1-59. MARK HOLD SWITCHING
The mark hold switching circuit compares the noise in the unfiltered, audio input signal with the filtered tty signal.
When signal to noise is excessive, the channel is squelched. Unfiltered signal plus noise is applied to a voltage divider
consisting of potentiometer A4A2R1 and either resistor A4A2R30 or A4A2R31, depending on the setting of front panel
MODE SELECTOR switch S4. Resistor A4A2R30, used during the 850-Hz mode, is a much lower value than resistor
A4A2R31, to compensate for the greater noise that is present in the wider bandwidth signal.
A portion of the voltage that appears across potentiometer A4A2R1 is applied through coupling capacitor A4A2C2
to detector network A4A2CR1, A4A2CR2, A4A2C3, and A4A2R2. The detector network provides a positive dc voltage at
the junction of A4A2C3 and A4A2R2 that is proportional to the signal-plus-noise input. This dc voltage is applied to
differential amplifier A4A2Z1.
Dc voltages proportional to the filtered and rectified signals present in tty channels 1 and 2 are applied to the
anodes of diodes A4A2CR5 and A4A2CR6, respectively. Diodes A4A2CR5 and A4A2CR6, and cathode load resistor
A4A2R3 form an auctioneer (OR gate) circuit referenced to + 5.6 volts dc. The greater voltage applied to diodes
A4A2CR5 and A4A2CR6 will cause the appropriate diode to conduct, and the resulting voltage drop across A4A2R3 is
applied to differential amplifier A4A2Z1.
Differential amplifier A4A2Z1 is an integrated circuit differential amplifier identical with A4A2Z2 with internal
circuitry arranged so that pin 8 is low as long as the dc signal on pin 1 does not exceed the signal on pin 6 by more than
a small amount. Because the dc voltage applied to pin 1 also includes the noise component of the input signal and the
dc voltage applied to pin 6 is derived from a signal with the noise filtered out, when excessive noise is received the
voltage at pin 1 will exceed that at pin 6, and pin 8 of differential amplifier A4A2Z1 will approach the + 10.7 volt dc
supply voltage. This causes zener diode A4A2VB1 to conduct. The amount of noise which is considered excessive
depends on the mode of operation. Because of the wide-band operation in fsk, the signal-to-noise ratio which initiates
the squelching action is between 3 and 6 db. In all 85-Hz modes, the ratio is between 6 and 9 db. The sensitivity of the
tty squelch depends on whether low value resistor A4A2R30 (85-Hz mode) or high value resistor A4A2R31 (all 85-Hz
modes) is used as the path to ground for tty squelch adjust A4A2R1.
When zener diode A4A2VR1 conducts, the voltage drop across bias resistor A4A2R4 causes tty squelch switch
A4A2Q1 to conduct, presenting a low level to dc bistable multivibrator A4A1Q5, Q6. Capacitor A4A2C23 bypasses
resistor A4A2R4 to enhance rapid turn on of switch A4A2Q1. With front panel AUTO MARK HOLD switch S6 set at OFF,
the base of switch A4A2Q1 is grounded, preventing the transistor from conducting.