Transmitting Circuits

TM 11-5895-1141-34

means of the lamp brightener control R1243 located on the front panel of the microphone amplifier/lamp brightener control

module assembly, and is variable between +12 and +22 volts. This output is delivered to the selector module assembly

voltage control regulator which supplies the panel lamps in that module. The power supply return is connected to J105-B,

and is extended to each module.

b. Transmitting Circuits. Refer to figures 2-2, FO-1 and FO-2. We see that the audio signal originates at one of

three microphones: the headset microphone, low-level microphone or the high-level microphone. The signal proceeds

from the jack panel to the microphone amplifier to the selector module before reaching the transmitter where it serves to

modulate the carrier. Beginning at the headset/microphone jack panel, the signal enters the panel via either jacks J301

and J302, J303, or J305, depending on which microphone is used. Keying is accomplished by a switching transistor

circuit. From the headset/microphone jack panel, the signal is delivered via connectors P301 and J101 to input

transformer T1201, input level adjust potentiometer R1202, and preamplifier stage U1201. The preamplified signal is then

fed to an operational amplifier stage U1202 via a resistor and a gain control shunt FET transistor Q1201. Q1201 is

controlled by the dc output of U1204 which rectifies and filters the signal output of U1202. This arrangement results in the

U1202 output remaining fairly constant despite a wide range of audio input levels. This regulated signal is further amplified

by stages U1203, Q1202, Q1203, U1205B and Q1204 and drives the transmitter audio bus common to all of the selector

modules at the operator's position. Transmit level adjust potentiometer R1230 adjusts the audio level. Amplifier stage

U1205B is gated by the transmitter channel status signal so that its output is blocked unless one or more transmitters are

being keyed by the operator at this position. At each selector module transmitter audio is fed to the input of the channel's

radio transmitter via buffer amplifier stage Q1010, Q1011, transformer T1002 and a pair of contacts on relay K1001. The

relay is energized, passing the output signal only when the channel is keyed from this operator's position. The audio signal

is also routed to the headset via gated stage U1205D plus additional circuitry in the volume control module assembly,

when one or more transmitters are keyed. This sidetone signal gives the operator a sense of "presence" when he speaks,

as well as confirmation that the equipment has been keyed into the transmitting mode. A portion of the microphone signal

is also diverted via U1205C to tape recorder output terminals in the recorder monitor module (if used) from the microphone

amplifier module.

c. Keying and Interlock Circuits. Refer to figure 2-3 and figure FO-1. Closure of the microphone push-to-talk (P/T)

switch grounds P301-C. The four sections of integrated circuit U1001 are interconnected to perform as logic gates and

provide required interlocking and status indication functions. The selected channel is enabled by moving front panel switch

S1002 from its center-off position to either the NONLOCK or the LOCK position. The switch may be locked in the LOCK

position but in

2-2