Eddystone 990R - The Audio Section

Muting
Circuit
.
(TR26-TR29)
Noise
voltages
at
the
output
of
the
PM
Discriminator (in
the
absence
of
a
signal)
are
passed
via a
high-pass
filter
to
the two-stage
Noise
Amplifier comprising
TR26
and
TR27
(2
x
GET880) .
Circuit
constants
are chosen
to
give
maximum
amplification
at
frequencies
above
the
speech
and
music
range,
RV7
provides
a
means
of
setting
the
gain
of
TR26/TR27,
while
TH1
ensures
sensibly
consistent
performance
with
changes in
ambient
temperature
.
The
second
of
the
two
Noise
Amplifiers
feeds
a
voltage-doubler
Noise
Rectifier
(D9/D10
1
2
x
0A202
)
via a
(vinkor)
transformer
tuned
to
approximately
27
kHz
by C170.
The
output,
load
of
this
stage
(R135)
is
in
series
with
the base
return of
TR28
(2N3704)
which
is
the
input
stage
of
a
Schmitt
Trigger.
With
the
MUTING
SWITCH
(S8)
in
the
"ON"
position,
all
transistors
in
the
muting
circuit
are
operative
and
a
negative
voltage
is
produced
across
R135
in
the absence of
a
received.
'signal
(i
» e
•
under
noise
conditions).
This
puts
TR28
in
the '’off"
condition
•and
so
causes
TR29
(2N3704)
to
conduct
heavily
due
to
the reduced
voltage
drop
in R140.
The. relay
RLA/2
is
energised
and
audio
output
to
both
line
and speaker
cut
by contacts
A1 and
A2
(See
"Audio
Section"
below)
.
On
receipt
of
a
signal,
noise
voltages
are
removed from
TR26/27,
the
voltage across
R135
falls
rapidly
to
zero
and
TR29 is
tripped
into the
"off"
condition
by
the increase
in
current
through
R140.
RLA/2
falls
out,
restoring
audio
output
and also
opening
the
circuit
across
the
MUTE
INDICATOR
terminals
at
the rear
of
the
set.
THE_AUDIO_SECTION
General
.
Audio
from
TR30
on
the
Video
Board is
routed
to
two
separate
gain
controls
(RV8
%
AUDIO
GAIN
and
RV9
1
LINE
LEVEL)
which
feed
the
two
independent
audio
channels.
Both
amplifiers
are
usable
simultaneously
in any
mode
of
operation
and
provide
separate
out-
puts
for
local
and
remote
use.
Relay
contacts
A1
and
A2
are
arranged
to
mute
both
channels
in
the absence
of
a
signal
when
the
muting
facility
is
being
used.
A1
short-circuits
the
6000
line
output,
while
A2
open-circuits
all
local
monitor
outputs
and closes
the
circuit
between the
two
MUTE
INDICATOR
terminals.
A
remoto
lamp
can
be
controlled
by
thi»
circuit
to
indicate
that the
receiver
is
operational
but
in
the
muted
condition.
A separate lamp supply
is
required.
Low-level
Audio
Channel
.
(TR32
&
TR33)
TR32
(GET880)
and
TR33
(OC72)
form a
low-level
audio
amplifier
for
connection
to
6000
line
circuits.
Gain
adjustment
is
by RV9
which is a
pre-set
control! maximum
out-
put
is
limited
to
lOmW.
The
output
transformer
(T12) has a
centre-
tapped
secondary
and
is
electrostatically
screened
from
the
primary
winding.
Output
can
be
arranged
to
suit
balanced
or
unbalanced
lines.
High-level
Audio
Channel
.
(TR34-TR38)
This
amplifier
employs
a
total of
five
transistors
(TR34-TR38)
and
provides
output
for
an
external
loudspeaker,
an
internal
low-level
monitor
speaker
and
low-impedance
telephones
(headset).
The
monitor
speaker
can be
switched
off
by
S9
and
the
external
speaker
by
insertion
of
the
telephone
plug
at
JK1.
THa
high-level
audio
stages
operate
from
a
+12V
supply
line.
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10
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