DASHES
Dash Clamp
Dash clamp Q6 operates the same as did the
dot clamp. Transistor Q6 keeps the flip-flop
multivibrator circuit turned off by applying a
positive bias voltage to the base of Q4. This
bias voltage will cause Q4 to be cut off.
Fl ip-Flop Multivibrator
The flip-flop multivibrator consisting of tran-
sistors Q4 and Q5 and their associated circuit
are similar in construction and appearance to
the dot generator multivibrator. However, in-
stead of having cross-coupling (collector of one
to the base of the other) capacitors, it uses re-
sistors. Therefore, it cannot oscillate as didthe
dot generator multivibrator. This circuit will
stay in either of two states, where one tran-
sistor
is
cut off (fully off) and the other
is
saturated (fully on).
When the base of Q4 is positive with respect to
the emitter, it cannot conduct. The collector
voltage of Q4
is
then maximum negative and
is
coupled through resistor R41 to the base of Q5.
This negative voltage on the base causes Q5 to
conduct, thus keeping it at saturation.
In
this
condition, the multivibrator cannot switch. It
will
stay in this condition until a positive-going
pulse
is
fed to the base of the transistor that
is
in the saturated condition.
In
this case it
is
Q5. A positive pulse on the base of Q5 will
turn it off, causing its collector current to de-
crease and its collector voltage to become more
negative. This increase in negative voltage at the
collector of Q5 is coupled through resistor
R51 to the base of Q4 to turnit on. The negative
voltage will continue until Q4
is
driven tomaxi-
mum collector current (fully conducting), while
the collector current of $5
is
cut off (noncon-
ducting). Nothing further can happen until Q4
gets a positive pulse at its base to turn it off,
causing the cycle to repeat itself in the opposite
direction. This switching action takes place very
rapidly.
As mentioned before, the flip-flop multivibrator
cannot switch as long as the bias voltage from
clamp Q6
is
present at the base of Q4. Moving
the key lever to the dash position removes the
bias voltage from Q6, letting
Q4
operate. Si-
multaneously, diode Dl turns off dot clamp Q3,
allowing the dot generator multivibrator to run
free.
In
its starting condition it will be re-
membered that Q2 was off (maximum negative
voltage at the collector). As soon as the dot
multivibrator started to operate, the Q2 col-
lector voltage decreased rapidly from maximum
to near zero. This produces a positive (less
negative) going pulse from the collector of Q2
to the junction at the bottom of capacitors C41
and C51. This waveform
is
shown at points TO
and T2 in Figure 13.
0
VOLTS
-16
VOLTS
TO
TI
T2
T3 T4
TIME
02
COLLECTOR VOLTAGE
Figure
13
These positive
going pulses trigger the flip-
flop multivibrator. They are coupled through
capacitor C41
and diode D40 to the base of Q4.
These pulses are also coupled through capac-
itor C51 and diode D50 to the base of Q5. These
pulses affect only the transistor that
is
in the sat-
urated condition.
In
this instance, it
is
Q5. The
waveform of the pulse at the junction of C5l and
the diode D50
is
shown in Figure 14.
FOR
REFERENCE ONLY
0
VOLTS
-16
VOLTS
TO
TI T2
T3
T4
TlME
TRIGGER PULSE AT JUNCTION OF C51
AND D50 WITH
KEY
LEVER IN DOT POSITION.
Figure
14
As Q5 turns off, a negative going pulse
is
pro-
duced. This is because Q5 stops drawing cur-
rent, permitting the collector voltage to be-
come more negative. Capacitor C50 is used to
pass this pulse to the base of Q4 to speed up
the switching action. Capacitor C40 performs
the same task in the opposite cycle.
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