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CHAPTER 1. BLOCK DIAGRAM THEORY
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TM-11-5820-222-35 Radio Sets AN/VRC-24 and AN/TRC-68 Manual
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Figure 1. Receiving signal path, simplified block diagram.


range without disturbing the preset chan-
the frequency range of 225.0 to 399.9 mc.
nels.
The input signals are heterodyned with the
200- to 370-mc signal supplied by the
b. Frequency Selection.
ultrahigh frequency (uhf) injection subunit
(1) The frequency selector subunit in-
to produce a 20.0 to 29.9-mc first inter-
cludes four automatic positioners:
mediate frequency (if.) signal. The 20.0 to
a 20-position channel selector; an
29.9-mc if. signal is amplified and heter-
18-position, 10-mc selector; a 10-
odyned with a 17.0 to 26.0-mc signal in the
position, 1.0-mc selector; and a
first if. amplifier subunit to produce a 3.0
10-position, 0.1-mc selector. The
to 3.9-mc if. signal. The 3.0- 3.9-mc if.
20-position, channel selector auto-
signal is heterodyned again to produce a
matic positioner, controlled by the
500-kilocycle (kc) if. signal. The 500-kc
setting of the CHAN SEL selector
signal is amplified by the 500-kc if. am-
switch on the front panel to the
plifiers and demodulated. The audio sig-
CHANNEL selector switch on the
nals are amplified in the 500-kc if. ampli-
radio set control, positions a chan-
fier and audio amplifier and modulator
nel selector memory drum to
subunits and applied to the local, remote,
select the desired channel.
and fixed level outputs or to the broadband
equipment. Decoded signals from the
(2) The 10.0-, 1.0-, and 0.1-mc auto-
broadband equipment are returned to the
matic positioners drive seeking
audio amplifier and modulator for ampli-
switches which select the fre-
fication before being applied to local, re-
quencies preset on the memory
mote, or fixed level audio outputs.
drum or on the MANUAL FRE-
QUENCY controls. The 10.0 -,1.0-,
5. Simplified Transmitting Signal Path
and 0.1-mc automatic positioners
also drive five tuning shafts
through a series of gear trains.
The rt units transmit am. radiofrequency
(rf) signals in the frequency range of 225.0
(3) The frequency selector shafts are
to 399.9 mc. During transmission, the first
coupled to the tuning shafts within
and the 3.0- to 3.9-mc if. amplifier sub-
the rf and power amplifier, the uhf
units function as signal generators. The
injection subunit, and the first and
3.0- to 3.9-mc signal is heterodyned with
3.0- to 3.9-mc if. amplifier sub-
a 17.0- to 26.0-mc signal from the first
units.
if. amplifier subunit to produce a 20.0- to
(4) When each seeking switch reaches
29.9-mc signal. This signal is heterodyned
its proper position, the corre-
with a 200- to 370-mc signal from the uhf
sponding  automatic  positioner
injection subunit to produce an rf carrier
stops. When all automatic posi-
that can be varied from 225.0 to 399.9 mc.
tioners have stopped, the rt unit is
The rf carrier signal is amplitude modu-
ready for operation on the selected
lated by audiofrequencies from the audio
channel frequency.
amplifier and modulator subunit and cou-
pled to the uhf antenna.
7. Power Supplies
6. Frequency Selector System
a. Radio Set AN/VRC-24 Power Supply.
a. General. The frequency range of the
The internal power supply of Radio Set AN/
rt units is covered by 1,750 crystal-con-
VRC-24 consists of Dynamotor DY-151/U
trolled channel frequencies in 0.1-mc
and appropriate filters which provide the
steps. Nineteen channels can be preset for
+125-, +300-, and -30-volt direct current
automatic selection from the front panel
(dc) operating voltages. Dynamotor DY-
or from the radio set control. A manual
151/U operates on an input voltage of 26.4-
system permits selection of any channel
volt dc. Relay and filament voltages are
frequency within the 225.0- to 399.9-mc
supplied directly from the input voltage.


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