Technics SL-P202A - Page 37

2-2.CDM4 mini Optical Pick-up
rafoc unit CDM4mini
1, FPC 3
2, magnets
3. focus coil leaf spring
4. photodiode PCB
5. LASER protection PCB
6. focus unit
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.
DISC
OBJECTIVE
LENS
COLLIMATOR
LENS
' SEMI REFLECTIVE
IRROR
MIR
eur PRISM
ryyt
\y iM
p+} PHOTO DETECTOR
CDM4 Mini
Furthermore, a semi-transparent
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.
| FOTO
| FOTO ON
MONITOR &
DIODE LASER LIGHT
SPLIT PRISM
DETAIL OF LASER BEAM
3.Detailled Explanation of the
Functions in the Servo.
3-1-1 The LASER supply
The LASER supply circuit has a working
principle like the basic schematic
below.
ANVNV
The
LASER
supply
function
integrated
in
the
TDA8808
has
as
an
extra
an
overload
protection
and
a
turn
on
transient
protection.
The
LASER
current
is
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.
48