L-*--*=*:**Gaa
Page
271
remains at the same digit. interchange the memory
latch and observe the result. If the fault
still
remainsat
the same digit, interchange the counter
IC.
B.
To use voltage checks on a display string, refer to
Figure
4-4
which illustrates the right-hand (sixth)
digit of the display.
(
)
Check voltages at the segment driver output pins. The
voltages at pins connected to lighted segments should
be
lower than at pins connected to unlighted
segments.
( )
If you know that a
3
should be displayed but segment
G
(for example)
is
not lighted, check the voltage at pin
14
of the segment driver. If this voltage
is
similar in
value to the voltages at pins
13, 12,
11
and
10
(all of
whose tube segments are lighted), the tube
is
obviously
at
faoult. If the voltage at pin
14
is
higher
than the other four pins, then the segment driver may
be faulty. However, before making this decision, make
the following test.
(
)
Check voltages at the memory latch output pins. The
decimal value of the decoded
BCD
output voltage
should be equivalent to the displayed number. Refer
to the memory latch truth table in Figure
4-4
to
determine the decimal value from the high and low
I
voltages at the four memory latch outputs. For
example, if pins
10
and
15
are high and pins
16
and
9
are low, the value column shows a
3,
and the number
displayed should be a
3.
If so, the segment driver and
the display tube are operating
sat~sfactorily.
NOTE: The counter outputs cannot be checked with a
voltmeter as their outputs change rapidly with the count.
Use either an oscilloscope or the substitution method
described to check the counter outputs.
VlSl
IPARTl
D
4
LOW-
LIGHTED
+SVDC~L6
SEGMENT
DRIVER
LC151
I
MEMORY
LATCH
+5VDtd
IC103
I
-
t5VDC
14
COUNTER
1
IC102
I
MEMORY
LATCH
PINS
VALUE
L
H
L
H
MEMORY
LATCH
TRUTH
TABLE
Figure
4-4
L=
LESS
THAN
0.8VOC
H-
MORE
THAN
1.5VDC
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