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TM 11-5820-509-35
the grid circuits of rf amplifiers V1 and V2 in signal data
dard signal from the frequency control circuits.  The
translator A3.
resulting output from the balanced modulator is a
double-sideband,  suppressed-carrier  signal  that  is
2-6. Transmit Signal Path (fig. 7-3)
amplified by alc amplifier Q1 and if amplifier Q2 before
being applied to bandpass filter FL1.  Only the lower
sideband remains after this signal is applied to FL1 and
a. In the transmit mode, the audio signal is
this single sideband is further amplified by Q3 through
obtained from the microphone of Handset H 33G/PT or
Q5 before being applied to signal data translator A3. If.
from the transmit output circuits of an external fsk unit.
amplifiers Q3 through A5 and bandpass filter FL1 are
This signal is connected to the transmit audio input of
shared with the receiver circuits of the receiver-
audio frequency amplifier A1 where it is amplified by
transmitter, but during the transmit period the agc bias
microphone amplifier Q1 and Q3, clipped by limiters
otherwise applied to Q2 and Q3 is removed so that the
CR5-CR6, further amplified by output amplifier Q4, and
if.  gain of this circuit will not vary as a function of
applied to the audio input of the balanced modulator of
modulation.
amplifier-modulator A2.  Part, of the amplified audio
output from Q4 is also applied to sidetone gate CR10.
d. Transmit relay A2K1 fin amplifier-modulator A2)
This gate is enabled during tansmit periods by the bias
is energized for transmit operation by closure of vox
from sidetone rectifier CR101 in the power amplifier
relay A1K1. The output of if. amplifier Q5 is routed
compartment so that the sidetone audio from Q4 is
through the closed contacts of A2K1 to the transmit
routed through driver Q10, audio output amplifier Q1
mixer (CR3-CR8) in signal data translator A3. The 2.5-
1W-QI2X and VOLUME control R11-R12 to the
to 12.499-MHz hf oscillator signal from the frequency
connected audio output device. The sidetone audio is
control circuits is applied to the transmit mixer through
then available for monitoring purposes.
follower Q13.  The difference-frequency obtained by
heterodyning this signal with the 500-kHz if. output of
b. Another part of the output signal from Q4 is
Q5 is a 2.0to 11.999-MHz single-sideband rf signal that
applied to vox detector CR7-CR8.  When this signal
is buffered by follower Q14 before being applied to
level exceeds a minimum threshold value, vox relay K 1
amplifier QI5 and subsequently to rf amplifiers V1 and
transfers circuit functions in the RT-671/PRC-47 from
V2. The amplified hf output of V2 is further amplified by
the receive mode to the transmit mode. It the audio
driver V3 and power amplifier V101. The output circuit
signal drops below this threshold level, or disappears
of the power amplifier is connected to the antenna
completely for a predetermined period of time, these
system when t/r relay K101 is closed.  This stage is
circuits  will  automatically  return  to  the  receive
tuned to the operating frequency shown on the
configuration.
(Where  the  vox  mode  is  not
KILOCYCLES indicator and is loaded to the antenna
implemented, vox relay K1 cannot cause operation of
system by the adjustment of POWER AMPLIFIER
the ptt relay (A5K1) in power supply module A5. This
TUNE and POWER AMPLIFIER LOAD controls on the
relay operates when the ptt switch on the operator's
front of RT-671/PRC-47. Optimum adjustment of these
handset is pressed and energizes the high-voltage
controls is indicated by maximum deflection (maximum
power  supply  circuits.)  If  continuous  wave  (cw)
power output) of XMTR OUTPUT meter M 101 on the
telegraphy is desired, the circuit functions described
front of the receiver-transmitter.
above apply except that Telegraph Key J-45 turns on,
and shuts off, the audio tone generated by oscillator Q5.
This tone replaces the voice audio at the input of
2-7. Frequency Control Circuits (fig. 7-4)
microphone amplifier Q3, and is amplified by the
remaining circuits of audio frequency amplifier A 1. In
a. The temperature-compensated crystal-oscillator
the cw mode, the CW-FSK/VOICE switch on the front
of radio frequency oscillator A6 is an extremely stable
panel of RT-671 /PRC-47 is placed in the CW-FSK
standard that provides a 3-MHz output signal for the
position so that vex relay A1K1 and ptt relay A5K1 are
frequency divider (locked oscillator) circuits of this
both energized The CW-FSK/VOICE switch must be
module.  The crystal is contained in a temperature-
returned to the VOICE position when it is desired to
controlled oven to minimize frequency drift caused by
receive incoming telegraph or teletypewriter signals (i.e.,
changes in ambient temperature, and the circuit is
break-in operation is not possible).
equipped with an adjustment that permits compensation
c. That part of the audio signal from Q4 that is
for long-term crystal aging effects. Transistor Q1 is the
applied  to  the  balanced  modulator  of  amplifier-
oscillator, and transistors Q2 and Q3
modulator A2 is mixed with the 500-kHz stan-
2-9


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