Operating instructions

6.2.6 Erase Current Adjustment
1. Turn off the power to the machine.
2. Extend the BIAS CONTROL PCB Assembly (Board #7).
3. Turn on the power to the machine.
4. Use an oscilloscope to monitor the waveform at TP2 on the BIAS
CONTROL PCB Assembly with the group lead of the scope connected to
a GND test point on the PCB Assembly.
5. Adjust VR-4 on the BIAS CONTROL PCB Assembly for 5 V P-P
waveform.
6. Turn off the power to the machine.
7. Remove the Extension PCB Assembly and reinstall the BIAS CONTROL
PCB Assembly to its correct slot.
8. Extend the CH 1 AUDIO PCB Assembly.
9. Turn on the power to the machine.
10. Thread the machine with tape, engage the servo system and put all
tracks into Record Mode.
11. Connect an oscilloscope to TP2 and the adjacent GND point on the
AUDIO PCB Assembly.
12. Turn VR401 on the AUDIO PCB Assembly (located near the bottom rear
of the PCB Assembly) slowly clockwise until a triangle wave just starts to
appear. Then turn VR401 counterclockwise until the top and bottom
peaks just round out. This should set the peak to peak amplitude to
approximately 3 V.
NOTE: If a triangle waveform is already present at TP2, turn VR401
counterclockwise first and then adjust for a triangle waveform as above.
13. Repeat Steps 8 through 12 for all 24 channels to complete the
procedure.
6.2.7 Gain Structure
Figure 6-17 is a block diagram of the audio signal path for one channel for
the 24-track MTR-90III (8 and 16 track versions may differ slightly). On it
are indicated the nominal signal levels in dBu (ref. 0 dB=0.775 V RMS) given
a 1 kHz signal with NAB EQ at 30 ips.
Section 6 Alignment
6 - 24
October 1990
MTR-90III Operation and Maintenance Manual