Page
99
Heafhkit
VFO
FET (field effect transistor)
Q102
and its associated
components make
up a
Hartley oscillator.
Part of coil
LI
18, Main Tuning capacitor Cl, and fixed
tempera-
ture-compensating
capacitors
C182
through
C188
are used in the frequency-determining
circuit of the
VFO (variable
frequency oscillator). The remaining
part of LI 18 is used
to
couple back
part of the gener-
ated signal to the
drain (D) of
Q102
to help sustain
oscillation. The VFO generates frequencies
between
5.9993
MHz and 5.7493 MHz.
Diode D119 clamps
the positive-going half of the sig-
nal to prevent
Q102
from reaching high peak operat-
ing currents.
This helps to keep the VFO from gener-
ating harmonic signals.
The signal
at the output of
Q102
is coupled through
C194
to the gate
(G)
of source
follower
Q105,
which
acts
as a buffer and impedance-matching device.
Q106
is
a
fixed-tuned
amplifier that is followed
by
a
low-pass
filter, which reduces the
harmonic output
of the VFO.
The VFO signal is then
coupled to gate
1 (Gl)of
1st
mixer
Q107.
RIT
The RIT
circuit is in parallel with Main Tuning
capacitor Cl of the VFO.
This circuit, which is con-
trolled
by a +12 VDC switching voltage
(
+
R in re-
ceive and
+T
in transmit)
through
Q103,
causes
a
small
change in the capacitance of D118
by varying
the voltage
across it. In the receive mode,
RIT control
Rl is
used to
vary this
voltage, which results in
a
tuning range of ±
1 kHz (with respect to the receiver
frequency).
In the transmit mode,
Transmit Return
control R131
ensures that a constant voltage
is ap-
plied across D118, regardless of
the setting of the
RIT
control.
HFO
The
HFO (heterodyne frequency
oscillator) operates
on any
of eight crystal-controlled
frequencies, de-
pending on which band position
is selected by Band
switch
SW1. The HFO signals for
80,
40,
30,
and
20 meters are
produced by
Q108
and its associated
circuitry.
Q109
and
its
associated components
pro-
duce the HFO
signals for
17, 15,
12,
and 10 meters.
The signal from either HFO transistor is coupled
to
the base of HFO emitter follower QUI. The
HFO
signal is then coupled through
C175
to gate 2
(G2)
of
1st
mixer
Q107.
1ST MIXER
Q107
produces the appropriate premix signal
for the
selected band
by
mixing
the
VFO
and
HFO
signals.
The premix signal is used in both the receive and
transmit modes of operation.
PREMIX FILTERS
The unfiltered premix signal is applied
to
one of
the eight diode switched filters and tuned circuits
selected by Band switch SWl.
The
selected filter
suppresses all unwanted signals; only the desired
premix
signal passes
through
C101 to the
base
of
amplfier
Q101.
The filtered and amplified signal is
then coupled through
C103
to 2nd mixer
U401
on
the T/R circuit board. This circuit will be discussed
later.
BFO
This circuit produces an 8.8314 MHz signal in
the
receive mode, while an 8.8307 MHz signal is gener-
ated in the transmit mode. To accomplish this
task,
Q112
is used in a VXO (variable crystal oscillator)
circuit.
In the receive mode, crystal Y109, coil L135, and
capacitors
C206
through
C208
determine the oscil-
lator frequency.
D142 clamps the positive-going half of
the
signal
to
prevent
Q112
from reaching high-peak operating
currents. This helps
to
keep the oscillator from gen-
erating harmonic signals.
C211
couples
the
oscillator signal
from the source
(S)
of
Q112 to
the base of emitter follower
Q113,
which
acts as a
buffer
stage.
Part
of the
signal
from
Q113
is
coupled to product detector U303, which
will be discussed later.
In
the
transmit mode,
a
+
12
volt
switching voltage
(
+ T) is fed
to resistor
R151 to
turn
on D141. This
allows the circuit,
which is
made up of
L134 and
C205,
to
produce a fixed
offset frequency
of
700
Hz
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