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STAR*TECH JOURNAL/NOVEMBER 1982
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SEGMENT
IDENTIFICATION
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FIGURE 2
The display control signals originate at the
Al control board and are wired directly to the
displays. There are 16 digit control lines and
three groups of 8 segment lines that are common
wired to three groups of displays. The common
wiring is actually a combination of cross wiring
between digits, segments, and displays.
The digit lines are always sequentially
strobed, so only one line at a time will be high.
The processor will fill each segment group with
the data or blanking information that is neces-
sary for each digit.
For instance, if there is a first and second
player up, then the "B" segment group will
always be filled with blanking information. So
when the digits are strobed for the 3rd and 4th
player, the segments will be low, and the
display will be blank. If first player only is up,
then the processor will fill the "A" group with
blanking information when the digits D7 through
D 13 are high. The microprocessor scans each
digit 61 times a second so displayed data will
look constant to the eye.
Most problems that will develop in displays
will be shared by two or more displays because
of the signal configuration. A segment-related
problem would be shared by the first and
second player displays or third and fourth
player displays. Digit problems will be shared
by player displays 1 and 3 or 2 and 4. When
checking display problems, remember that one
bad display can make others appear bad
because of the common wiring. Disconnect one
display at a time and observe what happens to
the other displays. CAUTION: Do not plug or
unplug displays with the power on. Damage to
the control board will result.
If the problem can be determined to be on
the display board itself, it may be possible to fix
the display board, saving the expense of
purchasing a whole new display.
The voltages can be easily checked on the
display board to determine what component
may be bad. The +60vdc can be checked by
placing your meter leads across the 1 rnfd filter
capacitor or across pins 9 and 10 of the
UDN6118 buffer chip. The 5vac filament
voltage can be checked on the fluorescent
display tube by placing the meter across pins 1
and 37 on the tube. The +8vdc filament offset
voltage should be checked from either one of
the pins 1 or 37 to the negative side of the 1 rnfd
capacitor or any ground. The 4-digit display
voltages are checked using the same procedures.
The display board components can be
checked individually using just a meter. All
components should have standard readings
that can be checked to verify their integrity. On
the fluorescent display tube itself, there should
be infinite continuity readings between all
segment, digit, and filament pins. Make all
measurements on the pins of the tube using the
display schematic from the game manual (see
Figure 2). Some of the pins are not connected
to anything and the digits have two pins each.
There are other minor variations between 4-
digit, 6-digit, and 7-digit tubes. There should
be continuity readings between the two filament
pins and also between the individual two digit
pins, but again, not to each other.
The two main IC chips on the board are
high voltage, Darlington output buffers,
UDN6 l l 8A. These I Cs isolate the high voltage
segment anode and digit grid lines from the low
level signal source. To check them with a
meter, first disconnect the plug from the display.
Then put your ground lead on pin 9 of the
UDN6118 chip. From pin 9 to pin 10, Vdd
source reference, there should be infinite or
very high resistance. Then checking pin 9 to all
the input pins, 1 through 8, there should be a
resistance of from 30K to 45 K ohms. From the
ground, pin 9, to the output pins, 10 through
18, there should be a high impedance reading
of from 11 OK to 130K ohms. The readings
should be the same if the meter leads are
reversed.
On the 7-digit and 4-digit display boards
there is a 7432 quad OR gate that is used to
generate the comma on the 7-digit display and
the "I" on the 4-digit display. Continuity
readings on this chip would give indications of
forward and reverse bias voltage drop. With
the ground lead on pin 7 (ground), there should
be a high resistance of impedance reading to all
pins, except to the tube connected outputs, pins
3 and 6. Placing the positive lead on pin 7 will
give readings of 25K to 30K ohms, showing
forward bias, on all pins.
The large filter capacitors on the +60vdc
input line can be checked for continuity by
placing the positive lead on the negative side of
the capacitor and the negative lead on the
positive side. There should be a reading of 25 K
to 30K ohms and infinity in the reverse direction.
Please note that all these readings may
vary, depending on the type of meter you are
using. These readings will greatly differ if you
are using an old meter or an analog meter. If
your measurement differ greatly, or if in doubt,
verify your readings by checking against a good
display.
The glass tube can also break down intern-
ally or bum out over the years. The fluorescent
display tube can be replaced, saving money
over the cost of the whole display board. First
unsolder all the display tube pins from the
display PC board. Then carefully insert a
utility knife between the PC board and the top
of the display tube. The glue should be sliced or
loosened and the pried up. Gottlieb does not
recommend prying the glass off, as the PC can
crack or the glass could shatter in your hand.
Gottlieb also recommends using some work
gloves to protect your hands in case the glass
shatters or the knife slips.
Cour11sy of Gottliob's 1lldlnical newslelte< ·on Target".