Willianis Switch Matrix: Troubleshooting
Mark McCain
Case 1: Column Shorted To Ground
NDI
When a Column wire is shorted to ground,
and any switch on that Column is closed,
switch-test will show all switches in the Row
of the switch that is being closed. If no
switches are closed, switch-test will show no
switches closed.
Houston, Texas
OUTLINE
Troubleshooting Williams switch problems
that are caused by shorts in the playfield or
cabinet wiring can be a real pain in the neck.
These problems are usually caused by
people pinching wires under metal brackets
and splashing solder on the wiring harness.
Switch-test usually shows these types of
problems as Row failures, not as Column
failures. To save you some wasted time and
effort, I have included eight cases where Row
and Column wires have been shorted in
various w~ys.
Before you assume that your problem is on
the playfield and not on the logic board,
however, you might try the following tests:
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• Unplug connector JS on the logic board.
• Probe each of the male pins of JS on the board.
• The correct reading should be High Pulsing
Low (or approx. 4. 7 to 4.9 volts DC with your
meter).
• If any of the pins read something different
from the other pins, replace the associated
transistor. (I recommend an MPSA06 instead of
a 2N3904).
• Unplug connector JlO on the logic board.
• Probe the male pins on the board. A good
reading will be High (or about 4.8 volts).
• If you get something different, check the
associated resistor pack and MC1401 l.
If everything checks out ok on the
board, your trouble is probably in the
wiring.
To fmd the location of the short, you could go
to the end of the Column and break the daisy
chain one switch at a time until the short no
longer affects the switch-test. You will then
know which two switches to look at (or
between) for the short.
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Case 2: Row Shorted To Gnd (diode anode)~
When a Row wire (anode end of diode) is
shorted to ground, the switch-test will show
the entire Row, whether the switch is closed
or not. To find the location of the short, go to
the end of the Row and break the daisy chain
one switch at a time until the short disap-
pears. You will then know which two
switches to look at (or between) for the short.
Case 3: Row Shorted To Gnd (diode cathode)
When the Cathode (banded end) of a switch's
diode is shorted to ground, that switch's
entire Row will show closed in switch-test
with the switch either opened or closed. To
fmd the location of the short, use the method
described in Case 2.
Case 4: Column Wires Shorted Together
When two Column wires are shorted to-
gether, and none of the two Column's
switches are closed, switch-test will show no~
switches closed.
When two Column wires are shorted to-
gether, pressing any switch on either Col-