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AUDIO
OSCILLATOR ASSEMBLY
19-0847
(Fig.
15)
Introduction
4.121
The Audio
Oscillator
Assembly
provides three fixed frequencies of
400Hz, 1kHz
and 5kHz,
together with ancillary amplifiers
in
the
meter
circuit.
For
an
understanding of
the circui
t
operation
and
functions
it
is
necessary
to
refer
to
the front
panel switching
diagram.
Fig. 20, as well
as
to
the circuit Fig. 15.
Audio
Oscillator Circuit
4.122
Referring
to
Fig. 15,
transistors
Q4,
Q5
and
Q7,
together with
R15/C9
and
R1
1/C4
form a
400Hz Wien
Bridge
oscillator.
Frequencies of
1
kHz
and
5kHz
are
obtained by shunting R15 and
R1
1
by
resistors
on
the
Modulation Frequency
Switch
Assembly S10
(Fig.
20)
.
To
obtain
1
kHz R23/R24
are
switched
in
parallel
with
R15and
R19/R20
with
Rl
1
.
For
5kHz,
the parallel
switched
resistors
are
R21/22 and R17/18
respectively.
4.123
Transistors
Q8,
Q9
and
Q6
provide a
feedback
loop
for
automatic
level
control
with potentiometer
R20
providing a
bias control
for
output
level setting.
The
oscillator
output
is
fed via the amplifier
Q10,
and
the
back-to-back
electrolytic
capacitors
06
and C17,
to
the front
panel
Modulation
Level
potentiometer
(Fig. 20)
via
pin 11.
An
output
is
also fed
via
04/0
5
and
resistors
R25/R26
to
give a
600Qoutput
at the rear panel
Audio Output
socket.
4 124
Referring
to
Fig.
20,
the
audio
signal at the slider
of the
Modulation
level
control
R33b
is
fed
to
switch S8/1
R
for
use
on
AM
(internal)
modulation.
The
signal
is
also fed
back
into the
Audio
Ascillator
Assembly
at pin 18
where
it
is
buffered
by
Qll
and
fed
out
via pin
20
to
the
FM/fiM
Control switch
(Fig.
20, switch
S9)
.
4.125
With
switch S9
at
INT
the
audio
signal
is
fed via tag 12
to
the
Range Switch
S2/3R where
it is
attenuated by
the
network R2-R6, according
to
the
frequency
range selected.
From
tag 18
of
S2/3R
it is
fed via the
FM
COARSE
potentiometer
R33a
to
pin
5
of the
Audio
Oscillator
p.c.b.
(Fig.
15)
where
it
is
buffered
by
Q1
and
then fed
via pin 6
to
the
Peak
Deviation switch
S5/1 R
for
appropriate ranging
by
the
network R8-R10.
4.126
From
S5/1
R the
audio
is
fed via tag
22
to
pin
11
of the
FM
Loop p.c.b.
(Fig.
9)
where
it
is
buffered
and
fed
to
the
modulating
line
of the
main
oscillator,
VCOl,
when
FM
mode
is
in
use.
4.127
Pin
15 of
the
Audio
Oscillator
p.c
.b.
receives
the
meter
drive
signal
on
AM
and'
FM,
as
follows:-
(1)
AM.
The output
signal
described
in
para
4-80
is
fed via
meter
switch
S11F/1F
tags
C
and
E.
(2)
FM.
The demodulated f.m
.
audio
(see
para 4.
129)
is
fed via
SI
1
F/l
F
tags
F
and
E
.
The
signal
entering
the
p.c.b.
at pin
15
is
fed via
C2
to
the
operational amplifer
ICI
and
Q2,
from
which an
output
via the
meter
setting
potentiometer R8
is
fed
to
the
Meter
Switch
SI
1/1 R,
and
thence
to
the meter.
9081
&
9082
Vo
1
.
2
4-23
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