Onkyo TX-2500 - Muting Circuit

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4. Muting Circuit
The
muting
circuit
is
operated
through
utilization
of
functions
of
the
IF
carrier,
the
O-point
detection
(variance
in
detected waves between IF and Quartz) and the noise component.
(At time of tuning)
For reasons that the base potential of Q 128 and 129 is in the state of OV in terms of direct current as can be noted
from the description given of the T-Meter Circuit at the time of tuning, the emitter potential of Q 128 and 129 is
rendered equal to each other. As for Q 131 and 132, these are cut off to cause OV output to be brought forth.
At the same time, the IF carrier portion, at the time of tuning, is at a lower level in voltage than the standard level
for
the
Schmitt
trigger
of
Q
133
and
134
as
muting
regulating
output
inside
the
quadrature
detector
IC.
For
this reason, Q 133 is turned OFF while Q 137 is turned ON, which, in turn, causes Q 109 and 110 to be turned OFF.
(At time of detuning)
When the voltage of the O-point (the variance in output between the IF detector and quartz detector) has turned
positive or negative, if it come to stay on the plus side, for instance, the base potential of Q 128 is caused to drop,
that of Q 129 to pick up, with Q 132 and 133 being turned ON. Again Q 109 and 102 are turned ON at the same
time,
with
the
signal
being
caused
to
drop
to
the
earth.
Again,
simultaneously,
Q
133
is
turned
ON,
Q
134
OFF,
Q
135
ON,
causing
the
AFC
circuit
to
be
closed.
In
addition,
the
IF
carrier
portion,
too,
is
caused
to
act
upon
Q
133,
turning Q 133 ON. :
Again,
the
L
106
resonance
point
is
arranged
at
a
higher
level
as
the
noise
portion
enters
into
the
Q
112
base,
so
that
Q 112
may
act
on
the noise
amplifier,
causing
the
Q
113
to
be
turned
OFF. This
passes
through
D 109
and
is added
to
the
base
of
Q 131.
It
also
passes
through
D
105,
causing
the
transistor
of
Q
130
to
be
turned
ON,
and
thus,
causing,
in
turn,
the
meter
circuit
to
be
shortcircuited,
thus
ensuring
against
any
error
in
performance.
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