easily . Try it and you 'll see what I
mean'
Decade counter U3 (7490) is
clocked by the output of U2 . The
binary coded decimal output of U3 is
decoded by U4 (7442) to provide a
one -of-ten output. The individual
outputs are used to strobe the digits
of the display unit one at a time . Be-
cause the display has six digits and
the counter and decoder are set up
for a count sequence of ten. four of
the outputs of U4 are unused . Six of
the U4 outputs are inverted by hex
converter US (7404) to provide the
proper output to drive the score
display . The outputs of US are wired
to connector J 1 for interface with the
display module .
BCD INPUTS
The score display knows what
numerals to show by the four-bit
code at the inputs at pins 16-19 .
In order to make the test fixture as
inexpensive and easy to build as
possible. a simple system of fou r
switches is used to set up the binary
coded decimal (BCD) input to the
display module . By flipping the
switches in different combinations. all
ten of the numerals (0 - 9) can be
displayed . S2 through SS are
connected to pull-up resistors on one
side and to ground on the other .
When a switch is open . a high
signal is sent to the input of the
display unit. Closing a switch
grounds the pull -up and brings the
signal low . I have found it much
easier to troubleshoot displays with a
static numeral display. as compared
to the "counting" test mode available
wh n using a game as a test bed for
repairs .
LATC H E NABLE, BLANKING
The latch enable switch (S6) is
connected to pull -up resistor R6 on
one side and ground on the other . As
long as the switch is open . the latch
enable is high and the score display
will reflect any change in the numeral
input code . I.e .. changing the input
code from 0 111 to 1000 will change
the display from Ts to 's . Closing
S6 brings the latch enable low.
locking in the last code presented to
the BCD inputs .
Any further change at the BCD
inputs will now be ignored. and the
score display module will continue to
show the numeral that was displayed
when the latch enable was brought
low .
Blanking switch S7 and pull -up
resistor R7 operate in much the same
way as the latch enable switch . When
S7 is open. the blanking signal is high
and the display is turned off. Closing
S7 brings the blanking signal low.
allowing the display module to show
the numeral indicated by the BCD
the physical size o f both transformers
can be reduced and the whole unit
installed in a chassis box . In such a
case. T1 can be rated as low as SOO
milliamps. which is quite small.
The four BCD input switches , and
the latch enable and blanking
switches (S2 - S7) are all on a single
unit . THis dual in -line switch package
is familiar to all of us as the "option
switches" found on most every game
produced today. By using this
switch . construction is simplified and
costs are reduced over individual
toggle switches .
Simply ground one side of all the
switches . and solder pu ll -up resistors
to the others . Of course. a neat little
test fixture with a row o f six toggle
switches looks better and is easier to
use . But I just can 't see spending a
buck each for mini-toggle switc hes.
Then again. I save used paper bags.
Another slick way to p rovide the
BCD input is to use an angle encoder
from Midway's Sea Wolf II game .
The angle encoder normally provides
code at the inputs .
Note that the blanking input works
by making all of the segment in p uts
low . The blanking signal does not
affect the digit strobe in any way.
CONSTRUCTION
I will freely admit that I'm not too
skilled at making projects that are
cosmetically appealing . My score
display test fixture is made from a
small piece of vectorboard on which
the four ICs are mounted. screwed to
an old piece of 3/ 8-inch plywood .
Terminal strips are mounted for
point -to-point construction of the S
and 190 volt power supplies. T he
voltage regulator is operating at a
fraction of its rated outpu t current. so
it's not necessary to provide a heat
sink for it.
The two transformers are also
screwed to the plywood. as well as
the fuse holder . Bleeder resistor R 1
should be mounted with space on all
sides to help dissipate its heat. With
thge proper high voltage transformer
PO WER
Fl
St
SUPPLY
T1
=~:111
PARTS
LIST
Fl -1/2 AMP FAST- BLO
Cl - 2300MF, 16VOC
51 - l A , 125 VAC SPST
C2- 25 MF, 10 VOC
Tl -18 VOLT, 2 AMP CENTER - TAPPED
• T2- 12 VO LT, 3 AM P (SEE TE XT!
Ul- L M 3 4 0T- 5
0 1- 0 6 - IN4004
+5
1K
l t-4
lK
C3 -160 MF, 350 VDC
Rl- 25 K OHM , 10 WATT
DIGIT
STROBE
OR 7 805