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Amplifier Assembly (cont)
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TM-11-5825-270-23 Monitor Radio Frequency R-2176/FRN (NSN
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Figure 3-2. Schematic Diagram, Switch Bracket 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.
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


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