Amplifier Section - Tascam 122 Service Manual

122
is muted. Since the bias oscillator is work­
ing at this time, the tape runs with no
signal recorded on it.
d. On the other hand, when 0610 turns on,
base current flows through 0612, turning
it on (0611 is off during record mode).
When 0612 turns on, the REC MUTE LED
lights.
The R EC/MUTE mode is released by re ­
setting the REC MUTE flip-flop. There are
three ways this can be done as below.
i) Depress the PLAY button.
An L-Ievel reset signal will be applied
to the base of 0609 via path
CD .
ii) Depress the PAUSE button.
When the PAUSE button has been de­
pressed, the deck changes from the re ­
cord/reproduce (mute) mode to the re ­
cord/pause mode. Therefore, pin 12 of
U601 changes from H level to L, apply­
ings L level signal to the base of 0609
via path
® .
iii) Depress any of the STOP, F.FWD, and
REW buttons.
The record/reproduce (mute) mode will
be released and pins 12 and 10 of U601
drop to L level. As a resu I t, a reset
signal will, be applied to the base of
0609 via path
®
or @.
When an L-Ievel reset signal has been appli ­
ed to the base of 0609, 0609 turns off as
its base current is cut off. Since the collec ­
tor of 0609 rises to H level, base cu rrent
flows through 0608 and it turns on reset ­
ting the REC MUTE flip-flop. When 0609
is off, 0610 turns off as i,ts base current is
cut off. (Note: , I n this case, the base cur ­
rent of 0608 flows via path @ . The volt­
age drop across R647 caused by this cur ­
rent (the emitter-base voltage of 0610)
is approximately 0.16 V so 0610 cannot
turn on.) When 0610 has turned off, 011 11,
0110, and 011a turn off in sequence to
release the record amplifier from muting.
Note that, when the STOP, F. FWD, or
REW button is depressed during REC/
MUTE mode (case (jii)l, the record mode
will be released. In this case, when pin 10
of U601 drops to L level, 0604 turns off
as its base current is cut off, while 0611
turns on as base current is applied to it.
Therefore, even if 0609 turns off with the
REC MUTE flip-flop reset, 0610 remains
on because its base current flows via R648
and 0611. This means that the record
amplifier is not released from muting.
30
AMPLIFIER SECTION
3-11,. REPRODUCE CIRCUIT
See Fig. 3-26.
a. The signal picked up by the reproduce head is
amplified by reproduce equalizer amplifier
transistors 0101 and 0102. The reproduce
equalizer amplifier negative feedback loop
determines the compensation characteristic
(time constants) for reproduction as given in
Table 1. The compensation curve at high fre­
quencies is determined by the composite
resistance of R301-R304, the combination
of which is determined by the settings of
SPEED switches S4-10 and S4-8 and EO
switch S5-2, and the capacity of Cl04. Table
2 summarizes the composite resistances in
relation to the settings of the SPEED and EO
switches.
Table 1
Compensation characteristic for reproduction
~
EQ
STANDARD
HIGH
METAL 3180 + 70 3180+ 35
Co (Cr02) 3180 + 70 3180+ 35
NORMAL 3180+120
3180+50
(unit: ,usec)
Table 2 Composite resistance
~
EQ
STANDARD HIGH
METAL
R30Z
=
S.2k.
R301 IIR302 ~3.1 k
Co (Cr021 R302
=
S.2k R301 //R302 ~3.1 k
NORMAL
R302 + R304 ~10 .9k.
R30111R302
~
+ R30311R304
~4.2k
("II" represents parallel connection of resistors. unit: ohms)
b. Capacitor Cl07 and coil L1 01 connected in
series are a bias trap with a resonance fre­
quency of 100 kHz.
c. Semi-fixed resistor R 11 adjusts reproduction
level.
d. Capacitor Cl08 and semi-fixed resistor Rl0
compensate for characteristics at high fre­
quencies.
e. Thermistor RT10 compensates for variations
in the sensitivity of the reproduce head due to
temperature changes.
The standard resistance of RT10 is
10k ohms at room temperature (25°C). The
resistance decreases (increases) when the
temperature rises (falls). On the other hand,
the sensitivity of the reproduce head rises