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 transis-
tor Q2. Diode D30 isolates Q3 from Q2.
DOT GENERATOR MULTIVIBRATOR
Transistors Q1 and Q2 and their associated cir-
cuitry make up the basic timing dot generator
which is a free running multivibrator. Capaci-
tors C10 and C20 with Speed control R11 and
R21 control the switching speed of the multivi-
brator. Control R21 (screwdriver adjust) is ad-
justed 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 sections 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 gener-
ator multivibrator turned off. In the off condi-
tion, 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 resistor R12, R21
and R22. This causes Q1 to conduct to a satu-
rated condition. Therefore, its collector voltage
is practically zero or very near ground potential.
At the positive bias voltage is removed from
the base of Q2, it immediately becomes less
negative (more positive) as shown at point T0
on Figure 10. (T0 is at the start of the first dot;
T1 ia at the start of the following space.)
Figure 10
This positive rise is coupled through capacitor
C20 to the base of Q1. This drives the base of
Q1 to collector cutoff. With the collector current
of Q1 cutoff, the collector voltage increases to
its 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 volt-
age across C10 requires time to change. When
the collector voltage of Q1 goes more negative,
capacitor C10 must charge to this higher collec-
tor voltage. The charge path is trough the base
of conducting transistor Q2 an through collec-
tor load resistor R10. The collector voltage of
Q1 will rise with a slight rounding off. This is
shown between points T0 and T1 on the lead-
ing curved line in Figure 11.
Figure 11
Although Q1 is now off and Q2 is on, the circuit
cannot remain in this inactive (quiescent) con-
dition indefinitely because of the action of ca-
pacitor C20. This capacitor has previously been
charged to the Q2 collector voltage. Since the
Q2 collector voltage has very rapidly become
less negative C20 must now discharge. This is
shown by the flat top line in Figure 10. The dis-
charge path is through control R21. The time
constant of the discharge curve is determined
by capacitor C20 and control R21. The charging
time of C20 is very short with respect to the
discharge time. This is due to the high resist-
ance value of control R21 as compared with re-
sistor R20. As C20 discharges, the voltage at
the base of Q1 becomes less positive (ore neg-
ative).
When the conducting level of Q1 is reached
and the transistor starts to conduct as shown
at T1, the complete cycle is now reversed, with
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