audio amplifier V16B, which produces the power to
first mixer V13B. The output frequency of this oscil-
operate a speaker, headphones, or a phone patch.
lator is always 3.155 MHz higher than the lower edge of
the desired band. This high-frequency injection signal
is the crystal fundamental frequency for all desired
The transceiver contains the tone oscillator, the beat-
signals below 12 MHz. For operating frequencies above
frequency oscillator, the variable-frequency oscillator,
12 MHz, the crystal frequency is doubled in the plate
the high-frequency oscillator, and the crystal calibrator.
circuit of the oscillator. Instructions for calculating
a. Tone Oscillator. The tone oscillator operates
crystal frequencies for the desired bands are given in
when the EMISSION switch is at LOCK, TUNE, or
It is a phase shift oscillator operating at
e. Crystal Calibrator. The output of crystal
approximately 1,750 Hz. Its output is fed to the
calibrator V12B is coupled to the antenna and applied to
transmitter audio circuits for cw operation. Some of the
the receive circuits. The output is used when calibrating
output from the tone oscillator is applied to the receiver
the tuning dial.
audio circuits for sidetone monitoring in cw operation.
3-18. Vox and Antivox Circuits
Because of the 1,750 Hz tone applied to the balanced
Audio output voltage from second microphone am-
modulator during cw operation, the actual transmitted
plifier V1B is coupled through the VOX GAIN control
signal will be 1,750 Hz above the Collins KWM-212A
to vox amplifier V14B. The amplified voltage is applied
to vox rectifier V14A. The positive d.c. voltage output
b. Beat-Frequency Oscillator. The bfo is crystal-
of the vox rectifier is applied to vex retry switch V4B,
controlled at either 453.650 or 456.350 kHz, depending
causing it to conduct and actuate vox relay K2.
on whether Y16 or Y17 is selected by the EMISSION
Contacts of K2 switch the antenna connection to the
switch. The unused crystal is shorted out by the switch.
transmit circuits, the other relay coils, and bias voltage.
These crystal frequencies are matched to the passband of
Other relays switch the metering circuits from receive to
mechanical filter FL1 so that the carrier frequency is
transmit, the low plate voltages from receive to transmit
placed approximately 20 db down on the skirts of the
tubes, and the AVC and ALC leads. The antivox circuit
filter response. This 20-db carrier attenuation is in
provides a threshold voltage to prevent loudspeaker
addition to the 30-db suppression provided by the
output (picked up by the microphone circuits) from
tripping the Collins KWM-2/2A into transmit functions.
c. Variable-Frequency Oscillator. The vfo uses
Some of the receiver output audio voltage is applied
fixed capacitors and variable inductance to tune the
through the ANTIVOX GAIN control to antivox
range of 2.5 to 2.7 MHz. Its output is applied direct to
rectifier V14A. Negative d.c. output voltage from the
second receive mixer V17B, and is applied to second
antivox rectifier, applied to vox relay switch V4B,
transmit mixer V6 through vfo cathode follower V2A.
provides the necessary antivox threshold. The
d High-Frequency Crystal Oscillator. High-
ANTIVOX GAIN control adjusts the value of the
frequency crystal, oscillator V 13A is crystal-controlled
antivox threshold so that the loudspeaker output will not
by 1 of 14 crystals selected by bandswitch S2. The
produce enough positive d.c. output from the vox
output from the high-frequency crystal oscillator is
rectifier to exceed the negative d.c. output from the
applied to the transmitter second mixer and to crystal
antivox rectifier and cause V4B to actuate K2.
oscillator cathode follower V12A. The cathode follower
However, speech energy into the microphone will cause
provides isolation and impedance match between the
the positive vex voltage to overcome the negative
crystal oscillator and receiver
antivox voltage and produce the desired action of K2.
Section V. OPERATOR'S MAINTENANCE INSTRUCTIONS
Refer to TM 750-244-2 for proper
24 are applicable for the operator's
procedures for destruction of this
equipment to prevent enemy use.
a. Operator/crew preventive maintenance is the
3-19.Scope of Operator's
systematic care, servicing
and inspection of
equipment to prevent the occurrence of trouble to
reduce downtime and to maintain equipment in
The maintenance duties assigned to the operator of
serviceable condition. To be sure that your
the RT-718/FRC-93 follows: