Heathkit HD-10, Manual

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Q1 conducting to saturation and turning off Q2.
The circuit then reaches its second inactive
(quiescent) condition. Capacitor C1 having pre-
viously been charged, now discharges at a time
constant determined by the value of C10 and
control R11. As long as the key lever is held in
the dot position, the dot generator multivibra-
tor continues to run free. Each stage is then al-
ternately on and off for relatively long periods
(determined by the value of C10 and R11, and
C20 and R21), followed by a very rapid switch-
over to the opposite stage.
The collector output signals of Q1 and Q are
fed to other sections of the circuit. The output
from the collector of Q1 is fed through R30 to
the base of Q3 and through R79 to the base of
Q7. The output is fed to Q3 to make the multi-
vibrator action self-completing. Once the multi-
vibrator is started, any movement of the key
lever will not have any effect until the dot and
following space have been completed. The op-
eration of Q7 will explained in the following
section.
Driver Follower And Switch
The base of switch transistor Q8 draws more
current than can be supplied by the dot gener-
ator transistors. Therefore driver transistor Q7
is used as an emitter follower to supply the
necessary drive for Q8. Resistors R73, R74,
R75, R76 and R78 divide the power supply vol-
tages to set the proper bias voltages to fully
control Q7 and Q8.
Transistors Q7 and Q8 have a positive base bias
voltage which keeps them cut off (no current
flow). Each time the collector voltage of Q1 is
at –16 volts, transistor Q7 is biased to conduc-
tion. This action then biases Q8 to a point
where it will also conduct. The output from Q7
is fed through resistor R111 to the base of
audio clamp Q11. The operation of Q11 will ex-
plained later. The collector output of Q8 is used
to key the transmitter. This waveform is shown
in Figure 12.
Figure 12
Manual keying may be accomplished by con-
necting a hand key between the Hand Key and
GND terminals. This operates Q7 and Q8 inde-
pendently of the Keyer operation by shifting
the bias voltage on Q7. The Hand Key terminal
is also grounded when the slide switch is
placed in the Hold position. This allows continu-
ous transmitter operation for tuning or adjust-
ment purposes.
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.
Flip-Flop Multivibrator
The flip-flop multivibrator consisting of transis-
tor Q4 and Q5 and their associated circuit are
similar in construction and appearance to the
dot generator multivibrator. However, instead
of having cross-coupling (collector of one to the
base of the other) capacitors, it uses resistors.
Therefore, it cannot oscillate as did the dot
generator multivibrator. This circuit will stay in
either of two states, where one transistor is cut
off (fully off) and the other is saturated (fully
on).
When the base of Q4 is positive with respect to
emitter, it cannot conduct. The collector volt-
age 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 in 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.