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FM LOOP ASSEMBLY
(1)
Range
Switch
Range
switch
S2/3R
receives
the internal
or
external
audio
signal via
switch S9/1 R
when
on
FM.
This
is
fed
through
the
attenuating chain
R2,
R3,R4, R5
and
R6,
and
the
appropriate
level
(in
the ratio
1
-2-4-8)
is
picked
off
by
the
switch
according
to
the
frequency range
selected
(see
para.
4.143).
The
audio
at tag 18
of
the
switch
is
fed via the
FM COARSE
potentiometer R33a*
to
a buffer
in
p.c.b.
19-0847,
thence
via pin
6
to
the
Peak
Deviation switch
S5
for
routing
to
the
modulation
line
of the
main
oscillator.
(2)
Peak Deviation
switch
(a)
Wafer
S5/1
R:
The
attenuating network R8,R9, RIO and
R1
1
with switch
S5/1R
taps
down
the
audio modulating
level
according
to
the
deviation selected.
In
the
100kHz
deviation
position the signal
is
fed via
tags 18
and 22
to
the
FM
audio
line
at the highest level, but
in
the
remaining deviation
positions the
signal
is
attenuated
in
the
ratio
of 3, 10
and 30
to
1
respectively.
In
the
0M
position
Cl and R7 provide
a small
amount
of
roll
off
on
the
audio
signal
.
(b)
Wafer S5/2R
.
The
resistor
network
R13,R14,R15 and R16
is
connected
to
amplifier
Q38-Q27
in
the
FM
Loop p.c.b.
19-0844,
and
defines the amplifier gain
in
the ratio
of 3, 10
and 30
to 1,
relative
to
the
100kHz
deviation
position.
R
1
2
and C3
provide
roll
off
in
the amplifier
gain
when
in
0M
mode
(para 4.
1
48)
.
9081
&
9082
Vol
.
2
4-29
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