Operating instructions

6.1.10 Capstan Servo Adjustments
NOTE: Refer to the following illustrations:
Figure 6-11 for Capstan Tachometer Adjustment
Figure 6-12 for CAPSTAN CONTROL PCB Assemblies Adjustments
6.1.10.1 Capstan Tachometer Adjustment
1. Turn off power to the machine. Extend the CAPSTAN CONTROL PCB
Assembly (board #2). Turn on the power.
2. Check the output of the capstan tach with a dual-trace oscilloscope
connected to TP1 and TP2 of the TACHO SENSOR AMP PCB Assembly
mounted under the capstan motor (Figure 6-11A). The signals at TP1
and TP2 should be no more than 50% different in amplitude from each
other. If they are correct, skip Step 3 and continue on to Step 4.
3. Adjust the position of the capstan tach sensor to match the
specifications in Step 2. Adjustment of the amplitude will also affect the
phase relationship between the two signals (Figure 6-11A). The tach
sensor is accessible by opening the meter panel and removing the
capstan motor. Do not confuse this small circuit board attached to the
front of the capstan motor with the tachometer roller and related
circuitry located on the Supply side of the head assembly. To remove
the capstan motor, remove the Transport Cover Plate and the three allen
screws that secure the motor to the deck plate. Leave the capstan motor
connected to the machine and support it with an object to allow it to be
operated.
4. Initialize the reel servos and put the machine into Play mode. Observe
the waveforms at TP1 and T 2 on the TACHO SENSOR AMP PCB
Assembly with the dual-trace oscilloscope. Trigger the scope trace from
the signal at TP1. Adjust VR101 and VR102 until the tach pulse
waveform has a 50% duty cycle (Figure 6-11A).
The phase of the signal at TP1 should lead the signal at TP2 by 90° when
in Play mode. If it does not, remove the capstan motor and adjust the
position of the capstan tacho sensor (Step 3). Adjustment of the phase
will also affect the amplitude of the signal. It may be necessary to repeat
Steps 3 and 4 several times until both amplitude and phase relationships
are correct.
TP-1
TP-2
50% 50%
90
90
TP-1
TP-2
A) OUTPUT WAVEFORM
APPROX. 6-14 VP-P
APPROX. 5 VP-P
Figure 6-11
Capstan Tachometer Adjustment
B) LOCATION OF THE CAPSTAN
TACHOMETER PCB ASSEMBLY
Section 6 Alignment
6 - 16
April 1991
MTR-90III Operation and Maintenance Manual