CHAPTER IV
THEORY OF OPERATION
4-1. GENERAL.
The
TR-4C
is a
300
Watt HF single sideband trans-
ceiver which covers the 80 through 10 meter ama-
teur bands.
AM
and CW modes are also included.
The
TR-4C
requires either an R. L. Drake AC-4,
120 V AC
power supply, or an R. L. Drake DC-4,
12
V DC
power supply. The TR-4C features a
high-stability linear permeability tuned VFO and
two 8 pole crystal lattice filters for sideband selec-
tion. Some of the circuits are common to both the
transmit and receive functions. Refer to the block
diagram figure 4-l and the schematic diagram
figure 5-5 as required to supplement the following
discussion.
4-2.
RECEIVER CIRCUITRY.
A signal entering the antenna terminal passes
through the antenna switching contacts of the relay
and is applied to the grid of the RF amplifier V7
through the selectivity of the L/C network formed
by T9, T 10 and a section of the RF TUNE capacitor
C37. After being amplified, it is passed through an
additional L/C network consisting of T7, T8 and
the remaining section of C37, to the grid of the
mixer V3B. At this point it is combined with a sig-
nal from the premixer system of the required fre-
quency to yield a 9.0 MHz IF. The premixer system
consists of a 4.9-5.5 MHz solid state permeability
tuned VFO, a buffer Q2, a switchable overtone
crystal oscillator Vl A, the premixer pentode Vl B
and a cathode follower V3A.
The VFO signal output is applied to the grid of the
premixer pentode through the buffer Q2 and its
associated circuitry. For 80 and 20 meter operation,
the VFO signal bypasses the premixer and is con-
nected through the cathode follower to the mixer.
On 40, 15 and 10 meters a signal from the crystal
oscillator heterodynes with the VFO in the pre-
mixer,
Vl B, to produce the desired injection
frequency. On 40 meters, for example, a 21.5 MHz
overtone crystal and the appropriate coil Ll are
switched into the crystal oscillator circuit. The
output from the oscillator is coupled into the pre-
mixer pentode where it heterodynes with the 4.9-
5.5 MHz VFO to produce an output frequency of
16.0-16.6 MHz. This output is coupled through
the 16.0-16.6 MHz bandpass coupler, T3, and to
the cathode follower, V3A. On 15 meters, a 35.5
MHz crystal is used with a 30.0-30.6 MHz coupler,
T2, and on the three
10
meter ranges, 42.5, 43.0
and 43.6 MHz crystals are used with a 37.0-38.7
MHz coupler, Tl .
The 9.0 MHz output of the mixer, V3B, passes
through the impedance matching transformer T6
into the upper or lower sideband crystal filter. The
setting of the SIDEBAND knob determines which
crystal filter is used. From the crystal filter the
signal passes through the impedance matching
transformer, T13, and is amplified by the 9 MHz
receiver IF amplifier system, V 11 and V 12 and the
IF transformers T1 1 and T12. The output of T12 is
applied to the AGC amplifier,V13A, to the product
detector, V16, and to the diode detector, V2.
The AGC amplifier V1 3A is biased beyond cutoff
to provide an AGC delay. When sufficient RF volt-
age from T12 is applied to its grid, plate current
flows
during part of the cycle. This causes ampli-
fied negative voltage to appear across its plate load
resistor R63, thus charging Cl 15. This negative
control voltage is applied to the grids of V7,
Vl
1
and V12. Cl 15 discharges through R63 with a time
constant of approximately one second. Rotating
the RF Gain control counterclockwise applies in-
creasingly more negative bias to the AGC control-
led grids, thus limiting their maximum gain.
The product detector tube, V16, consists of a 9
MHz crystal oscillator formed by the cathode, grid
1 and grid 2. A product detector is formed by the
cathode, grid 3, and the plate. The IF signal is
applied to grid 3 where it heterodynes with the
BFO voltage in the tube. The resulting audio signal
is of sufficient amplitude to drive the audio pre-
amplifier transistor, Q5, which drives the audio
output tube, V1 7.
4 - l
