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|  THEORY
Section I. BLOCK DIAGRAM ANALYSIS
mixer stage is coupled through the 11.5-
mc crystal filter, and amplified by the
receiver first, second, third, fourth and
Receiver-Transmitter, Radio RT-505/
fifth if. amplifiers. The receiver fifth if.
PRC-25 is tunable in 50-kilocycle (kc)
amplifier output is applied to the receiver
increments over a frequency range of
discriminator driver. The receiver dis-
30.00 to 75.95 megacycles (me) to provide
criminator driver output is applied to the
920 channels. The frequency range is
receiver discriminator. The five receiver
covered in two bands: the low band is
if. stages (A21) have a power gain of ap-
30.00 to 52.95 mc; the high band is 53.00
proximately 85 decibels (db). The driver
to 75.95 mc. The front panel tuning con-
output is fully limited. The limited if. sig-
trols and the BAND switch are used for
nal is applied to the receiver discrimi-
tuning the RT-505/PRC-25 to the desired
nator, which demodulates the if. signal.
frequency.
The audio output of the receiver discrimi-
a. The incoming radiofrequency (rf) sig-
nator is applied to the receiver first audio
nals are coupled from the antenna through
amplifier.
the antenna loading network to the power
amplifier tank. The antenna loading net-
output is applied to both the VOLUME
work matches the antenna circuit imped-
control and to the squelch input ampli-
ance to the long or the short antenna.
fier. The VOLUME control controls the
The power amplifier tank couples the rf
audio signal level to the receiver audio
signal to the receiver first rf amplifier.
output amplifier. The amplified signal
The power amplifier tank tunes both the
from the receiver audio output amplifier
receiver and transmitter signals. Relay
is fed to the handset. The two receiver
K1 protects the receiver first rf amplifier
audio amplifiers also amplify sidetone
from the high power during transmission
signals from the speech amplifier limiter
by grounding the input to the receiver
of the transmitter section during trans-
first rf amplifier. The amplified signal
mission.
from the receiver first rf amplifier is
applied to the receiver second rf amplifier,
e. During squelch operation, when no
where it is amplified and applied to the
signal is being received, the audio out-
receiver mixer.
put circuit is shorted to ground through
b. The output from the variable freq-
contacts of squelch relay K3. If an rf
uency oscillator (vfo), ranging in frequency
carrier having a s qu e 1 c h tone is re-
from 41.50 to 64.45 mc in 50-kc incre-
ceived, K3 is energized by an output from
ments, is applied through the vfo buffer
the relay driver, the short circuit is re-
to the receiver mixer. The receiver mixer
moved, and audio signals are fed to the H-
heterodynes the two signals to produce the
138/U handset. Relay K3 will be energized
11.5-mc intermediate frequency (if.). The
only if the output from the receiver first
vfo frequency is stabilized by the output
audio amplifier includes a 150 cycle-per-
of the frequency synthesizer system (para
second (cps) tone signal. During trans-
mission, the transmitter tone generator
tioned by the mc and kc tuning controls
provides a 150-cps signal to the squelch
through a gear train.
input amplifier to prevent shorting the
c. The 11.5-mc if. output of the receiver
sidetone.
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