Heathkit RA-1 - Circuit Description; RF Amplifier, Mixer, and Oscillator Stages; Intermediate Frequency (IF) Amplifier Stages; Detector, Automatic Volume Control (AVC), and Noise Limiter

.
CIRCUIT DESCRIPTION
To
assist
you
in
underatancing
the
following
circuit
description,
we
suggest
that
you
refer
frequently
to
the
Block
Diagram
(Page
33}
and
the
Circuit
Diagram.
As
the
BAND
awitch
is
quite
complex,
the
circuit
will
be
discussed
with
the
switch
in
the
160
metre
position.
RF AMPLIFIER - MIXER OSCILLATOR
Assuming that the BAND switch is in the 160 metre position, the
through
the
BAND
switch,
to
the
grid
of
the
RF
amplifier
valve
V
¥1
amplifies
the
RF
signal
which
is
induced
through
coil
L7
to
the
gvid
in
the
heptode
section
of
valve
V2.
Valve
V2
is
a
combination
mixer
and
oscillator.
This
valve
heterodynes
or
mixes
the
incoming
signal
frequency
with
the
oscillator
frequency
to
obtain
a
difference
frequency
of
1621
Ke/s.
The
position
of
the
BAND
switch
determines
which
coils
and
capacitors
are
usetl
in
conjunction
with
the
triode
sec-
tion
of
the
valve
V2B
in
the
oscillator
circuit,
These
coils
and
capacitors,
including
the
main
tuning
capacitor,
are
used
to
set
the
oscillator
frequency
1621
Ke/s
higher
than
the
desired
radio
station
frequency,
to
which
the
RF
and
mixer
circuits
are
tuned,
The
difference
frequency
of
1621
Ke/s
ia
called
the
intermediate
frequency
or
IF
signal.
For example:-
Frequency of oscillater.... fo Wale bare eae ++ 3485 Ke/s
(Subtract)
Frequency
of
signal
at
antenna
......-.-0.ees000.
1864
Keolg
Frequency
of
difference
signal
.....
ieiewareeniy
se.
LOZ)
Kels
IF AMPLIFIER
From
the
anode
of
V2A,
the
signal
is
coupled
through
IF
transformers
IFA
via
a
half
lattice
crystal
filter
in
the
secondary
winding,
to
the
grid
of
the
first
IF
amplifier
¥3,
The
half
lattice
crystal
filter
is
included
to
provide
a
narrow
bandpass
for
suppression
of
unwanted
adjacent
signals.
This
provides
the
Receiver
with
exceptionally
good
selectivity.
The
amplified
IF
signal
from
the
anode
of
V3
is
coupled
through
the
second
IF
transformer
JFB
to
the
grid
of
the
second
IF
amplifier
valve
V4A.
Valve
V4A
again
amplifies
the
IF
signal,
At
this
point
in
the
circuit,
the
Beat
Fre-
quency
Oscillator
(BFO)
signal
from
V4B
may
be
beteredyned
(beat)
with
the
IF
signal
by
turning
the
BFO
switch
ON,
The
introduction
of
the
BFO
signal
{1621
Ke/s
t
i
Ke/s)
produces
an
audible
signal
for
Continuous
Wave
(CW)
Sideband
(SSB)
reception,
The
output
from
V4A
is
coupled
through
IF
transformer
IFC
to
diode
D1,
DETECTOR,
AUTOMATIC
VOLUME
CONTROL,
NOISE
LIMITER
STAGES
Diode
D1]
is
the
detector
stage.
This
diode
detects
{or
rectifies}
the
IF
signal,
and
after
the
filtering
out
of
the
RF
component,
the
audio
signal
passes
through
the
noise
limiter
stage
V5,
which
when
in
use,
removes
peak
pulses
caused
by
ignition
etc.
The
diode
D2
igs
the
AVC
diode
which
causes
some
of
the
voltage
from
large
signals:
to
be
fed
back
to
previous
stages,
in
the
farm
of
negative
bias,
to
reduce
sensitivity.
AUDIO STAGES
The
audio
signal
controlled
through
a
manual
gain
control,
is
applied
to
the
grid
of
the
triode
section
of
valve
V6
andis
amplified
before
being
passed
from
the
anode
of
valve
V6B
to
the
grid
of
the
pentode
section
V6A.
This
sec-
tion
further
amplifies
the
audio
signal
and
feeds
it
through
the
transformer
OT
to
the
speaker
terminais.
POWER SUPPLY
The
power
supply
uses
valve
V7
as
a
full
wave
rectifier
with
power
transformer
MT1,
Filtering
of
the
DC
voltage
is
provided
by
resistors
R31,
R32,
R33
and
capacitors
C55,
C56,
C57,
C5R.
Voltage
regulation
for
the
cecillaters
¥2B
and
V4B
is
provided
by
V8,
LOU
ESTE
BR
cme
y
DA
Ph
pt
Py
—_—_—
ees
ENGLAND