Fostex CR200 Service Manual page 21

[ How to find the null point ]
When you insert the small screwdriver into the slit for the grating adjustment and change the grating angle, the amplitude
of the tracking error signal at CN104 (TP1), Pin 3 (TE) changes. Within the range for the grating, there are five or six
locations where the amplitude of the wave reaches a minimum. Of these five or six locations, there is only one at which
the envelope of the waveform is smooth. This location is where the three laser beams divided by the grating are all right
above the same track. (See Fig. 13.)
This point is called the null point. When adjusting the grating, this null point is found and used as the reference position.
Turning the grating counterclockwise
Grating adjustment position
4.2.5 DPP (Tracking Offset) Adjustment
• Objective
• Symptom when out of adjustment
• Measurement instrument connections Oscilloscope to CN104, Pin-3 (TE)
• Disc
• Player state
• Adjustment location
[ Procedures ]
(1) Move the pickup to the midway across the disc (R = 35mm) with the MANUAL / TRACK SEARCH FWD or REV keys.
(2) Press the FINALIZE key and the PLAY key in the order to close the focus servo and then spindle servo.
(3) Line up the bright line (ground) at the center of the oscilloscope screen and put the oscilloscope into DC mode.
(4) Adjust VR112 (TE. OFS) so that the positive amplitude and negative amplitude of the tracking error signal at CN104,
Pin-3 (TE) are the same (in other words, so that there is no DC component).
Turning the grating clockwise
Null point
Waveform of
CN104 (TP1), Pin 3 (TE)
Fig. 13
To correct for the variation in the sensitivity of the tracking photodiode.
The unit does not playback, track search is impossible, tracks are skipped.
[This connection must be via a low pass filter (15kW + 0.001µF). ]
GND probe can be connected to CN104, Pin-4 (AGND).
[ Settings ]: 50 mV/div., 5 mS/div., DC mode
TCD-785
Test mode, focus and spindle servos: closed, tracking servo: open.
VR112 (TE. OFS) (HEAD BOARD assy)
A = B
A
GND
B
When there is a DC
component
Null point waveform
Note : If the difference between the amplitude of the error signal at the
innermost edge and outermost edge of the disc is more than 10%,
adjust the grating again.
Fig. 4
A
B
When there is no DC
component
Fig. 14
Maximum amplitude Waveform other than the
waveform null point
A = B
GND
CR200
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