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® When the power supply is turned off
The —B, voltage level diminishes relatively rapid-
ly, and
Q,
is turned
on
by the forward biasing
(because
—B,
diminishes
rather
slowly).
Conse-
quently, the potential at point A is reduced to —B,,
and the FET is tumed
off. This muting status is
achieved within 0.1 second after the power supply
is turned off.
e When FUNCTION selector is operated
When
the FUNCTION
selector switch is at any
of the selector positions, Q, will remain off due to
+B, being applied to its base via the FUNCTION
switch. But since this is a non-shorting type selec-
tor switch, the +B, being applied to the base of Q,
will be interrupted whenever the position of the
2.2
TAPE MONITOR
I
SWITCHES
-Bi (-22v)
selector
switch
is changed.
Both
Q, and Q, will
thus be turned
on
temperarily.
As a result, the
potential
at point A will be reduced
to —B,, fol-
lowed by the FET being turned off and the power
amplifier
stage being muted.
However,
once
the
FUNCTION
selector
switch
has been completely
switched
to the next position, +B,
will again be
applied to the base of Q,, resulting in Q, and Q,
being turned
back
off. The potential at point A
will consequently
increase (at a speed determined
by the R,, C, time
constant)
until the FET
is
turned on, and the muting condition terminated.
Note that when the TAPE MONITOR
switch is
ON,
Q,
will
be
disconnected
from
Q,,
so the
power
amplifier
stage
will
no
longer be muted
when the FUNCTION selector is operated.
POWER
AMPLIFIER
(-34.5v)
Fig. 1
Basic circuitry of muting circuit
POWER
ON
t——-
Oo — +
POINT
A
0
(-Bl)
MUTING
oN
——
———_
——___——-
Fig. 2
Voltage changes of different points
POWER
OFF
+82
j
FUNCTION
CHANGE
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