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TM-11-6625-564-45 Maintenance Kits Electronic Equipment MK-731/ARC-51X Manual
Difference in Models
C2, TM 11-6625-564-45
of transistor Q1. The output of Q1 is devel-
may be injected to the modulator and audio
module in the receiver-transmitter in this
oped across collector load resistor R8 and cou-
pled through capacitor C6 to the base of tran-
sistor Q2. The signal is then consecutively
amplified by and coupled to succeeding tran-
1-28. Auxiliary Audio
sistor stages Q3 and Q4 and finally to out-
put transformer T2.
a. The +27.5 volts dc supplied to audio am-
The auxiliary (aux) audio signal produced
plifier 3A1 is applied to the collectors of tran-
in the receiver-transmitter is applied through
sistors Q1, Q2, Q3, and Q4 through voltage-
pin L of connector J2 to a simulated adf load
regulating circuit resistor R6 and Zener diode
consisting of capacitor C3, resistors R4 and
CR1 and voltage-dividing network R2, R3,
R5, and diodes CR2 and CR3. AUX AUDIO
R4, and R5. Capacitors C2 through C5 are
jacks J10 and J11 (J11 is ground) serve as test
decoupling capacitors used with resistors R2
points to check aux audio voltage across the
through R5 to keep the audio stages from in-
teracting with each other. Capacitors C6, C7,
C8 are coupling and dc blocking capacitors.
1-29. SENSITIVITY Control and Squelch
Resistors R8, R10, and R12 are collector load
b. Resistors R7 and R14, R9 and R15, R11
and R17, and R13 and R18 are voltage dividers
SENSITIVITY potentiometer R4 is con-
to provide base emitter forward bias for tran-
nected to the sensitivity control in the receiver-
sistors Q1, Q2, Q3, and Q4, respectively. Re-
transmitter through pins B of plug P1 and
sistors, R20, R22, R23, and R25 and capacitor
connector J6 and pin Z of connector J6. The
Cl2 form an automatic volume control (avc)
sensitivity of the receiver-transmitter can be
network for the emitters of transistors Q1
varied by rotating SENSITIVITY potentiome-
and Q3. When the ac signal output increases
ter R4. When SENSITIVITY potentiometer
at the output of Q4, the net forward bias of
R4 is rotated to its extreme clockwise posi-
Q1 and Q3 is reduced, thereby automatically
tion, SQ. DIS switch S5 is closed and applies
decreasing the gain.  Capacitor C9 suppresses
a ground to squelch relay K1 in the receiver-
noise that may be present at the base of Q4.
transmitter through pins H of plug P1 and
Resistors R21, R16, R24, and R19 are emitter-
connector J6 and pin a of connector J2. This
swamping resistors, used to temperature stabi-
disables the squelch action of the receiver-
lize the emitter current. Capacitors C13, C10,
C14, and C11 are for the purpose of bypassing
the signal around the emitter-swamping
1-30. Frequency Selection Action
c. Output transformer T2 applies the audio
The control head portion of the test unit
signal, to MIC SELECT switch S2. When this
switch is set to HDSET, the audio signal from
contains three FREQ SELECT switches: S1,
S2, and S3. These switches perform identically
audio amplifier module 1A3 is connected by
MIC SELECT switch S2 to the modulator
as their counterparts in the radio set control.
and audio module A4 in the receiver-trans-
The switches apply a ground potential to the
mitter through pins E and V of connector
relays of the mechanical tuner in the receiver-
transmitter through pins K, W, V, U, Z, X,
J2. When MIC SELECT switch S2 is in the
DUM MIC position, the audio amplifier mod-
M, a, d, T, c, b, g, f, j, Y, h and s of plug,
ule is disconnected from, and DUM MIC
P1 and connector J6, and pins X, P, V, U,
jacks J16 and 517 are connected to, pins E and
T, c, b, L, M, K, N, J, Y, F, Z, H, a, and
S of connector J1.  F R E Q S E L E C T s w i t c h S 1
V on connector J2. An external audio signal,
changes the receiver-transmitter frequency in
such as that produced in an audio oscillator,

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