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+22-VOLT PREREGULATOR
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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85-HZ OPERATION
TM 11-5805-387-34-2
1-67. OPERATIONAL CONTROL CIRCUITS
(fig. FO-15)
MODE SELECTOR Switch . Wafer F of MODE SELECTOR switch S4 switches regulator +20 volts (para 1-66)
and preregulated +22 volts (para 1-65) to relays and circuits within modules that must operate only in certain modes.
The routing of +20 and +22 volts is discussed below and in paragraphs 1-68 through 1-71.
850-Hz Operation. When MODE SELECTOR switch S4 is set at 850 HZ, + 22 volts, + 20 volts, and ground are
applied to the appropriate circuit as follows:
The +22 volts dc from the preregulator is applied through contacts 12 and 7 of MODE SELECTOR switch S4F to pin
25 of receiver module connector XA4, A4J1.  The +22 volts energizes relay A4A1K1, modifying operation of
frequency discriminator A4A1Q9, Q10 for the fsk tones in the 850-Hz mode. The +22 volts also forward biases
channel 2 switch A4A2Q4, effectively grounding the input to receiver module tty channel 2. The +22 volts also
energizes nsk/fsk filter relay K1.
The +20 volt dc output from the regulator is applied through contacts 6 and 1 of MODE SELECTOR switch S4F to
pin 1 of receive audio module connector XA1, A1J1. The +20 volts energizes the bfo generation circuitry. The +20
volts is also applied to a voltage divider containing BFO control R2. The setting of R2 determines the voltage
applied through XA1, A1J1-2 to the rc circuit connected to the emitter of BFO A1A3Q1, which In turn determines the
Ground is applied through S4A, S4C, S4D, and S4G as follows:
Switch S4A grounds receiver module terminal XA4, A4J1-5, which connects tty squelch adjust A4A2R1 to ground
through A4A2R30. The value of A4A2R30 is lower than the corresponding resistor used in the nsk modes,
modifying the tty squelch circuitry for 850-Hz mode of operation. This is necessary because the wideband width
of fsk operation results in a higher noise level than that experienced in nsk operation.
Switch S4C grounds the output of 600-ohm voice amplifier A3A2Q8 through contacts 12 and 7, and the output of
the transmitter module 50-ohm mike circuitry through contacts 1 and 6.
Switch S4D grounds the input to nsk input filter FL4, providing a grounded input to receiver module tty channel 2
(A4J1-15), which is unused in 850-Hz mode.
Switch S4G grounds transmitter module pin XA3, A3J1-7. When pin 7 is grounded, mixer oscillator A3A2Q3, Q4
is switched to produce an output of 26,050 Hz through 850-Hz switch A3A2CR5. Diode-by-10 bypass switch
A3A2CR10 is biased into conduction through isolation diode A3A2CR12, bypassing the divide-by-10 circuitry to
apply the 26,050-Hz from the mixer oscillator direct to mixer A3A2T1.  Diode A3A2CR4 is forward biased
because of the ground from pin 7 applied through A3A2R26. Output mixer amplifier A3A2Q2 is switched to
2,000-Hz fsk filter FL2 through diode switch A3A2CR4.
1-56

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