Tascam 122 Service Manual page 27

122
+23V
REPRODUCE
EOUALIZER
REPRODUCE AMPLIFIER
HEAD 0101,0102
DOLBY DECODER
0509
UIOI
LD
MONITOR
CIRCUIT
R660
0103
I
RI13
PINCH ROLLER
0695
R ch
SOLENOID
R213
L693
Fig. 3-24 Reproduce muting circuit
3-10-2 Reproduce muting circuit
See Fig. 3-24.
The reproduce muting circuit mutes the repro ­
duce equalizer amplifier to prevent the genera ­
tion of undesirable noise in modes other than
the reproduce mode.
1) Muting on
In a mode other than the reproduce mode
(stop mode, for example), the reproduce
muting circuit operates as follows to mute
the reproduce equalizer amplifier.
a. In the stop mode, the pinch roller sole­
noid drive transistor 0509 is off.
b. Because of this base current isnot supplied
to transistor 0614 and it remains off.
c. As 0614 is off, base current of 0113 flows
via path
CD ,
therefore it is on.
d. As 0113 is on, base current of 0112 flows
via path
®
and it is on. At this time, capa ­
citor C 136 is charged to +23
V.
e. As 0112 is on, the base current of 0103
flows via path @ and it is on so the
reproduce equalizer amplifier is muted.
Because the signal level is low in the 0103
circuit its collector is grounded to reduce
muting transistor switching noise ..
2) Muting off
When pinch roller solenoid drive transistor
0509 turns on in the reproduce mode, mut­
ing is released by the following process.
a. As 0509 is on, base current is applied to
0614 and it turns on.
b. As 0614 is on, 0113 turns off as its base
current
CD
is cut off.
c. When 0113 is off, the base current @
applied to 0112 is cut off. 0112 does not
go off immediately since C136's discharge
current
®
flows to its base.
d. As C136 discharges, its discharge current
®
(the base current of 0112) decreases
gradually, causing the collector current of
0112 to decrease gradually. Therefore, the
base cu rrent of 0103 decreases an d it goes
off grqdually.
0103 turns off approximately 400 msec
after 0509 is turned on.
e. Once 0112 turns off, the base of 0103 is
fixed at a negative voltage applied through
R 150 to lock it in the off state. Reproduce
muting is completely released in this condi­
tion.
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