exasperating to
solve, for short circuits have the tendency to occur
in areas almost impossible to find. First, try
inserting a new fuse, as old fuses age and may blow
without cause. If the new one also blows, you
definitely have a short.
The best way to approach this problem is by
turning the power off and disconnecting devices
that may be causing the problem, such as the TV,
transformer, and PCB. Disconnect the devices by
pulling off their connectors, but do not allow them
to touch. If necessary, insulate them with small
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pieces of electrical tape. Then, connect your VOM
across the terminals of the fuse block ( all electrical
power shut off), and set it to one of the resistance
scales. This will save blowing a fuse each time you
want to check the circuit.
If the VOM reveals that disconnecting the
devices removed the short, reconnect the devices
one by one until the short returns. The last device
connected is the one that is at fault. If the VOM
reads a short even after the devices are discon-
nected, the fault must lie in the harness itself, and
only patient exploration will reveal its location.
First, carefully examine all the wiring, looking for
terminals that may be touching, metal objects such
as coins shorting connections or burned insulation.
If necessary, use the VOM to check each suspected
wire.
Ma/fanctioning Controls
One of the most common problems here is a bad
potentiometer. Typically, a bad pot will cause the
image to jump as it reaches a certain point. The
only cure for this one is to install a new pot.
If a feature that is operated by a switch ( for
example, joysticks, foot pedals, control panel
buttons) does not operate at all, check the switch
with a VOM or continuity tester to verify its
operation. If the switch does not check out, replace
it. If the switch is OK, you should suspect the input
to the switch from the PCB. In this case, get out the
harness and logic schematics and check to see
what kind of input it is. In many cases, the input
will be +5vdc. If so, use the VOM to check its
presence. Normally, the switch is used to pull a
+5vdc line LOW to GND or to pull a LOW line
HIGH. If the PCB output is missing, check the
wire length from the PCB. If you find the signal at
the PCB trace, the wire length or connection is at
fault. If not, begin exploring the PCB using the
logic schematics.
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