Eddystone 990R - The IF Section

The
Calibrator
is
brought
into
operation
by
means
of
S4 which
completes
the +10V
supply
via
R37
to
both
TR8
&
TR9-
Correction
for
scale
errors
is
carried
out
mechanic-
ally
with
an
adjustor
which
provides
limited
lateral
movement
of
the cursor
independ-
ently
of
the
TUNING
CONTROL,
Calibration
checks
can
be
carried
out
in
any
position
of
the
MODE
SWITCH,
greatest
accuracv
being
obtained
either
at
"CW"
where
the
receiver
can
be
tuned
to
zero-beat
with
the
Beat
Oscillator
crystal,
or
at
"Mi"
whore
the
centre-zero
tuning
meter
can
be
used
to
advantage.
Checks
should
be
made
with
the
SELECTIVITf
SWITCH
at
NAltrfO
1
.
THE
IF
SECTION
General
.
The
standard
intermediate
frequency
of
10.7
MHz
is
employed
in
the
IF
Section
which
provides
detection
facilities
for
AM,
F1VI, CW
and
video
reception.
Two
IF
bandwidths
are
available
selected
by a
panel
control.
Associated
circuitry,
including
the
AGC
system,
muting
circuits
and
video
amplifiers
will
be
considered
in
this
Section.
IF
Pre-amplifier
and
Filter
Unit
.
(TR10-TR14)
IF
output
from
the
Mixer
Stage
is
fed
via
PL7
and
a
short
length
of
coaxial
cable
to
the
base
of
the
IF
Pre-amplifier
(TRIO
1
GM2$)0)
which is
one
of
five
transistors
located
in
the
IF
Pre-amplifier/Filter
Unit
at
the
rear
of
the
RF
Tuner
Unit.
The
four
remaining
transistors
(TRII-TRI
4
)
are
operated
in
the
emitter
follower
configura-
tion
and
serve
as
switches
to
select
input
and
output
of
the
two
lx
1
Filters.
Only
three
of
the
five
transistors
in
this
unit
will
be
functional
at
any
time.
The
IF
Pre-amplifier
utilises
the
common
emitter
configuration
with
its
base
tied to
the
RF
Amplifier
manual/auto
gain
control
line
via
FC5.
Signal
output
is
taken
via
C77
and
C83
to
feed
both
the
input
switches
in
parallel
from
a
tap
on
T2.
TR11
(GM290)
and
TR12
(GM378)
switch
the
input
and
output
respectively
of
the
crystal
filter
used
in
the
"NARROW"
selectivity
position.
Overall
bandwidth
is
30
kHz
at
6dB
down.
C78
and
C
79
allow
accurate
matching
to
the
input
and
output
of
the
crystal
filter.
An
L/C
filter
comprising TJ
,
T
4
and
T5 is
used
in
the
other
leg
9
inpub
being
switched
by
TR13
(GM290)
and
output
by
TR14
(GM378).
Bandwidth
on
this
channel
is
200
kHz
(SELECTIVITY
SWITCH
at
"WIDE").
Selection
of
the
appropriate
input
and
output
"switch"
is
by
S5A
and
S5B,
the
former
removing
base
and
emitter
voltage
from
the
input
switch
on
the
inoperative
channel,
the
latter
removing
base
and
emitter
voltage
from
the
output
switch.
The
+10V
supply
is
also
applied
via
S5B
and
R47
to
D2
(0A47
)
i
n
the
"WIDE"
position
to
damp
the
crystal
filter
and
suppress
minor
distortion
of
the
200
kHz
response
which
would
otherwise
occur
due
to
stray
coupling
through
TR12.
C80
completes
the
diode
circuit
for
i.f.
voltages.
Provision
is
made
in
the
form
of
SK10
for
direct
connection
to the
input
of
the
IF
Pre-amp
(TRIO)
when
it
is
desired
to
use
the 10.
7
MHz
IF
Stages
to
amplify
an
externally
derived
signal
at
this
frequency.
An
external
converter
with
IF
0/Pat
10
7
MHz
could
easilv
be
connected
at
this
point
to
extend
the
tuning
coverage
of
the
receiver
proper.
S
2
allows
the
normal
RF
Tuner
Unit
to
be
shut
down
to
prevent
noise
and
signal
breakthrough
from
the
main
channel
.
SK10
can
also
be
used
to
extract
a
low-level
output
at
10.7
MHz
for
connection
to
external
ancillary
equipment
(panoramic
display
etc.).
Bandwidth
is
of
the
order
1
iv z
at
the
higher
frequencies
in
the
tuning
range.
-
7
-