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Deviation Monitoring
FM LOOP
ASSEMBLY
Note:
Model
9081 instruments
with
serial
numbers
from
1710
to
1909 and
Model 9082
instruments
with
serial
numbers
from
3010
to
3109
are
fitted
with
a 'piggy
back'
board 19-0953.
This
board
carries
Q44,Q45,
Q46,
Q25
and RLA.
The
circuit
and
interconnections are
shown
In
Fig.
9a.
The
operation of the
circuit
is
identical
to that
for
the
later
models,
which
is
described
below.
4.140
The
common
output from discriminators
Q6
and
Q8
is
fed via
C44
and
Q24
to the
low
pass
filter
L2, L3
with associated capacitors
which
eliminates the
400kHz
and
800kHz
ripple.
The demodulated audio
is
then routed
via the
Set
Deviation
potentiometer R85
to
an
amplifier
formed by
Q38, Q39,
Q26
and
Q27.
4.141
The
overall
gain of
the amplifier
between
Q38
and
Q27
is
partially
determined
by
a resistor/capacitor
network on
the
Peak
Deviation switch
(Fig.
20).
This
switch
is
connected
to pins 18
and
15
(marked 'Gain
Select
1*
and 'Gain
Select
2'),
and
in
0M
mode
causes the gain of the amplifier
to
roll
off
with
increasing
frequency,
thus
ensuring
a relatively constant
phase modulation
level
.
4.142
From
Q27
the
demodulated
f.m.
is
fed
to
Amplifier
IC5,
whose
gain
depends
upon
the selected carrier range, as follows:
Carrier
Range
5
4
3
2
1
Amplifier
Gain
x
8
x 4
x
2
x
1
x 4
4.143
The
difference
in
gain
is
determined by
C66
with
either
R112,
RII3
or
RII4
in
series,
depending on whether
Q31,
Q32
or
Q33
is
switched
on by
the
Range
Switch
via
IC6a, IC6b
or
IC6c.
4.144
The demodulated
f.m.
is
fed
from IC5
via
Q30
and RLA—
1
to
the
meter
switch
(Fig.
20)
whence
it
is
routed via the
FM
contacts
to
the
Audio
Oscillator
Assembly
19-0847.
Here
the signal
is
fed
through
meter
input circuitry
to
provide a reading
on
the
meter
of the actual deviation
on
the
carrier.
This
deviation
can be
adjusted
by
the
operator
by
combined
use of the
MODULATION
LEVEL and
FM COARSE
controls.
4.145
When
the
equipment
is
being tuned
by
spinwheel
or Step
Switch
pulses are fed to
pin 16.
With
the
OUTPUT
switch
at
NORMAL
or
X2
pin
21
is
at
+12V
and
Q45
is
turned
off.
The
pulses are
passed
by
Q44
to
pin 2 of IC7
which
changes
state
and
turns
on
Q29.
Q25
is
turned
off
and RLA-1
breaks the
FM
audio
line.
With
the
OUTPUT
switch
at
OFF
Q46
is
turned
off
and
Q45
turns
on.
This
provides a
low
at
IC7/2
to
operate
the
muting
circuit.
RLA-1
is
remade approximately
1
second
after
tuning ceases or the
OUTPUT
switch
is
moved
from the
OFF
position.
4-27
9081
&
9082 Vol
.2
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