Play Meter

By Randy Fromm
Building a score display
•
•
cheaper
IS
easier,
test fixture
Repairing Bally score display units is
really pretty easy. Because more
operators are now repairing their
own score displays, replacement
parts (including the glass display
envelope itself) have become easier
to find and even a little cheaper'
By building this score display test
fixture , field mechanics can bring
defective modules to the shop for
repair. The modules can be repaired
and tested without the necessity of
having a Bally or Stern game in the
shop to use as a test bed. It's easier to
fix things sitting down anyway.
instead of standing up to work on a
score display that's hanging off the
back of the lite box insert.
The test fixture checks all of the
functions of the score display. To
perform the test. the fixture simulates
the inputs that are normally provided
by the game: + 190 volts DC, + 5
volts DC. the six-digit strobes, the
four -bit code that determines the
displayed numeral. latch enable ,
blanking, and ground.
POWER SUPPLY
The power supply provides the
regulated + 5 volts DC for the four
ICs in the test fixture , and the single
IC in the score display module . It also
provides the + 190 volt supply that
fires the display envelope.
This power supply is a bit different
from most others that you have
probably seen. so be careful when
you put it together .
Tl is an 18-volt transformer in a
full wave , center tapped power
supply configuration. The cathodes
of each of two diodes (D 1 , D2) are
connected to the high and low tap of
the transformer secondary. The
anodes of the diodes are grounded to
provide the ground return of the
supply. The positive output is taken
from the center tap and filtered by
primary filter capacitor C 1. The
unregulated DC is passed to Ul. a 5-
volt, l-amp positive regulator. Th e
reference input is grounded. and the
+ 5 -volt DC output is filtered further
by C2. This capacitor should be
located as close to the regulator as
possible .
Th e + 190 volt supply is a stand-
ard full -wave. bridge rectifier con -
figuration. Diodes D3- D6 rectify the
output of T2 and pass it to C3 where
it is filtered and sent to teh display
module. T2 is a 12 -volt, 3-amp
transformer that is hooked up
backward and driven by the 18-volt
AC output of Tl . With 18 volts as an
input on what was once the
secondary, the transformer produces
about 145 volts RMS at the output
(formerly the 117 -volt primary) .
The 145 VAC output is rectified
by the bridge and filtered by C3 to
provide the + 190 volt supply . R 1 is
a 25 kohm. 10-watt resistor that
quickly discharges the high voltage
supply when the unit is switched off .
Without this "bleeder" resistor,
it would be possible to get a pretty
good tingle when you remove the
plug from the score display after
testing . The bleeder resistor will
discharge the + 190-volt supply to
safe levels within a few seconds after
power is removed from the test
fixture. If you are used to working on
a display module in a game, you're
probably familiar with the shock you
often receive if you unplug the
module immediately after turning off
the game .
Bally uses a 100 kohm bleeder
resistor. so it takes quite a bit longer
to discharge the high voltage capaci -
tor. If you wait about 15 or 20
seconds before handling the display
unit. the shock hazard will be
considerably reduced .
Before all you engineers out there
give me a hard time about "the folli es
of overdriving transformers and
hooking them up backward and
other stranqe stuff." let me explain
the reasons for the design and give
some
warnings
about
design
limitations .
I looked through every trans-
former manufacturer's catalog that I
could get my hands on . but I was
unable to come up with a suitable
transformer. All of the high voltage
transformers listed were designed as
plate transformers for vacuum tubes.
and were definitely overkill for the
test fixture . Most of them weighed 4
or 5 pounds . The "backward"
hookup allows the use of inexpensive
and commonly available transform-
ers to provide the necessary voltages .
Of course. if you can find a proper
transformer. don't hesitate to use it .
Because the 12-volt transformer is
being overdriven by the 18-volt
output ofT 1. the transformer will get
quite hot if left on continuously . This
test fixture should be considered an
intermittant duty fixture only. and
should not be used to "cook"
displays . If you want to have a fixture
that can be left on continuously.
you'll have to come up with a trans-
former (145 volts and at least 250
milliamps) somewhere and hook it
up in a more conventional way.
using the primary as the primary and
connecting it in parallel with the
primary input of Tl . (If you do find a
good transformer to use . let me
know and I'll pass the information
along to the other readers.)
DIGIT SCAN
During normal operation in the
game. each of the six digits in the
score display is illuminated individu -
ally . in sequence . The action is so
fast . we see it as if all of the digits are
on at the same time . The test fixture
does the same thing. U2 is a 555
timer IC that provides the clocking
signal for the digit strobe circuit.
Pontiometer R3 can be used to slow
down the speed of the digit scan. so
shorted digit driver transistors and
bad display envelopes can be spotted