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LOOP BATTERY MODULE CURRENT REGULATOR
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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LOOP BATTERY MODULE SEND SENSING
TM 11-5805-387-34-2
LOOP BATTERY MODULE RECEIVE SENSING
1-41. RECEIVE AND SEND SENSING CIRCUIT
Receive Sensing Circuit. The receive sensing circuit controls the flow of do loop current. The current causes loop
current to flow when a tty mark pulse is received and no current to flow when a space pulse is received. Loop current
may be supplied either by the Internal dc-to-ac Inverter and current regulator (para 1-40) or by an external power supply.
When a mark tty signal Is received, circuits of receiver module A4 develop a positive pulse. No pulse is developed when
a space tty signal is received. The positive pulse Is applied through connector A5J1-1 and rf noise filter A5A2L2,
A5A2R3, A5A2C8 to the base of receive oscillator transistor A5A2Q2. The receive oscillator turns on for the duration of
the positive Input pulse.  The signal from the oscillator Is applied by oscillator-isolation transformer A5A2T1 to an
envelope detector-doubler consisting of A5A2C3, A5A2CR6, A5A2CR5, A5A2R2, and A5A2C2. The resulting dc output
from the envelope detector-doubler is applied through resistor A5A2R1 to the base of loop current switch A5A2Q1. Thus,
when a normal mark pulse is received, loop current flows through steering quad diode A5A2CR4, the emitter and
collector of forward-biased switch A5A2Q1, zener diode A5VR5, and steering quad diode A5A2CR1 to operate tty
machines connected to the dc loop. When a normal space is received, switch A5A2Q1 is reverse biased and no current
flows through the dc loop zener diode A5VR5 duplicates the voltage drop of regulator zener diode A5VR6 in the send
circuit. This permits use of either current regulator 1 or 2 with either the send or receive circuit, as required when the
mode of operation is switched from duplex to one way receive. Steering quad diodes A5A2CR1 through A5A2CR4 allow
an external power supply to be used for loop current without regard for polarity. In the above discussion, if an external
supply had been used and had been connected backwards, loop current would have flowed through diodes A5A2CR3
and A5A2CR2 (instead of A5A2CR4 and A5A2CR1, respectively). However, current flow through switch A5A2Q1 would
not be affected.
1-28

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