Technics SL-P222A - Page 38

SL-P222A
2-2.CDM4 mini Optical Pick-up
SPLIT PRISM
rafoc unit CDM4mini
1. FPC 3
2. magnets
3. focus coil leaf spring
4. photodiode PCB
5. LASER protection PCB
6. focus unit
DETAIL OF LASER BEAM
3.Detailled Explanation of the
Functions in the Servo.
3-1-1 The LASER supply
A feature of the CDM4 mini pick-up
is its simple diffraction limited
optics. The laser point source is
focused on the information layer of
the disc by two lenses: a spherical
glass objective with a plastic
aspherical skin, and a spherical
glass collimator.
The LASER supply circuit has a working
principle like the basic schematic
DISC below.
| FOTO >
| FOTO nein >
OBJECTIVE MONITOR é y
LENS DIODE
LASER LIGHT <
COLLIMATOR
LENS
SEMI REFLECTIVE
R
MIRROR
SPLIT PRISM
Mi
PHOTO DETECTOR ;
The LASER supply function integrated in
CDM4 Mini the TDA8808 has as an extra an overload
protection and a turn on _ transient
Furthermore, a semi-transparent protection. The LASER current is
mirror is used instead of a
beam—splitter cube. Astigmatism
introduced into the reflected beam
by the mirror is corrected by a
Plastic component (the wedge) which
also dissects the beam into the two
halves from which the tracking and
focus
error
Signals
are
generated.
regulated by sensing the monitor
photodiode current.
The threshold voltage of the LASER
diode is approximately 1.75V and the
sense voltage is approx. 200mV. It is
not allowed to measure the _ sense
voltage. A distortion at this end of
the loop might cause severe damage to
the LASER diode.
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