CR200
[ 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 counterclockwiseTurning the grating clockwise
Null point waveformMaximum amplitudeWaveform other than the
waveformnull point
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.
Null point
Grating adjustment position
Waveform of
CN104 (TP1), Pin 3 (TE)
Fig. 4
Fig. 13
4.2.5 DPP (Tracking Offset) Adjustment
? ObjectiveTo correct for the variation in the sensitivity of the tracking photodiode.
? Symptom when out of adjustmentThe unit does not playback, track search is impossible, tracks are skipped.
? Measurement instrument connections Oscilloscope CN104, Pin-3 (TE)to
[This connection must be via a low pass filter (15kW + 0.001?F). ]
GND probe can be connected to CN104, Pin-4 (AGND).
[ Settings ]: 50 mV/div., 5 mS/div., DC mode
? DiscTCD-785
? Player stateTest mode, focus and spindle servos: closed, tracking servo: open.
? Adjustment locationVR112 (TE. OFS) (HEAD BOARD assy)
[ 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).
A = BA = B
AA
GND
GND
BB
When there is a DCWhen there is no DC
componentcomponent
Fig. 14
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ACEC
