1-68. 85-HZ OPERATION
When MODE SELECTOR switch S4 is set at 85 HZ, +22 volts, +20 volts and ground are applied to the appropriate
circuit as follows:
The +22 volts from the preregulator is applied through contacts 12 and 8 of MODE SELECTOR switch S4F to pin
17 of receiver module connector XA4, A4J1. The +22 volts forward biases channel 1 switch A4A2Q3, effectively
grounding the input to receiver module tty channel 1.
The + 20 volts dc output from the regulator is applied through contacts 6 and 2 of MODE SELECTOR switch S4F
to pin 15 of transmitter module connector XA3, A3J1. The + 20 volts energizes divide-by-10 circuit A3A1Q11-Q18.
Thus, mark-space tones generated by mark-space oscillators A3A2Q19-Q22 are divided by 80 in 85 Hz operation rather
than by 8 as in 850 Hz operation.
Ground is applied through S4A, S4C, and S4G as follows:
Switch S4A grounds receiver module pin XA4, A4J1-6, which connects tty squelch adjust A4A2R1 to ground
through A4A2R31 in all nsk modes. This action compensates for the difference in noise content between fsk
signals and band nsk signals caused by the difference in bandwidths.
Switch S4C grounds the output of 600-ohm voice amplifier A3A2Q8 through contacts 12 and 8, and the output
of transmitter module 50 ohm mike circuitry through contacts 2 and 6.
Switch S4G grounds transmitter module pin XA3, A3J1-8 through contacts 10 and 12. When pin XA3, A3J1-8
is grounded (as It is In all 85 Hz modes), A3A2C1 parallels A3A2C2, narrowing the pass band of sine wave
pass filter A3A2C1, C2, L1 for nsk tones.
1-69. VOICE OPERATION
When MODE SELECTOR switch S4 Is set at VOICE, +22 volts, +20 volts, and ground are applied to the
appropriate circuits as follows:
The +22 volts and +20 volts are not switched to any special circuitry in voice mode.
Ground is applied through S4A, S4D, and S4G as follows:
Switch S4A grounds receiver module pin XA4, A4J1-23 through contacts 3 and 6, resulting in a mark-hold output
from the modem for all associated receive tty equipment.
Switch S4D grounds the input to nsk input filter FL4 through contacts 6 and 3, providing a grounded input to
receiver module tty channel 2 (through XA4, A4J1-15), which is not used in the voice mode.
Switch S4G grounds the input of transmit module tty amplifier A3A2Q6 through contacts 9 and 12, preventing
spurious output of mark-space tty tones from the transmitter module in voice operation. Contacts 3 and 6
provide a ground for relay A5K1 through CR2, preventing interaction with units attached to the modem through
pin A of AUXI LIARY connector J5.