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The leading edge of this clock pulse takes IC2 pin 11 low. This will reset the dot NBM unless the
dot paddle is still closed. If the dash paddle has not been closed, then there is a low on IC3 pin 4
which holds IC3 pin 6 high, and the PBM cannot change state. Thus the next bit will be another
dot. If, however, the dash paddle has been keyed sometime during the dot bit, then IC3 pin 4 will
be high. When IC2 pin 11 goes low, IC3 pin 10 goes low, and IC3 pin 8 goes high. Then IC3B has
all inputs high so IC3 pin 6 goes low and holds IC3 pin 8 high following the clock pulse. On the
trailing edge of this clock pulse, the bit generator begins a dash, since IC4 pin 2 is high.
A necessary delay is provided by R3 and C1. Recall that when the PBM changed state, IC3 pin 6
went low on the leading edge of the clock pulse. IC3 pin 12 does not go high until the trailing
edge of the clock pulse. Were it not for the delay, IC2 pins 9 and 10 would be high, and the low on
IC2 pin 8 would remove the dash from this memory if one were stored there.
OUTPUT CIRCUITRY
When IC3 pin 12 goes high (mark condition), it turns on the Darlington pair, transistors Q6 and Q7,
which will key positive tines to ground. Transistor Q4 also turns on, thus turning on transistor Q5
and keying negative lines to ground through the power supply and resistor R25. Note that no more
than approximately 10 mA may be keyed by this circuit since it is a constant current sink which
limits at 10 mA. Diodes D4 and D5 isolate the two sections.
SIDETONE OSCILLATOR
The Sidetone Oscillator is made up of IC5 C and D, which are cross-coupled to form an astable
multivibrator. It is enabled when IC5 pin 6 goes low. Its Output is coupled to Q3, which drives the
speaker.
POWER SUPPLY
The Power Supply is a conventional series regulated bridge circuit that supplies +5 volts. Diode D6
protects the power supply against reversed battery polarity.
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