Muting Circuit - Yamaha CT-7000 Service Manual

CT-7000

MUTING CIRCUIT

This circuit eliminates weak signals and inter-station
noise during tuning, or in case of station drift. This
Operation
1. Muting operation due to a signal from terminal
a. DC voltage e1, which shows the strength of the
received signal between 10dBJ.! and 100dBJ.t is fed
to the h terminal and passes to the base of Tr4,
which has had its bias adjusted by VR1.
b. If e 1, is less than the set bias value, Tr4 switches
off, switching Tr5 and Tr6 on. This makes the voltage
at point� sufficient to turn on Tr8, Tr9 and Tr1 0,
grounding the composite
audio signal via Tr9 and Tr10.
c. On the other hand, if e1 is greater than the set
bias value, the Tr4 switches on, thus switching off
Tr5 and Tr6. This means that the voltage at point
<6)
will switch off Tr8, Tr9 and Tr10, thus passing
the composite and audio signals.
2. Muting operation due to a signal from terminal
a . Signal e2, which contains a
the tuning aberration, is fed to the
This is potential which can be obtained by the
discriminator S curve. When the received signal is
lower than the correct tuning point, negative volt-
8
II.
signal via Tr8 and the
12.
DC component showing
12 terminal.
circuit also functions to cut transient noises, a function
which will be explained in another section.
age is contained in e2; when it is higher, positive
voltage is present.
b. When the received signal is lower than the correct
tuning point, Tr1 is continually off, but Tr2 can
be switched on by voltage of below -0.5V, and
at this time Tr3 comes on. In this case potential at
point© becomes zero due to diode D1, while point
(ji) is grounded by 02, thus switching on the muting.
c. When the signal is higher than the muting point,
Tr2 and Tr3 are continually off.
when it receives more
potential at point ©. D2 thus
switching on the muting.
d. In addition to selecting DC voltage, this circuit works
in the same way to block unwanted
below 20Hz and over 50kHz through the actions
of C1 and L 1. This cuts the noises normally heard
when the tuning knob is turned quickly and passes
one or more stations.
e. When the muting effect is not desired it can be
bypassed by raising voltage at point
the Muting switch, thus turning Tr6 off.
Tr1 comes on
than -0.5V, creating E
®
grounds point
signals of
@
to +12V via