TRANSMITTER KEYING CIRCUITS

TM 11-5820-670-30

1-60. RADIO SET AN/ARC-131 INTERUNIT CIRCUIT DETAILS. (CONT)

The + 27.5 vdc primary power is supplied to the radio set from the aircraft electrical system. Primary

power is applied at pins C, D, and E of control unit electrical connector J9101. From pins C, D, and

E, power is applied to contacts 1,2,3, and 5 of wafer D of mode control S9105. Control S9105 is

shown in OFF position. In this position, no power is applied through the switch. When the switch is

rotated to T/R position, contacts 1, 2, and 5 are shorted to the switch segment, applying power

through contacts 9, 10, 11, and 12 to pins e, f, and F of J9101.

The primary power at pins e, f, and F of the control unit receptacle is connected through the aircraft

harness, to pins 18, 34, and 36 of the receiver-transmitter electrical connector P9601. From P9601,

power is coupled directly to the power supply through connectors P9501 and J9501 pins 1, 2, and 9.

The power supply converts primary power into regulated +24 vdc operating voltage. In addition, +27.5

vdc primary power is coupled through fuse F9502 and diode CR9504 to operate relays on the receiver-

transmitter chassis. Diode CR9504 protects the radio set from reverse voltage.

When mode control switch is rotated to RETRAN or HOME position, primary power is applied the same

way, except that it is shorted through S9105 at contacts 1, 2, and 3 instead of 1, 2, and 5. In HOME

position, primary power is also applied through diode CR9101. Diode CR9101 provides a reverse volt-

age protection for relays K4501 and K4502.

This homer primary power is coupled through the aircraft harness to pin 16 of receiver-transmitter unit

electrical connector P9601. From P9601, power is applied through P4501 and J4501, pin 12 to coils

of relays K4501 and K4502 on the rf control assembly.

Application of homer operate voltage energizes K4501 and K4502; relay K4501 disconnects the com-

munications antenna from the radio set, and relay K4502 switches the power to operate the homer cir-

cuits in the homer assembly.

See paragraph 1-58 for primary power distribution within the receiver-transmitter.

TRANSMITTER KEYING CIRCUITS

See FO-14 while reading this section.

Switching operations are initiated when the transmitter system is keyed. This transfers the radio set

from receive mode to transmit mode of operation. Switching operations are performed by relays in the

receiver-transmitter and within the rf control and audiofrequency amplifier assemblies. FO-14 shows

the radio set in transmit mode with the push-to-talk button pressed.

When the radio set is switched to transmit operation, the transmitter section is connected to the com-

munications antenna through the rf control assembly. The sidetone gate signal output to the rf control

assembly is supplied to the audio amplifier. The radio set audio section supplies all audio output

signals to the aircraft interphone amplifier and is also used by the transmitting system when the system

is keyed.

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