Marantz 2245 - Page 7

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‘ea
proportional
to
the
noise
components
in
the
audio
signals.
Audio
Muting
and
Stereo
mode
auto-selecting
circuit
The
muting
circuit
consisting
of
all
solid-state
electrical
switching
has
been
incorporated
in
the
Model
2245.
Three
inputs
control
the
muting
function.
The
first
is
related
to
signal
strength,
the
second
to
the
noise
condition
at
the
detector
and
the
third
is
derived
from
the
DC
component
of
the
detector
output.
These
inputs
are
properly
matrixed
and
gated
to
provide
muting
free
from
noise
and
transients.
The
first
input
of
DC
voltage
obtained
by
rectifing
a
part
of
IF
output
signal
from
the
H205
is
applied
to
the
base
of.
H306
and
turns
on
it,
if
the
1F
output
is
greater
than
predetermined
level
(muting
threshold
level).
When
the
H306
is
turned
on
the
H307
is
turned
off,
allowing
the
emitter-collector
resistance
increasing
and
the
collector
voltage
rises
about
9V.
The
increased
collector
voltage
increases
the
gate
bias
voltage
and
turns
on
the
switching
FET
H308,
decreasing
the
source-drain
resistance
to
near
zero
ohm
and
allowing
the
audio
signal
applied
to
the
source
to
flow
to
the
center
of
38KHz
switching
transformer
through
the
source-drain
path.
;
When
the
input
signal
is
lower
than
predetermined
level,
the
DC
output
obtained
is
small
and
can
not
turn
on
the
H306,
thus
the
H306
keeps
its
turn-off
state
and
this
makes
H307
turn
on,
decreasing
the
collector
voltage
and
turning
off
H308.
Thus
no
audio
signals
can
pass
through
the
FET.
This
is
the
fundamental
principle
of
the
muting
operation
but
for
more
elaborate
muting
.
operation
the
second
and
the
third
inputs
are
necessary.
The
second
input
is
used
to
protect
the
muting
operation
and
MPX
stereo
beacon
lamps
from
“misoperation
due
to
undesirable
noises.
The
high
frequency
noises
included
in
the
detected
audio
signals
are
separated.
by
a
small
capacitor
C551
and
amplified
by
the
noise
amplifier
transistor
H551
and
its
output
is
rectified
by
the
two
diodes.
The
rectified
DC
output
is
When
there
are
excessive
noises
in
the
audio
signals
such
as
obtained
with
a
station
incorrectly
~
tuned
in,
the
rectified
DC
output
turns
on
the
transistor
H522,
decreasing
the
emitter-collector
resistance
to
zero.
This
means
the
collector
of
H307
is
short-circuited
to
the
ground,
therefore
.
the
H308
is
turned
off
and
any
audio
signals
having
excessive
high
frequency
noises
can
not
go
through
the
FET’s
sourcedrain
path.
The
transistor
H303
connected
in
series
with
the
19KHz
pilot
signal
amplifier
transistor
H302
is
also
turned
off
(when
the
transistor
4522
or
H307
are
turned
on.)
and
no
current
flows
in
the
H302,
resulting
in
turning
off
the
stereo
beacon
lamps.
Thus
misoperation
due
to
undesirable
noises
is
also
avoided.
The
third
input
is
obtained
from
the
EM
discriminator
circuit.
The
DC
output
so
called
“gr
curve
is
applied
to
the
gate
of
H553
through
a
resistor
R523
and
deviding
network
(R565
&
R566).
The
DC
output
is
zero
with
a
station
correctly
tuned
in,
but
will
vary
from
negative
to
positive
values
or
vice
versa
when
the
tuning
point
is
deviated
toward
either
plus
or
minus
frequency
from
the
correct
tuning
frequency.
When
the
DC
output
is
increased
to
a
greater
level
than
that
of
predetermined,
the
increased
source
potential
of
H558
makes
the
transistor
H561
turn
on,
and
this
makes
the
H306
turn
off,
...
H307
turn
on,
....
H308
turn
off,
....
1303
turn
off
(this
means
no
19KHz
pilot
signal
is
amplified
and
no
stereo
beacon
is
turned
on.)
When
the
DC
output
is
increased
to
the
negative
predetermined
level,
the
decreased
source
potential
turns
off
the
H559
which
in
turn
makes
the
4560
turn
on
and
the
H306
is
turned
off.
The
subsequent
changes
are
exactly
the
same
as
that
just
described
above.
Thus
when
the
tuning
is
shifted
or
deviated
to
the
certain
frequencies
in
which
undesirable
noisy
side-audio
signals
are
produced,
both
muting
and
19KHz
switching
transistors
are
operated
automatically
and
open
the
circuits.
-
oa
ae
With
the
station
correctly
tuned
in,
the
bias
current
of
the
FET
H558
is
adjusted
so
that
both
transistor
H560
and
H561
are
not
turned
on,
giving
no
effect
on
the
transistor
H306.