Tascam 122 Service Manual page 28

122
3-10-3 Record muting circuit
See Fig. 3-25.
Record inhibit switch S694 operates by mechani­
cally detecting the presence or absence of a re ­
cord protection tab on the cassette loaded in the
deck.
S694 is in the N.O. position if the loaded cas­
sette is protected by the tab being removed. In
this case, pin 9 (AR IN) of system control IC
U601 is fixed at L level. The deck will not enter
the record/reproduce mode when the P LAY and
RECORD buttons are depressed simultaneous­
ly if terminal AR is at L level. (The reproduce
mode will be enabled in this case.)
S694 switches to N.C. when a recording-enabled
cassette tape (i.e. protection tab not removed)
is loaded. Now current from U601 charges capa ­
citor C607 and terminal AR switches to H level.
When terminal AR is at H level, recording
can occur. Hereafter it is assumed that terminal
A R is at H level.
1) Record/reproduce mode
When the deck is in the stop mode, the cir­
cuits shown in Fig. 3-25 are in the following
conditions.
Pins 12 and 10 of U601 : L level
Transistors: Q604, Q609, Q612, Q114, and
Q115: off
Transistors: Q608, Q610, Q611, Qll0, and
Qll1: on
Record muting Q116: on
Record muting flip-flop: reset
Bias oscillator U103: not oscillating
If the PLAY (
~
) and R ECOR D buttons
have been depressed simultaneously, H-Ievel
signals are output from pins 12 and 10 of
U601. The reproduce mode signal output
from pin 12 is mainly for the control of
mechan ical tape transport operations (see
3-9-1). The record mode signal output from
pin 10 in itiates the fol/owing operations.
a. When an H-Ievel signal is output from pin
10 of U601, base current floll\>3 through
Q604 and it turns on. As Q604 is on, cur ­
rent flows through the RECORD LED
and turns it on. At the same time, Q611
turns off as its base current is cut off.
b. As Q611 is off, Q610 turns off as its base
current i,s cut off. Similarly Qlll turns off,
as its base current is cut off.
c. As Q 111 is off, discharge current from
C 135 flows through the loop consisting of
C135, the emitter of Qll0, the base of
Qll0, and C135. As the discharging of
C135 proceeds, the discharging current (the
base current of Q 110) decreases and Q 110
turns off slowly.
d. As Q 11 0 turns off gradually, the base cllr­
rent of Q 116 decreases gradua lIy and it is
turned off slowly. As a result, the record
amplifier is smoothly released from muting.
e. On the other hand, if an H-Ievel signal is
output from pin 10 of U601, base current
flows through Q 115 and it turns on. There­
fore, base current is applied to Q114 turn ­
ing it on.
f. When Q114 has turned on, bias oscillator
Q103 begins to oscillate, its output being
delivered to the erase head. At the same
time, it is added to the output Signal (audio
signal) from the record amplifier after its
level has been adjusted by ·the bias adjust
circuit and the resulting signal is supplied
to the record head.
2) REC/MUTE mode
If you depress the R EC MUTE button when
the deck is in the record/r r eproduce mode, the
recording signal is muted to leave a blank
section on the tape.
When the deck is in the record/reproduce
mode the circuits shown in Fig. 3-25 are as
in the following condition.
Pins 12 and 10 of U601: H level
Transistors: Q604, Q608, Q114, and Ql15:
on
Transistors: Q609-Q612, Qll0, Qll1, and
Q116: off
REC MUTE flip-flop: reset
Bias oscillator: oscillating
Record muting: released
If the REC MUTE button has been depressed
in the record/reproduce mode, the circuits
operate as described below to enable the
REC/MUTE mode .
a. When the REC/MUTE button has been de ­
pressed, the REC MUTE flip-flop is reset
by an L-Ievel signal. Q608 turns off as its
base current is cut off.
b. As Q608 is off, its col/ector rises to H level
and base current flows through Q609
turning it on. When Q609 is on, its col ' l ec ­
tor is at L level. Therefore, if the REC
MUTE button is released and the L-Ievel
signal is removed from the REC MUTE
flip-flop, Q608 stays off and Q609 on.
c. As Q609 is on, base current flows through
Q610, turning it on. When Q610 turns on,
Qlll, Qll0, and Ql16 turn on in sequ ­
ence, as opposed to on in the record/
reproduce mode, and the record amplifier
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