DRAKE TR-4CW - Chapter IV: Theory of Operation; General Theory and Receiver Circuitry

DRAKE
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 4-1 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, T10 anda 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 VIA, the premixer pentode V1B
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, V1B, to produce the desired injection
frequency. On 40 meters, for example, a 21.5 MHz
overtone crystal and the appropriate coil L1 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, T1.
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, V11 and V12 and the
IF transformers Tl] 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
V13A
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
C115.
This
negative
control
voltage
is
applied
to
the
grids
of
V7,
V11
and
V12.
C115
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 a9
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, V17.