Headphones Output Level; Bias Trap Adjustment; Bias/Rec Calibration Check - Tascam 122 Service Manual

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4) If the response decreases at high frequencies,
adjust semi-fixed resistor R36 for NORMAL
tape and R37 for Co (Cr02 ) tape.
7-14. HEADPHONES OUTPUT LEVEL
1)
Connect an 8-ohm load and a VTVM to
the PHONES jack.
2) Load test tape MTT-150 and play it back to
measure the headphones output level.
3) The value should be 0 dBV ±2 dB (100 mW)
when the PHONES knob is set to maximum.
7-15. BIAS TRAP ADJUSTMENT
Bias trap adjustment is only necessary when :
* the record/reproduce head is replaced,
*
parts in the record amplifier circuit (especially
the bias osc illator and. trap coil) are replaced,
*
too much bias leakage is observed.
1) Set the deck to the record/pause mode.
2) Connect the VTVM or oscilloscope's HOT
terminal to T.P.3/T.P.4 (junction point be ­
tween L 1 04/L204 and R 176/R276 of the bias
trap) and its COLD terminal to TP (GND).
3) Adjust L 104lL204 so that the level (bias
leakage) reading is minimum.
NOTE: A fter adjusting the bias trap, check the
bias setting, record level and overall
frequency response.
7-16. BIAS/REC CALIBRATION CHECK
Prior to this check, set the deck as follows.
The tape to be used for calibration is Co
(Cr02) MTT-5061.
a. Set the EQ and BIAS switches to Co
(Cr02 ).
b. Set the INPUT switch to TEST.
c. Set the ADJUST/P RESET switch to AD ­
JUST.
d. BIAS/REC CALIBRATION:
BIAS L/R (R35/R45)
fully counterclockwise
REC LEVE L L/R (R30/R40)
fu IIy clockwise
1) Set the deck to the RECORD/PAUSE mode
and the MONITOR switch to SOURCE.
2) Apply a -34 dBV /1 k Hz signal to II N E 1,
and
~djust
the INPUT (LEFT/RIGHT) level
control so that the VU meter indicates
o
VU. (When observed at the OUTPUT
terminal, adjust the OUTPUT level control
so that the level is -30 dBV.)
Variable BIAS CAL. range
3) Record and play back a 1 kHz input signal,
setting the MONITOR switch to TAPE.
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4) While recording and reproducing the signal,
turn the BIAS LlR (R35/R45) clockwise to
obtain the maximum value (Lmax) of the
VU meter. (When observed at the OUTPUT
terminal, obtain the maximum level.)
5) Also obtain the indication value (LI ) of the
VU meter when the BIAS L/R (R35/R45) is
turned fully clockwise. (When observed at
the OUTPUT terminal, obtain the OUTPUT
level at that time.).
6) Confirm that the variable range is Lmax ­
LI :;? 4 VU (or 4 dB at the OUTPUT
terminal).
Variable REC CAL. range
7) As in the variable BIAS CAl. range, record
and reproduce a 1 kHz signal, setting the
MONITOR switch to TAPE.
8) While recording and reproducing this signal,
turn the BIAS L/R (R35IR45) clockwise to
obtain the maximum indication value
(Lmax) of the VU meter (when observed at
the OUTPUT terminal, obtain the maximum
level value.).
9) Obtain indication value
L~
of the VU meter
when REC LEVEL L/R (R30/R40) is turned
fully counterclockwise. (Obtain the OUT­
PUT level when observed at the OUTPUT
term ina I. ).
10) Confirm that the variable range is Lmax ­
L2 :;? 10 VU (or 10 dB at the OUTPUT
terminal).
BIAS/REC CALIBRATION setting
1)
Set the deck to the RECORD/PAUSE mode
and the MON ITO R switch to SOU RCE.
2) Apply a -34 dB/400 Hz signal to LI NE 1.
Adjust the INPUT (LEFT/RIGHT) level
control so that the VU meter indicates 0 VU.
(When observed at the OUTPUT terminal,
adjust the OUTPUT level control to obtain
-30 dBV.)
3) Record and reproduce the input signal. Adjust
REC LEVE L L/R (R30/R40) to obtain iden­
tical VU meter levels (or OUTPUT levels),
switching the MON ITOR switch between
SOURCE and TAPE.
4) Record and reproduce the input signal with
the MONITOR switch set to TAPE. Adjust
BIAS L/R (R35/R45) to obtain the identical
VU meter values (or OUTPUT levels), varying
the input signal frequency between 400 Hz,
6.3 kHz and 12.5 kHz.
NOTE: After adjusting BIAS L/R, adjust the
RECORD level again. Also check the
overall distortion.
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