Tascam 122 Service Manual page 33

[ 122
R713
R714
r
0703
+~.3V----------~---------------------------------------,
FROM
METER
AMPL.
C708
R716
PEAK
Fig. 3-28 Peak indicator circuit
When INPUT switch S1-1 is set to LINE 1 or
LINE 2, base current flows through 0701 and
it is on. As 0701 is on, resistor R702 is
inserted in parallel to the input circuit of the
meter amplifier. This setting provides the
standard meter sensitivity.
When the INPUT switch is set to TEST and
the deck is in the record/reproduce or record/
pause mode, 0701 is off as its base current is
cut off. When
0~01
is off, R702 is discon­
nected from the input circuit of the meter
amplifier. As a result, the signal input level of
the meter amplifier rises by about 20 dB.
This rise in the input level results in a rise of
the output level by the same amount so that
meter sensitivity is raised by 20 dB.
2) Peak indicator circuit
See Fig. 3-28 and 3-29.
Semi-fixed resistor R72 determines the
operating level of the peak indicator.
When the signal level supplied from the meter
amplifier is lower than the threshold level
of 0704 determined by R72, the condition
of the circuit shown in Fig. 3-28 is as follows.
0704: off
C708: charged
Pins 12 and 13 of U701 : H level
Pins 11, 9, and 10 of U701: L level
Pin 8 of U701 : H level
Peak indicator: off
When a high-level, pulse-I ike signal is output
from the meter amplifier, the circuit operates
as follows.
a. When the input signal level exceeds the
threshold level of 0704 determined by
R72, base current flows to 0704 and it
turns on.
®
HIGH
LOW
HIGH - - - - ;
®
LOW
HIGH
---,!,..,
LOW
HIGH
---I--.,
@
LOW
®
HIGH
LOW
_--;-+....
HIGH --~.,
®
LOW
OFF
PEAK
LED
ON
I---~ PROLONGED
*
W HEN VOLTAGE AT
®
IS VIEWED AS LOGIC LEVEL.
Fig. 3-29 Peak indicator circuit timing diagram
33