122
I
3-10.
MUTING
CIRCUITS
3-10-1 Power
muting
circuit
See
Fig. 3-23.
The power
muting
circuit
prevents transient
click
noise
from
being
output
from
the
OUT-
PUT and PHONES terminals and the
VU
meter
needles
from
deflecting when the deck's power
switch
is
turned on
or
off.
1)
Power-on
muting
When the
deck
power switch
has
been
turned
on, the power
muting
circuit
operates
as
fol-
lows.
a.
When power
has
been
turned on, the power
muting
circuit
is
supplied
with
7 V AC.
Diodes
0516
and
0515
half-wave-rectify
the AC voltage, charging capacitors C524
and C523 via paths
CD
and ® momentari-
ly.
The voltage across C523
is
divided
by
resistors R551 and R550 and
is
then
applied
to
the
emitter
of
transistor Q522.
b.
Capacitor C522
serves
as
a ripple
filter
together
with
H551.
c.
The charging
of
C521 starts via paths @
and ® when the
circuit's
power supply
starts. Since the current on path ®
is
the
base
cu
rrent
of
transistor Q522,
it
tu rns on.
d. As Q522
is
on,
base
current
flows
through
transistor
Ql04
via path @ ,
turning
it
on.
As
Ql04
is
on, the signal line
of
the moni-
tor
ci rcu
it
is
shorted
to
ground
to
prevent
the generation
of
click
noise when power
is
switched on.
e.
When C521
is
fully
charged, Q522 turns
off
as
its
base
current
is
cut
off.
Ql04
turns
off
in
the
same
way
and,
as
a result,
muting
is
released.
Releasing power
muting
takes place ap-
proximately
3.5 seconds
after
the deck
power switch
is
turned
on.
f. When Q522
is
off,
-5.3
V
is
applied
to
the
based
of
Ql04
via R128 and R553. This
ensu
res
the
off
state
of
Q 104.
2) Power-off
muting
When the deck power switch
has
been tu rned
off,
the power
muting
circuit
operates
as
fol'lows.
a.
When power
has
been turned
off,
the sup-
ply
of
7 V AC stops. C524 and C521 dis-
charge
rapidly
through
the meter lamp
circuit
via paths ® and
(J)
.
b. Therefore, the
base
voltage
of
Q522 falls
and its
base
cu
rrent flows
from
C523 via
path
@,
turning
it
on.
c.
As
Q522
is
on, C523 discharges via path
® ,
tu
rn ing on Q
104
so
that
the power am-
plifier
is
muted.
,.
-
--,
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@
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CD
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O~19
-8V
0~17
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MONITOR
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TP.
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OUTPUT
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R34
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PHONES
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Rc:Io
R22B
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PHONES
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R72
LEO
INDICATOR
DRIVER
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0704
R5S4
PEAK
LEVEL
-2.8V
INDICATOR
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Fig. 3-23 Power muting circuit
26
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