Eddystone 990R - Page 24

2.
Install
the
crystal(s)
in
the
socket(s)
situated
within
the Crystal
Oscillator
Unit
which is
located
centrally
behind
the
front
panel.
Access
is
by
removal
of
the
rear
cover
which
reveals
the
eight
crystal
holders.
Crystal
positions
are
marked
on
the
outside
of
the
cover
plate,
the
numbering
being
the
same as
that
concentric
with
the
panel
control
(l-8).
Make
a
note of
the signal
frequency
obtaining
in each
crystal
position
used
and
record
this
information
on
a small
card
to
be
kept
with
the
receiver.
Ensure
that
the rear
cover
plate
is
replaced
securely after
fitting
the
crystal (s).
3.
Tune
the
pre-set
crystal
oscillator
circuits
by
following
the
procedure
outlined
below
s-
(a)
Connect a
multi-range
testraeter
between the
OSCILLATOR
TEST
POINT
(FC1
s z
near
top
of
Unit
at
right-hand
side
viewed from
front) and
earth
(negative).
Set
to
10V
range.
Reading
should be of
the order
8.5V.
(b)
Select
each
crystal
position
in
turn and adjust
the appropriate
core
or trimmer
(see
below)
for
a
dip in
meter
reading.
This should
be
of
the
order IV
to
1.5V
with
normal
crystal
activity.
Access
to trimming
and
core adjustments
is
by
removal
of
the
bottom cover
from
the Crystal
Oscillator
housing.
Numbering
is
repeated
to
correspond
with
the crystal/panel
marking.
(c)
The
eight
crystal
oscillator
tuning
circuits
are
all identical
and
are arranged
to
cover the
whole
37*7-90*0
MHz band when
full
use
is
made
of both trimmer
and
core.
There
is
thus
no
restriction on
the combination of crystal/signal
freq-
uencies
employed.
When
using
crystals
in
the lower
part
of
the
range,
trimming
should be
by means
of
the core
(L17-L24)
with the
appropriate
trimmer set to maximum
capacity. On
the
higher
frequencies,
set
the core
well out
and tune
with
the
trimmer
(C47-
C54).
Slight
jockeying
between
trimmer
and
core is
quite
permissible
and inter-
mediate
settings
of
the
trimmer
should
be
used
at middle
frequencies
in
the
range
.
NB
TRIMlERS
ARE
AT
MAXIMUM
CAPACITY WHEN
THE VISIBLE
SILVERED
PORTION ON
TOP
OF
THE
TRIMMER
IS
TOWARDS
THE
FRONT
PANEL .
(d)
Check
that
crystal
fires
readily
by
subjecting
the
receiver
to
several on/off
switching
cycles
whilst
observing
the
multi-range
meter.
4.
Operationally,
the
receiver
is
tuned
in
the
normal
manner
but
with the
SYSTEM SWITCH
set
to the
appropriate
M
CRYSTAL"
position
(l-8).
The
RANGE
SWITCH and
TUNING must
be
set to
align
the
signal
frequency
and
oscillator
circuits
correctly,
the latter
being
arranged
to
tune
the
output of the
frequency
multiplier
stage. It
may be
noted
that
crystal
frequencies
can
be pulled
slightly
by
careful
adjustment
of
the
appropriate
crystal
oscillator
circuits.
Advantage can
be
taken
of
this
when using
a
receiver
with
non-standard
filter
in the "NARROW"
position of
selectivity.
Adjust
under signal
conditions
using
built-in
meter at
"FM"
for
best
accuracy.
Operation
with
External
Oscillator
.
This
mode
of
operation
is
available
for
those occasions
when
high-stability
working
is
required
but
with
greater
flexibility
in
frequency
selection
than
obtains
when using
the
internal
crystal
oscillator.
Any synthesiser
with
low-impedance
output
capable
of
supplying
some
500mV
of
drive
can
be used. A
low-impedance
isolated input with
BNC
socket
is
provided
at
the
rear of
the
receiver
("EXT
OSC
INPUT").
The
drive
frequency
must
at
all
times
lie
10.7
MHz
higher
than
the required
signal
frequency.
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