Receiving Circuits

TM 11-5895-1141-34

the NONLOCK position it must be held to maintain contact. In either case, S1002 must be off the center position before

closure of the P/T switch can control the action of U1001. Closing the P/T switch causes the output of U1OO1C to go high

if, at the same time, the voltage on the interlock bus is also high. The interlock bus is common to all selector modules and

allows no more than one operator to work the transmitter at any time. The interlock bus voltage will be low if another

operator has keyed on the transmitter. When the U1OO1C output is high Q1008 conducts, energizing relay K1001. This

closes a set of contacts through P1001 pins 12, 13, keying on the transmitter. A second set of contacts on K1001

connects the audio line to the transmitter via T1002 and P1001 pins 10, 11. When the relay is deenergized, the transmitter

audio line is connected to 620 ohm terminating resistor, R1051. U10OO1D, B are also high when U1OO1C is high

causing Q1006 to illuminate DS1004, the channel keyed indicator and also opening Q1009 which disables the busy tone

generator. The function of this generator is to sound a buzz in the operator's headphone only if the transmitter is engaged

by another operator.

d. Receiving Circuits. Refer to figures 2-4, FO-1 and FO-2. Voice signal appearing at the channel's radio receiver

output, is delivered to the pin pair 15 and 16 of the selector module assembly assigned to this channel at the operator's

position. This signal is transmitted via transformer T1001 to a lamp driver circuit consisting of transistors Q1001, Q1003

and Q1004, causing amber indicating lamp DS1002 to flash at a rate comparable to the interval between speech syllables

and referred to as the syllabic rate. This gives a visual indication of audio from the receiver, if present. The threshold at

which received audio is capable of flashing the lamp may be adjusted through potentiometer R1003. The received signal

is also delivered to the HEADSET/LOUDSPEAKER selector switch S1001 via amplifier stages Q1002 and Q1005. S1001

is located on the selector module assembly front panel. This toggle switch is a three position, center-off type which allows

the operator to route received audio to either the headset amplifier bus or the loudspeaker amplifier pins. Q1002 is gated

by the interlock bus voltage so that received signals are blocked whenever the transmitter of this channel is keyed. The

amplifier stages U11O1D and U1105 located in the volume control module, are fed respectively by the headset and

loudspeaker amplifier buses. These amplifiers are of the operational type in a fixed gain configuration and are capable of

receiving and amplifying the sum of the audio signals delivered from all the selector modules engaged. The total headset

audio signal at the output of U1OO01D is fed to amplifier U1102 via a resistor and shunt FET transistor Ql101. The

conductivity of Ql101 is controlled by U1104 so that the output remains fairly constant over a wide range of audio input

levels. This regulated signal is further amplified by stages U1103, U11O1B, Q1102, Q1103, and Q1104 and fed via pins E

and F of connector J101 to the jack panel and to the connected headset. The HEADSET VOLUME control, located on the

module's front panel, permits the operator to adjust the headset sound level. The total loudspeaker signal at the output

U1105 is adjusted by the SPEAKER VOLUME

2-7