Overall Signal-To-Noise Ratio; Overall Distortion; Erasure - Tascam 122 Service Manual

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1122
Co{Cr02) and NORMAL
Take the same procedures as for metal tape.
Refer to Table under paragraph 7-5 for the tapes
to be used and the switch settings.
7.7. OVERALL SIGNAL-TO-NOISE RATIO
Obtain the standard setting according to proce ­
dures 1) - 6) of 7-5. to check record/reproduce
level.
1) Recording input signal of -14 dBV/400 Hz on
a blank tape for a short time. Then mute the
signal and record with no input signal.
2) Rewind the tape to the start of the recorded
section.
3)
Play back the recorded section and measure
the output level of the section with the signal,
then, at the point where the signal stops,
increase the sensitivity of the VTVM and
measure the output level of the no-signal
section. The output level measurement should
be weighted.
4) Calculate the level ratio of noise with respect
to the recorded signal and confirm that they
are identical to the values shown in the table
below.
For the tapes to be used and the switch
settings, refer to Table under paragraph 7-5.
If the values are unsatisfactory, proceed as
follows:
*
Demagnetize and clean the erase and record/
reproduce heads.
*
Check that erasi ng is being done effectively.
* Check and adjust reproduce and record/repro­
duce frequency response.
*
Check and adjust the bias trap.
*
Replace the tape.
SPEED STANDARD
HIGH
METAL
Co(CrOl)
More than 48 dB
More than 49 dB
NORMAL More than 45 dB More than 46 dB
7-8. OVERALL DISTORTION
1) Connect the measuring instruments as shown
in Fig. 7-4.
Obtain the standard setting according to
procedures
2) -
6) of 7-5. and check record/
reproduce level.
2)
Record an input signal of -14 dBV/400 Hz on
blank tape and play it back to measure the
distortion of the output signal.
3) The value should be less than 1.0 % for any of
the tapes used.
For the tapes to be used and the switch
settings, refer to Table under paragraph 7-5.
If satisfactory values are not obtained, check
the bias setting again, also check the SIN ratio
and the overa II frequency response.
OSCILLOSCOPE
OSCILLATOR
0
DISTORTION
ANALYZER
AMP SPKR
O~
.. ..
0
AC VOLTMETER
UNIT
under TEST
OVTP\JT
o
0
Fig. 7-4
7-9: ERASURE
1) Connect the measuring instruments as shown
in Fig. 7-5. To check that erasing is effective,
use a 1-kHz bandpass filter.
2) Obtain the standard setting according to
procedures
2) -
5) of 7-5.
3)
With a -4 dBV/1 kHz (saturation level) signal,
increase the input signal level by 10 dB and
record it on blank tape. Reproduce this to
measure the OUTPUT terminal level decreas­
ing the sensitivity of the VTVM by about 10
dB.
4) Rewind the tape to the start of the recording.
Erase the recorded section with no input
signal.
5) Play back the erased section and measure the
output level, increasing the sensitivity of the
VTVM.
6) Calculate the ratio of the signal output level
to the output level obtained by playing the
erased tape.
7) The value should be more than 65 dB for any
of the tapes to be used.
For the tapes to be used and the switch
settings, refer to Table under paragraph 7-5.
If the erase effect is unsatisfactory, check
whether the erase head is clean and the tape is
running correctly.
NOTE: When checking erasure, make sure the
blank tape used has been erased by a
bulk eraser before using it. Correctly
tu ne the frequency of the signal to be
recorded to match the bandpass filter
and compensate for measuring instru ­
ment loss.
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