Heathkit HD-10, Manual

Page: 6 / 8

from Q2 will be a negative going pulse, as Q2
turns off. This negative pulse from Q2 has no
effect on the flip-flop multivibrator, as only pos-
itive going pulses can reset it. After the dot
generator multivibrator goes through another
half cycle, a positive going pulse from Q2 will
result. This positive pulse is coupled to the
base of Q4 (saturated transistor), resetting the
flip-flop multivibrator.
Note in Figure 14 that the flat portion of th
waveform is equal to the length of a dot an a
space. This is because Q2 produces a positive
going pulse every time it switches on. Since the
positive pulses switch the flip-flop transistors
alternately, they operate at half the rate of the
dot generator multivibrator. The output from
the dot generator and flip-flop multivibrators
add at the base of Q7. This produces the first
third of a dash signal from the collectors of Q1
and Q5, as shown in Figure 15. The middle
third of the dash is produced by the collector
output of Q5 alone.
Figure 15
The last third of the dash is filled in by the next
dot signal from Q1 alone as shown in Figure
16.
Figure 16
The result of these two signals at he base of
Q8 is the waveform shown in Figure 17.
Figure 17
At this instant both Q1 and Q5 are on and a
space results. This completes a dash and a
space at the collector of Q8 to key the trans-
mitter. This waveform is shown in Figure 18.
Figure 18
As the key lever is moved to the center or dot
position, clamp Q2 is turned on and applies a
positive bias voltage to Q4. This voltage turns
Q4 off an Q5 on to set the flip-flop multivibra-
tor. Thus, it is ready to start a dash the next
time the key lever is moved to the dash posi-
tion.
MONITOR CIRCUIT
Audio Clamp
Each time Q7 is turned on by a dot or dash sig-
nal, a negative bias voltage is applied through
R111 to the base of audio clamp Q11. This in
turn starts the free running audio multivibrator.
The audio multivibrator consists of transistors
Q9, Q10 and their associated circuitry. It is
identical to the dot generator multivibrator, ex-
cept for component values. Both of these multi-
vibrator circuits works in the same way. The
only difference being the audio multivibrator
oscillates at an audio rate. This rate is deter-