Record Circuits - Tascam 122 Service Manual

122
b. When 0704 turns on, its collector drops to
L' level and C708 discharges momentarily.
Therefore, pin 13 of U701 drops to L level.
This inverts gate A, causing a H level at
pin 11 of U701 . After inversion by gate B,
an L level occurs at pin 8 of U701.
c. When pin 8 of U701 has dropped to L
level, current flows through the PEAK LED
and the peak indicator lights.
d. Alternately, the L level at pin 8 of U701
is transmitted to pin 12 of U701 via resis­
tor R716. If R716 is open, pin 1 '2 of U701
would not fait to L level when the collec­
tor of 0704 has dropped to L level be ­
cause of the presence of diode 0703. In
this case, operation would not be stable.
e. When the input level falls below the
threshold level of 0704, its base current
stops, turning it off.
f. When 0704 has turned off, current flows
into C708 via R715 to charge it. The
voltage across C704 (the voltage at @)
rises as shown in Fig. 3-29.
g. When the voltage at @ has risen to a cer ­
tain level (E1: approx. +2.0 V), U701
regards the input at pin 13 as H level.
Waveform
©
represents the voltage at
@
as logic level.
h. When the charging of C70B proceeds and
the voltage at @ reaches E2 which is ap­
proximately 0.6 V higher than E 1 (forward
voltage drop of D703),-U701 regards the
input at pin 12 as H level. At this moment,
gate A is inverted since pins 12 and 13 of
U701 are both H level, and the output
from pin 11 of U701 drops to L level.
i. Therefore, gate B is also inverted and the
output of U701 pin 8 turns to H level. As
a result, current supplied to the PEAK
LED is cut off and the peak indicator goes
out.
As described above, the peak indicator circuit in ­
. corporates a pulse stretching function that keeps.
the peak indicator lit for a certain length of time
even when the duration of the input signal is
very short.
34
3-13.
RECORD CIRCUITS
3-13-1
Line input circuits
See Fig. 3-30.
The deck has two line input circuits. LINE 1 is
provided at the back of the deck and LIN E 2
on the front panel. The INPUT switch deter ­
mines which line input is used. Signals from the
LINE 1 and LINE 2 input terminals cannot be
mixed. To use the BIAS/REC calibration circuit,
set the I NPUT switch at TEST and apply input
signals to the LINE 1 terminals.
With its level adjusted by the , R ECORD volume
control, the line input signal selected by the
INPUT switch is sent to Do' i by encoder U301.
MPX filter U305 connected to U301 removes
subcarrier and pilot signals which are mixed
with line input signals when recording FM
broadcasts in order to prevent the Dolby
encoder from mis-operation. The MPX filter
operates only when transistors 0301 and 0302
are both on. When they are off, the signals simp ­
ly pass through the fi!,ter. 0301 and 0302 are ·
on only when the N R SYSTEM switch is set to
NR or N R+HX: that is, when U301 is operat­
ing as an encoder.
Whatever the position of the NR SYSTEM
switch, an unmodified input signal is output
from pin 3 of U301 to the monitor circuit.
When the N R SYSTEM switch is set to N R or .
NR+HX, an encoded signal is output from pin
7 of U301. When the switch is at OUT, an un­
modified input signal is output. The output
from pin 7 is input to the line amplifier via the
HX variable equalizer. (Refer to 3-14 for the HX
variab ' l e equalizer.)
3·13-2 Line amplifier
See Fig. 3-31.
Recording level adjusting circuit BIAS/REC
CALIBRATION is included in the input circuit
of the line amplifier. When the CALIBRATION
switch is set at ADJUST, the recording level can
be adjusted with semi-fixed resistor R30. At this
time ADJUST LED 01 lights.
A temperature compensation circuit comprising
thermistor RT11 is located in parallel with the
output line of the line amplifier.
The output of the line amplifier is sent to the
recording level adjusting circuit after the record
signal level has been adjusted to match the type
of tape being used.