Buffer Amplifiers

TM 11-5895-1141-34

(3) Buffer Amplifiers. Refer to figure FO-8. The regulated audio signal is fed to amplifier U3 which has a non-

inverting gain close to 20 as set by the complementary pair Q2, 03. This complementary emitter follower is used to set the

module's audio output level. One audio signal path bypasses Q2, Q3 and potentiometer R30. Buffer amplifier U5D

receives the regulated audio out of U3 directly. U5D has unity gain, and its output is fed to the headset amplifier as a

sidetone signal. Resistor R23 serves to isolate the output of U5D from the headset amplifier in

the volume control module.

(4) Amplifier U5B is another buffer amplifier (gain is less than unity). The output of U5B is ac coupled to driver

stage Q4 and another buffer amplifier U5C. Q4 is a PNP transistor operated as a common emitter with the collector load

resistor R38, grounded. The regulated audio output of Q4 is sent to the selector unit via connector P1-21. The output of

U5C is sent to mixer amplifier U1301 in the recorder-monitor module via isolating resistor R42 and P1-3. Note that all

three buffer amplifiers U5B,C,D, are gated by the transmitter status bus signal. When the bus is high, indicating the

transmitter is not keyed on, the 24 v bus level is brought directly to the non-inverting inputs of amplifiers via CR6 and CR7.

This forces each of the three outputs to cut-off (+24 v) or a gated off condition. When the bus drops low (transmitter keyed

on) then the amplifier outputs will drop below 24 v and the amplifiers are gated on. Bias current is drawn through the 2.2

megohm resistors R24, R34 and R41 into the respective non-inverting terminals which forces the amplifier outputs below

24 v, back into their active region.

(5) Lamp Supply Bus Regulator. Refer to figure FO-8. Transistor Q5 is an emitter follower whose base voltage

may be varied between +12 and +24 v by LAMP BRIGHTNESS control potentiometer R43. The emitter voltage is

distributed via P1-35 and the lamp load bus, to the various selector modules to be used for the indicator display lamps.

Transistor Q5 also furnishes lamp current to DS1301, the syllabic indicator in the recorder-monitor module.

d. Selector Unit Assembly. This unit is the same size as the audio unit assembly for installation into the console.

The unit contains one air selector module assembly and seven blank panels. The positioning of the polarizing pins

prevents installation of any audio unit modules into the enclosure. The schematic diagram of the selector unit assembly is

shown in figure FO-15. This unit houses one selector module assembly and engages one only of the receptacles J204

through J211, located within the enclosure. Pins 3, 4, 5, 7, 8, 9, 19 through 23, and 26 of these receptacles carry power,

supply, control or signal functions common to both the selector module and the audio unit assembly. These functions are

bussed to the interunit cable, extending from the rear panel of the unit and terminate at connector P204. From the pin

group, 10 through 16, on the module receptacle, connections are extended to pin groups on receptacles J201 and J202,

so that individual radio channel functions provided

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