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E L E C T R I C A L
P R I N C I P L E S
95
trigger
8.
This negative pulse stops conduction in
tube
81
and causes trigger
8
t o go
ON
and the
counter stands at
8.
When trigger
8
goes
ON,
the anode of tube
81
is at high potential (+ 136 volts). Since this po-
tential is applied r o the grid of tube
B
through the
voltage divider R1R2, it means that the potential
at the grid of tube
B
will rise above cutoff and tube
B
becomes conductive.
This rube is used on the
tenth pulse.
A ninth entry pulse will again turn
trigger 1
ON
and have no effect on other triggers,
thus learing the counter at 9 (triggers 1 and 8
O N ) . A tenth entry pulse turns trigger 1
OFF
which in turn passes a negative pulse to trigger 2
and to the left side of trigger 8. This negative
pulse tends t o trip trigger
2
ON,
but such action in
trigger 2 demands a rise in potential at the anode
of rube 21. Since tube
B
is now conductive, it holds
the potential at the anode of tube 21 to t 4 0 volts
and overcomes the attempred rise in potential at
the anode of tube 21 and at the grid of tube 22.
Thus, with tube
B
conductive, trigger 2 is blocked
from turning
ON.
The negative pulse produced
by stage 1 is also applied ro the grid of tube 82.
This causes tube 82 to stop conducting; conse-
quently, trigger 8 goes
OFF,
and the counter has
restored to zero (no triggers O N ) . When trigger
8 goes
OFF,
the potemtial at the anode of tube 81
drops abruptly from + I 3 6 to t 4 0 volts, while
the potential
a t
the anode of tube 82 rises abruptly
from t 4 0 volts t o
+
1 3 6 volts.
A
t a p on the anode
resistor of tube 81 furnishes a -50 volt pulse t o
operate the carry trigger.
If
a positive pulse is
desired t o operate a switch when a carryover
occurs, a tap on t h e anode of tube 82 can be used.
Any digit can then be added into this counter
by applying the proper number of negative pulses.
If 6 pulses are applied, the counter will stand at
6, since triggers 2 and 4 will be
ON.
A
counter
will retain a reading as long as power is applied.
When a counter is to be cleared, it is merely a
matter of opening the -100 volt cancel line. This
applies + I jO volts to the right side of all the trig-
gers and they are all turned
OFF,
thus restoring the
counter to 0. Cancelling is always necessary before
reading into a counter, because when power is first
turned
ON,
the triggers may assume any status, de-
pending entirely upon chance or upon variations
in individual tubes.
Observe that on the tenrh pulse when trigger 8
goes
OFF
and the potential drops at the anode of
tube 81, triode
B
is rendered non-conductive, and
its anode potential immedia,tely rises, thus releasing
trigger 2 from the blocking action.
If this oc-
curred too soon, the tripping pulse produced by
trigger
I
on the tenth pulse might still be effective
to turn trigger 2
ON.
T o insure against this, the
blocking action of tube
B
is maintained for a short
time after ,trigger 8 goes
OFF
by maintaining con-
duction through tube
B
for a short period after
trigger 8 goes
OFF.
This insures that trigger 2 is
not unblocked until the tripping pulse from trig-
ger 1 is spent. I t is for this reason that the grid of
tube
B
is coupled to the anode of tube 8. through
capacitor C. During the reversal of trigger 8 to
the
OFF
status, the p'oten.tia1 at the anode of tube
8, is rising rapidly while that at the anode of tube
81 is dropping rapidly.
The rising potential is
transmitted by way of ,the capacitor C to grid re-
sistor R g of tube
B.
This rising potential counter-
acts the effect of the dropping potential on the grid
of tube
B
and maintains the grid of tube
B
above
cutoff until capacitor C is fully charged. Thus, the
grid of triode B does not follow the anode of tube
81 immediately, but is held above cutoff potential
for a definite delay period determined by rhe
charging time of capacitor C. Thereafter, the low
potential at the anode of tube 81 is effective t o hold
tube
B
cut off as long as trigger 8 is
OFF.
I n practice the blocking tube is one-half of a
12SN7 twin triode. The other half is used as the
blocking tube for another counter position.
For
this reason and to facilitate the handling of the
tube chassis, counter chassis are built with two
counrer positions per chassis as shown in Figure 93.
Each chassis also contains an indicator light block
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