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condition (conducting). When $3
is
on, a posi-
tive voltage
is
applied to the base of Q2, one of
the dot generator multivibrator transistors,
keeping it turned off (not conducting).
When the key lever
is
moved to the dot posi-
tion, switch S1 closes. This removes the posi-
tive supply voltage present at the base of Q3
by directing it to ground through resistor R6. A
negative supply voltage from the voltage divider
consisting of resistors R1, R2, and R5,
is
ap-
plied to the base of transistor Q3. The negative
voltage at the base turns Q3 off. This removes
the positive bias voltage at the base of tran-
sistor Q2. Diode D30 isolates Q3 from Q2.
DOT GENERATOR MULTIVIBRATOR
Transistors Q1 and Q2 and their associated
circuitry make up the basic timing dot gen-
erator which
is
a free running multivibrator.
Capacitors C10 and C20 with Speed control R11
and R21 control the switching speed of the
multivibrator. Control R21 (screwdriver adjust)
is
adjusted so that the spaces are the same
length in duration as the dots. Since the dot
and space ratio must be maintained for all
settings of the Speed control, the two sec-
tions of the control are clutched. Turning the
Speed knob turns both sections of the control
a like amount without changing the space and
dot duration ratio.
The positive bias voltage from Q3 that
is
present
at the base of Q2, keeps the dot generator mul-
tivibrator turned off.
In
the off condition, Q2
will not conduct or allow current to flow through
it. This allows the full negative supply voltage
to be present at its collector by way of resistor
R20. However, transistor Q1 has a negative
voltage
(with respect to the emitter) present
at
its
base through resistors R12, R21, and
R22. This causes Q1 to conduct to a saturated
condition. Therefore, its collector voltage
is
practically zero or very near ground potential.
As the positive bias voltage
is
removed from
the base of Q2, it immediately starts to con-
duct. Its collector voltage instantly becomes
less negative (more positive) as shown at point
TO in Figure 10. (TO
is
at the start of the first
dot; T1
is
at the start of the following space.)
(SATURATION1
-16
VOLTS
TO TI T2 T3
T4
TIME
Q2
COLLECTOR VOLTAGE
Figure
10
This positive rise
is
coupled through capac-
itor C20 to the base of Q1. This drives the
base of Q1 to collector cutoff.
With
the col-
lector current of Q1 cutoff, the collector
voltage increases to
its
maximum (most neg-
ative) potential. Instantaneously the Q2 collector
current reaches it maximum (saturation). The
result
is
to turn transistor Q2 fully on and Q1
off.
Although this on and off (switching) action
is
fast,
it
is
not repeated instantly, since the voltage
across C10 requires time to change. When the
collector voltage of Q1 goes more negative,
capacitor C10 must charge to this higher col-
lechr voltage. The charge path
is
through the
base of conducting transistor Q2 and through
collector load resistor R10. The collector volt-
age of Q1 will rise with a slight rounding off.
This
is
shown between points TO and T1 on the
leading curved line in Figure
11.
0
TUR
TO
TI
T2 T3 T4
TIME
Ql
COLLECTOR VOLTAGE
Figure
1
1
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