DEH-1400,14
6.2 CHECKING THE GRATING AFTER CHANGING THE PICKUP UNIT
Y Note :
The grating angle of the PU unit cannot be adjusted after the PU unit is changed. The PU unit in the CD mecha-
nism module is adjusted on the production line to match the CD mechanism module and is thus the best adjusted
PU unit for the CD mechanism module. Changing the PU unit is thus best considered as a last resort. However, if
the PU unit must be changed, the grating should be checked using the procedure below.
Y Purpose :
To check that the grating is within an acceptable range when the PU unit is changed.
Y Symptoms of Mal-adjustment :
If the grating is off by a large amount symptoms such as being unable to close tracking, being unable to perform
track search operations, or taking a long time for track searching.
Y Method :
Y Measuring EquipmentY Oscilloscope, Two L.P.F.
Y Measuring PointsY E, F, VREF
Y DiscY ABEX TCD-784
Y ModeY TEST MODE
CONTROL UNIT
L.P.F.
XchYch
E100k&
Oscilloscope
VREF390pF
F100k&
VREF390pF
VREFL.P.F.
EF
Y Checking Procedure
1. In test mode, load the disc and switch the 5V regulator on.
2. The display will change, returning to "81" on the fourth press.
3. As shown in the diagram above, monitor the LPF outputs using the oscilloscope and check that the phase differ-
ence is within 75! . Refer to the photographs supplied to determine the phase angle.
4. If the phase difference is determined to be greater than 75! try changing the PU unit to see if there is any
improvement. If, after trying this a number of times, the grating angle does not become less than 75! then the
mechanism should be judged to be at fault.
Y Note
Because of eccentricity in the disc and a slight misalignment of the clamping center the grating waveform may be
seen to "wobble" ( the phase difference changes as the disc rotates). The angle specified above indicates the aver-
age angle.
Y Hint
Reloading the disc changes the clamp position and may decrease the "wobble".
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ACEC
