Operating instructions

10. Turn the BIAS control on the AUDIO PCB Assembly counterclockwise
until the indication on the VU meter begins to drop. If the control is
already fully counter-clockwise, turn the BIAS control clockwise until the
indication on the VU meter peaks. Then continue clockwise rotation until
the VU meter reading decreases (overbias). Refer to the BIAS CHART
(Table 6-2) to determine the amount of overbias that is correct for your
tape and speed.
11. Turn down the test oscillator if the peak indication is over 0 VU.
12. Repeat Step 10 to re-check the bias alignment before proceeding to the
next channel.
13. Repeat Step 10 for each of the remaining channels 2-24.
14. When all channels have been aligned for proper bias level, unplug the
test oscillator from the TEST SIGNAL jack.
NOTE: For subsequent bias alignments where the only change is to a
different tape formulation, the above procedure need not be followed. Apply
a 10 kHz signal to the TEST SIGNAL jack or from the console, select the 2 or
3 BIAS SELECT switch position on the BIAS CONTROL PCB Assembly and
adjust the corresponding BIAS LEVEL control below the switch while the tape
is recording. In this way, all channels are biased properly with only one
adjustment.
SPEED
TAPE TYPE 30 IPS 15 IPS 7.5 IPS
OVERBIAS dB ALIGN FREQ kHz OVERBIAS dB ALIGN FREQ kHz OVERBIAS dB ALIGN FREQ kHz
SCOTCH 226 +4.0 20 kHz +3.5 10 kHz +6.5 10 kHz
SCOTCH 250 +3.0 20 kHz +3.0 10 kHz +6.5 10 kHz
AMPEX 406 +4.0 20 kHz +3.5 10 kHz +6.5 10 kHz
AMPEX 456 +4.5 20 kHz +4.0 10 kHz +6.5 10 kHz
BASF PEM 468 +1.0 20 kHz +2.5 10 kHz +5.5 10 kHz
BASF 911 +1.5 20 kHz +2.5 10 kHz +5.0 10 kHz
6.2.4 Record Alignment
1. Thread a reel of blank tape on the machine.
2. Connect a test oscillator or, preferably, the oscillator in the mixing
console to the LINE INPUT connectors of the machine.
3. Set the oscillator or console for 0 VU (or the operating level chosen to
correspond to 0 VU, i.e., +4, +6, or +8 dBu) at 700 Hz or 1 kHz.
4. Set the TAPE SPEED switch to the desired speed (HI or LOW) and set the
SPEED MODE switch on the Remote Control Unit to the FIX position.
NOTE: Record Bias alignment should be complete before performing the
following procedures. Refer to § 6.2.3.
5. Engage the ALL REPRO button on the Remote Control Unit, disengage
the ALL SAFE button and set all the READY/SAFE switches to the READY
position.
6. Press the RECORD and PLAY buttons to initiate recording of the test
signal.
AUDIO
BIAS
PHASE
EQ
PHASE
LOW SPD
Table 6-2
Bias Chart
Figure 6-15
Bias Section of Audio PCB Assemblies
Section 6 Alignment
6 - 22
April 1991
MTR-90III Operation and Maintenance Manual