integrated circuit (UlO), on the MPU
In simple
board (see figure 2).
terms, this means that both sides of
all the switch blade contact pairs in
the game have " computer data" on
them .
Perhaps you have observed that
resting your hand or arm on the
grounded side rails of the game
while touching a playfield contact
will make the game go crazy. This is
because you have created a ground
path for the data through your
body. This is an important fact to
remember when you get to a game
that seems to have a number of
" dead " playfield or front door
switches. Before you poke around
the MPlboard, run the switch test
(#4) on the game . Check every
switch in the game (including the
coin,credit, and tilt switches) and
note the results . If a switch is stuck
closed , the test will make it obvious
by showing its assigned number on
the player score display.
But if a switch bus (a bus is a
common line between electronic
components) is grounded or open,
the switch test w ill show a zero for
all switches on that bus. If you
compare your list of bad switch test
readings with the schematic, you
will probably find they all share a
common bus line into the MPU
board. As an example (and th is is
an actual problem that seems to be
recurrent in the Bally electronic
pins) , the spring steel backing blade
on the credit button switch is
separated from the contact blade
itself by a piece of fish paper. If this
insulating paper should shift or tear,
one side of the switch will become
grounded . In the electro-mechani-
cal games, this wouldn ' t normally
cause any problems because the
blade next to the door is tied to the
But in the electronic
common .
version, all of the switches that
share the accidentally grounded
switch bus will operate at best
erratically and usually not at all.
In the Night Rider game, this
grounded switch bus caused the top
saucer hole, lower bumper, center
target, and the credit button to
seem dead . The switch test showed
a " 0 " for all of the switches,
meaning the MPU never knew when
they were closed . The switches can
be easily tested with an ohmeter.
By pulling off the connectors at J2
and J3 on the MPU board, you can
isolate the switch matrix and test for
continuity and shorts between
various points and ground .
Don't forget that a diode is in
series with each switch , so you'll
have to observe the polarity of your
ohmeter when making this test.
(See Play Meter's service issue, July
1977).
~~
~10~--~--------~------~--------~-------.
Lm t:>ROP
"'
FIGURE 2