Muting Circuits - Tascam 122 Service Manual

122
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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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MONITOR
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OUTPUT
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R34
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PHONES
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R22B
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PHONES
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R72
LEO INDICATOR DRIVER
~V
-S-3V
U701 , 0704
R5S4
PEAK LEVEL
-2.8V
INDICATOR
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Fig. 3-23 Power muting circuit
26