Eddystone 990R - Page 9

Main
IF
Board
.
Output
from the
IF
Pre-amp/Filter Unit
is taken
via
a
short
coaxial
cable
to
the
large printed
board
which carries
the
greater
part
of the IF
circuitry*
Separate
branches
are provided for
AM and
FM,
the AM
branch
also feeding
a
product
detector
for
CW
reception. This
particular
circuit
is in
a screened
housing,
as
also
are
the
two
emitter
followers
TRIG and
TR13A.
AM
Stages
and
IF Output
.
(TR15-TR18)
Three
stages of
amplification are
provided in the
form
of TR15
(GM'378)
,
TR16
(GM378)
and TR17
(2S512), the
latter
being
an n-p-n
silicon
epitaxial
planar
transistor.
TR15
and TR16 operate
with manual
or automatic
gain
control
dependent
on
the
setting
of
the
MANUAL/AGC
SWITCH S6.
The
control line
is
separate
from
that
which
feeds
the
RF and IP
Pre-amp
Stages
5
it is switched
by
S6B.
All three
IP
Stages
operate
in
grounded-emitter
mode,
inter-stage
coupling
in
each
case
being single-tuned circuits
(T6
& T7)
tapped
to
feed the
base
of
the
following
stage.
A tuned-secondary
transformer
(T8)
is
used to
feed
the
AM/Video
Detector
D3
(OA47)»
This stage
has
a
wide frequency
response
and its
output
is
tapped
down
the
diode
load
to increase the
dynamic
range
of
the
following
stage.
R79/C119
form
a
normal
IP
filter
feeding the
"AM" position
of
the
MODE
SWITCH
wafer
S7B.
A
tap
on
the secondary of
T8
is used
to drive the
emitter
follower
TR18
(GM290)
which provides
a
low-impedance
10.
7
MHz output
at SK11
(BNC
socket).
Output
level
is
of
the
order
25mY
across
750
for an aerial input
of
lOfiV. Bandwidth
is
determined
by
the
setting of
the panel SELECTIVITY
SWITCH being ei
ther
30
kHz
or
200
kHz
overall.
The
IP output
is
available continuously
regardless
of the MODE
SWITCH
setting.
CW
Detector
Unit
.
(TR23-TR25)
IP
signal is
taken from the
collector
of the
final
IP
Amplifier
(TRI7)
to
feed
the
CW Detector via
R76/C159*
The
Detector
proper comprises
TR24
and TR25
(2
x
GM378),
signal
at
IP
being
applied
to
the
base of
TR25 and BPO
voltage
to the
base
of
TR24*
The
Beat
Oscillator
(TH23
%
GM378)
is
crystal
controlled
at
10.7
MHz and
delivers
output
to the
base
of
TR24
from
its
emitter
via C157*
The collectors
of
the
two detector
transistors
are
linked
together
and
work
into
a
common
load
(R12l)
from
which output
is
taken via the
filter
CH2l/Cl60
and
the
output
capacitor
Cl6l to
the
"CW"
position
of
S7B.
The
+10V
supply
is
applied
to
all
three
transistors via R124
and
CH22
by
S7A
when
set to
"CW".
Extensive
filtering
and
total
screening of
the complete
unit
prevent
harmonics
of
the
10.
7
MHz
oscillator
appearing
in
the tuning
range
of
the
receiver.
FT! Channel
.
(TR18A,
TR19
& TR20)
In
addition
to
feeding
the
two final
amplifiers
on the
"Ah"
path,
TR15
also
feeds
TR18A
which
provides isolation
for
the
FLi
Stages.
TR18A
is wired
as an
emitter
follower
and
coupled
to
the Fj'I
Limiter
Driver
Stage
(TR19
s GLI290)
by
C123A
and
C125.
The PM
Limiter
(TR20
s
GM290)
is fed from
the
previous
stage
through
a
double-tuned
transformer
(T9)
with
tapped secondary
to match
the
base
impedance.
Both
TR19
and
TR20
employ
common-emitter
configuration.
The
Discriminator uses a
pair of OA79
diodes
(D4/D5)
in
a
conventional
Foster-Seeley
circuit,
component
values
etc. being
chosen to
preserve
the
overall
response.
Output
is
taken
via CHI
8 to
the
"PM"
position
of the
MODE SWITCH
wafer
S7B.
A further
output
is provided via
R126
to drive
the
Noise
Amplifiers in the
carrier-controlled
muting
circuit.
The
+10V
supply
is
permanently
connected
to
the- PM
Stages
.
-
8
-