Audio Amplifier Assembly

TM 11-5825-270-23

b. The modulation signal to the audio amplifier is

pulse is applied to U2-D each time the modulation is

received at terminal 8 of TB2 (fig. 3-4). This input signal

removed (end of keying).

p. As long as the carrier level is normal, TP5 is

is taken from the primary of transformer T1 in the if. PCB

assembly (fig. FO-2), routed through P1-1 and the front

high. Thus, each time the modulation monitor circuit

panel AUDIO LEVEL control R1. The audio amplifier

produces a positive pulse, the output of U2-D goes low,

operates from the +28 V bus and comprises Q1 through

and master reset input of U3 goes high. Therefore, as

Q4 and associated components. Transistors Q1 and Q2

long as the on/off sequence of the modulation takes

operate in class A, while Q3 and Q4 form a class B,

place within the delay set by R32, pin 8 of U3 remains

push-pull complimentary output stage. The output (0.5

high and alarm condition is not produced. However, if a

W minimum) is taken through C3 and fed to the

malfunction results in continuous modulation or total loss

loudspeaker.

of modulation, pin 12 of U2-D assumes a steady low

c. The alarm oscillator consists of U1-A, U1-B, C4

state. As a result the output from U2-D becomes high,

and resistors R7 through R9. When an alarm condition

and U3-6 goes low through U2-C. This means that

is detected, terminal 7 of TB1 is grounded by the low

oscillator and counter in U3 are enabled and an alarm

from if. amplifier assembly, permitting the oscillator to

condition is produced at the end of the preset delay.

operate at approximately 3 kHz. Gates U1-C and U1-D

3-7. Audio Amplifier Assembly

buffer the oscillator output which is fed to the audio

amplifier section through R12, R13 and switch S1.

(fig. 3-4 and FO-3)

d. Potentiometer R13 provides means of setting

a. The audio amplifier performs the following

the alarm tone output from the audio amplifier. It is

normally set at the time of installation to the desired

functions.

volume. Switch S1, when closed, allows the alarm tone

(1) Generates a 3 kHz alarm tone upon

to reach the audio amplifier. Switch S1 may be left in

detection of an alarm condition by the if. amplifier

the off position in unmanned installations or during

assembly.

testing and troubleshooting.

(2) Drives the loudspeaker with modulation

audio or the alarm tone.

3-7