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TRANSMIT VOICE CIRCUITS
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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RECEIVE AUDIO MODULE A1
TM 11-5805-387-34-2
1-51. OUTPUT AMPLIFYING CIRCUITS
Output amplifying circuits (fig.  FO-10) include three conventional class A, common-emitter amplifiers:  tty
amplifier A3A2Q6, voice filter amplifier A3A2Q9, and transmit output amplifier A3A2Q7. Tty tones from nsk filter FL1 or
fsk filter FL2 are applied through A3J1-2 to the base of tty amplifier A3A2Q6. The amplified tty tone signal is applied
through coupling capacitor A3A2C29 and temperature compensating thermistor A3A2RT1 to the base of transmit output
amplifier A3A2Q7. In the 85-Hz plus voice mode of operation, audio from transmit voice filter FL3 is applied through
A3J1-25 to the base of voice filter amplifier A3A2Q9. The amplifier recovers filter losses and isolates the voice and tty
channels. The amplified 200to 2,260-Hz output of the amplifier is fed through capacitor A3A2C31 to the base of transmit
output amplifier A3A2Q7. Transmit output amplifier A3A2Q7 amplified both the voice and tty signals and applies them,
through coupling capacitor A3A2C30 and A3J1-1, to output transformer A6T1.
Section VII. STAGE ANALYSIS, RECEIVE SIGNAL PATHS
1-52. INTRODUCTION
This section presents detailed stage analysis of the modem receive circuits -including receiver module A4, receive
audio module A1, and scope module A2. Loop battery module A5, which is common to both transmit and receive signal
paths, was discussed in paragraph 1-43. The +20 volt regulator portion of receive audio module A1 is discussed in
paragraph 1-63. With these exceptions, the discussion of the circuits have been limited to a module basis. Signal flow
between the receive signal path modules is covered in the detailed block diagram discussion. Figure FO-8 is the overall
schematic diagram of the modem. Refer to it for the details of signal paths between modules and for switch connections.
1-53. RECEIVE INPUT ISOLATION AMPLIFIERS
(fig. FO-8)
Board A7 contains two identical Isolation amplifiers which insure separation of the receive voice channel from the
receive tty channel, and provide a constant input impedance. Input signal is applied through coupling capacitor A7C3.
Resistors A7R7 and A7R14, in conjunction with A7R15, provide channel separation and attenuate the input level. Base
bias is supplied through A7R8, R10 or A7R16, R12; collector bias through A7R9 or A7R17. Dc stabilization is insured by
emitter resistor A7R11 or A7R13. The output is applied to the tty channel through coupling capacitor A7C4 and to the
voice channel through coupling capacitor A7C5.
1-40

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