· Voice mode operation of 50-ohm voice signal path. In the voice mode, the modem filters and switches voice
signals, but does not operate on the signal in any other way. Keying level control and mike bias must be supplied
externally. Voice signals generated in the 50-ohm mike are applied through MICROPHONE input jack J3 to input
filter A6L6, C6, C13, through wafer S4B of MODE SELECTOR switch S4 to 50-ohm mike output filter A6L3, C3,
C11. The signal then passes out of the modem through RCVR TRANS AUDIO jack J2. Transmit voice filter FL3 is
not used in this mode.
1-25. TRANSMIT OUTPUT CIRCUITRY
The transmit output circuitry consists of tty amplifier A3A2Q6, voice filter amplifier A3A2Q9, 600-ohm voice amplifier
A3A2Q8 (voice mode only), transmit output amplifier A3A2Q7, and transmit output transformer A6T1. Tty amplifier
A3A2Q6, voice filter amplifier A3A2Q9, and 600-ohm voice amplifier A3A2Q8 feed a common buss through their
respective isolation resistors.
The buss is the input to transmit output amplifier A3A2Q7. In 85-Hz plus voice mode, the 600-ohm or 50-ohm voice
signal output of transmit voice filter FL3 is applied to voice filter amplifier A3A2Q9, which serves to recover filter losses
and to isolate the voice and tty signal paths. Filter FL3 limits the voice bandwidth to 2,260 Hz, preventing interference
with 2,805-Hz nsk tty tones that are applied to the buss through tty amplified A3A2Q6. Thus, mixing of voice signals and
tty tones occurs at the input to transmit output amplifier A3A2Q7. In all other modes, no mixing is necessary, and voice
(only) or tty tones (only) are applied to the transmit output amplifier. The unbalanced output of transmit output amplifier
A3A2Q7 is applied to transmit output transformer A6T1, which provides a balanced 600-ohm nominal output at RCVR
TRANS AUDIO jack J2. A 50-ohm nominal output is also provided through J2 in voice mode only.