Play Meter

Coin credit problem-- solved
By Robin Minnear of the Game Doctors
Dear Sir :
We have an ICC. Spitfire , with a
coin credit problem . About one out
of every five coins fails to give a game
credit. We have a similar problem
with a Ramtek Hit Me .
Your help in solving this problem
will be greatly appreciated .
Allen Beyer
Allied Distributors
Phoenix, Arizona
Both Hit Me and Spitfire are
processor games . The decision you
haue to make is whether the problem
is an input buffer type failure or
computer failure .
The easiest problems to remedy
are input buffer problems . This is the
place to start . Hit Me has some static
buffering and Spitfire has none . Both
could stand a little improuement .
Figure One is the coin input circuit
used in Spitfire . When a coin is
dropped , a low pulse is felt as Pin 7
of the edge connector. This low pulse
is fed d irectly into the I/O port of
CPU (Pin 16) . This is the first
problem area . There is no protection
for the CPU. This type of design is
asking for trouble .
Pin 7 is also connected to a 10 mfd
capacitor and the inuerter K2. The
capacitor stretches the coin pulse and
inuerter K2 driues some transistors
which pulse the coin counter. If
you 're lucky , the capacitor is bad .
Inverter K2 is also a suspect.
Whateuer you find bad, I suggest you
add some kind of protection . Some
Sp itfires haue 1K resistors in series
with the switch inputs .
There is also the possibility that the
static antenna circuit is causing
problems . When static enters the
antenna , Pin 2 of C8 (NE555) goes
low . This in turn causes a game ouer
signal. If you cut Pin 2 of C8, you
eliminate the static circuit . If this
improues the coin problem , you may
as well leaue the static circuit
disabled . Many times , static antenna
circuits cause more problems than
they solue .
If you still haue problems, substi-
tute CPUs . The CPU is at location L5
and is a Fairchild 3850. As a last
resort, substitute memory deuices.
Hit Me has a coin input circuit
which consists of a pull-up resistor, a
.1 mfd capacitor and an input
multiplexer (Figure Two) . Again , it's
a good idea to beef up the static
protection here . If there is a bad
deuice , chances are it's the .1 mfd
capacitor or the input multiplexer
(1M) . Pin 9 of 1M is the multiplexed
output containing coin information .
This information is further multiplex-
ed by deuice 4J (332) . The output of
4J (Pin 10) is connected to the CPU
data bus . Chances are that the data
bus is good so if multiplexer 4J isn 't
your problem , it's time to substitute
memories .
Both of these coin problems may
be nothing more than a bad coin
switch . Check that out first .
- Technical Editor
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FIE1 · 2
PLAY METER , January, 1978
73