Onkyo TX-2500 Service Manual page 5

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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. :
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