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as a closed switch in the matrix. When you use external paddles, transistors Q4 or Q5 are turned
off by a paddle closure, also resetting the associated flip-flop, producing the same results.
The oscillator also drives a "charge-pump". This circuit consists of capacitor C2 and C3 and diodes
D2 and D3. These components provide a negative potential to the negative (–) keyer output cir-
cuitry.
OUTPUT CIRCUITRY
OR Gate IC U9B allows the microprocessor to determine whether the output on port 5, bit 4, will
key the output circuitry and sidetone, or just the sidetone. If U9B is enabled, its output will go low
on each dot and dash, turning off transistor Q12. This allows transistor Q14 to turn on through re-
sistors R31 and R32, to key a positive keyline to ground. At the same time, the base of transistor
Q15 goes to near zero volts, turning it off and allowing transistor Q16 to key a negative keyline to
ground. Protective diodes across the two keyer output jacks prevent damage to to the output tran-
sistors if the wrong jack is selected for a particular transmitter. The transmitter would then be
keyed continuously.
POWER SWITCHING CIRCUIT
The power switching circuitry allows the keyer to be turned on and off with a momentary switch. It
also forces the keyer to be off whenever you plug it in and provides automatic turn-off after a pe-
riod of nonuse. Transistor Q8 is a series pass transistor between the diode bridge and the major
part of the circuitry. Note, that IC U6 is connected to the output of the bridge at all times. (It
draws virtually no current itself.) U6B and U6D form an R-S flip-flop which drives ICs U6A and C in
parallel to turn transistor Q8 on or off. When the keyer is plugged in, capacitor C9 will have dis-
charged through diode D13, which holds the output of IC U6D high. Both inputs of U6B will there-
fore be high, making the output of IC U6B low and the output of U6A and C high. This keeps tran-
sistor Q8, and thus the keyer, turned off.
When you press the
ON
button, one input of IC U6B goes low and causes its output to go high.
The output of IC U6A and C goes low. This turns transistor Q8 on and supplies power to the keyer.
At the same time, transistor Q9 is turned on and charges capacitor C9. Both inputs of IC U6D are
high and its low output holds the R-S flip-flop in the "on" state when the
ON
button is released.
When you leave the keyer on for a period of time, but do not use it, capacitor C9 discharges far
enough to take the input of IC U6D to a low state. The R-S flop-flop now toggles to turn the keyer
off. Transistor Q11 is connected to effectively "push" the
ON
button whenever the sidetone
sounds and resets the automatic shutoff interval.
MEMORY BACKUP CIRCUIT
ICs U2 and U3 are "256 × 4" CMOS RAMs connected as a 256 × 8 RAM. They have very low
power consumption in the standby mode. This allows them to retain data with a supply voltage as
low as 2 volts. This standby mode is selected by bringing chip-enable pin CE2 on ICs U2 and U3
low. To insure that data is not lost when the keyer is shut off or loses power, transistors Q6 and Q7
and their associated components sense drop in the supply voltage below the minimum required for
ICs U4 and U5 to remain in regulation and pull CE2 low.
As the collector voltage of transistor Q8 falls, capacitor C7 discharges quickly through diode D7.
When this voltage reaches approximately 8 volts, transistor Q6 and Q7 turn off, allowing CE2 to go
low through resistor R21. As the output of IC U5 falls below the battery voltage, diode D4 be-
comes forward biased and diode D5 reverse biased. (Diode D6 makes up for the voltage drop
across diode D5 when the keyer is on.) The RAM is now in the standby mode. The backup batter-
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