Eddystone 990R - Page 29

Set
all
controls
as
previously
with
the
exception
of
the
MODE
SWITCH
which
should
be
placed
at "PM".
Set
the top
core
of
T10
(Discriminator)
flush
with
the
top
of
the
can.
Peak
lower
core in
T10
and
both
cores in
T9
for
maximum
reading
on
output
meter
(all
cores tuned
on
"outer" peak).
Re-adjust
attenuator for
precise
output
reading
of
lOOmW
and
note
sensitivity.
A
figure
of
the
order
lOfiV
should
be
obtained.
Stage
by
stage
sensitivities
can
be
measured
if
the
overall
FM
sensitivity
appears
to
be
low.
Typical
figures
are
as
follows *
—
Generator
applied
at base of
TR18A
.
. .
.
50
|j.V
for
lOOmW
output.
Generator
applied
at
base
of
TR19
.
.
.
.
80|iV
for
lOOmW
output.
Generator
applied
at
base
of
TR20
.
.
.
.
lmV
for
lOOmW
output.
All
readings
taken
with
generator
applied
directly
at
base
with
series
capacitor
of
O.OOlpF
in
live
lead.
Remove
short
from
D5
and adjust
top core
of
T10
to
bring
the
needle
on
the
built-in
tuning
meter
to
centre-scale
coincident
with
the red
indicating
line.
Off-
tune
gener-
ator by
equal amounts
each way
from
centre-frequency
and
ensure
that
meter
deflections
are
symme tri cal
.
Re-alignment
of
the
IP
Filters
.
Either
filter
can
be
aligned
quite
satisfactorily
using
the
conventional
signal
generator
and
output
meter
technique.
Crystal
filter
alignment
however,
may
prove
somewhat
tedious by
this
method
and
use of
a v/obbulator
is
preferred
if a
suitable
instrument
is
available.
Control
settings
for
alignment
of the
two
filters
should
be
as
for
alignment
of the
main
IP
Stages,
accurate
centering
of the
generator
within
the
passband
being
achieved
as
before
by
comparison with the
crystal-controlled
beat
oscillator
on
10.
7
-MHz.
It
is
quite immaterial
in which
order
the
two
filters
are
aligned.
Wide
IF
Filter .
Signal
input
at
10.
7
MHz
is
applied via
the
IF
IUPUT
socket,
the
output
meter
being
connected
to the
30
output
as
before.
Access
for
trimming
adjustment
is by
removal
of
the
small
cover
at the
rear
end
of
the
right-hand
side-plate.
The
cores
nearest
to
the
printed
board
are
not
accessible
from
this
side but
can
be
adjusted
through
suitably
positioned
apertures in
the
inner
wall
of
the
unit
(above
the
Filtercons).
Put
the SELECTIVITY
SWITCH
at
"WIDE"
and
adjust
the
six
cores
in T3,
T4
and T5
for
maximum
reading on the
output
meter
(all
cores trimmed
on
"outer"
peak
at
10.
7
MHz).
It
is
convenient
at
this time
to
adjust
also the
single-tuned
transformer
T2
which
feeds
the
two
Filter
Switches TR11
and
TR13.
T2
is
housed
in
the
small
screening
can
adjacent to
T3/T5
and
is
accessible
through
the
side-plate.
MB
The first
10.
7
MHz
circuit (Tl)
is
adjusted later
when
carrying
out RF
alignment.
On
completion
of these
adjustments,
de-tune
the
generator
to
either
side
of
the
centre-frequency,
checking
the
response for overall
symmetry
down
to
some 50dB.
The
overall bandwidth
at
the
6dB
points
should be
200 kHz.
Narrow
IF
Filter .
Change
.SELECTIVITY SWITCH
to
"KARROW"
position,
time
generator
very
slowly
across
passband
and observe
output
meter carefully,
noting
degree
of
ripple.
In
a correctly
aligned
filter,
ripple'
will not
exceed
2dB
and
is
typically
1.5dB
in
an
average
set.
Ripple
proportions with
a
maladjusted
filter
will
rise
to
some
6-8dB
and
would there-
fore
be very noticeable even
when
tuning
across
a
normal
signal.
28
-