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When the PLAY key is pressed, the FOCUS ON
command
from the CONT
board is applied to the
VSOP
board,
while at the same time, the LASER
ON command
is applied to the LSPS board, result-
ing in the laser generator being switched on. Due to
the application
of the FOCUS
ON
command
the
Q5 inverter
output
is switched to H level, resulting
in this output combining with the Q7 H level out-
put to turn the Q2 analog switch on. And due to
the QS
output.
53 is inverted
and а *30mV
DC
voltage is applied to the Z2 (1/2) input. The circuit
constituted
Бу D7, 18. and C11
is for applying a
pulse differentiated
by a time
constant
of СІІ,
R52
and
R53
to Z2 (1/2). and overates for de-
creasing a static frictional forse of the focus motor
while beginning of the focus motor raising opera-
tion,
and
also
increasing
a
focus
motor
torque
lowered
by unbalanced
focus bridge. This *30mV
DC voltage is amplified in Z2 (1/2) in order to raise
the
focus
motor
to
the
required
position.
The
counter electromotive
force appearing between the
focus
motor
terminals
at this
time as detected
(level detection)
by the
Z2 (2/2) counter electro-
motive
force detector circuit via a bridge circuit,
and is then applied to the Z2 (2/2) amplifier as a
negative feedback signal. The focus motor can thus
be raised at a constant velocity.
When
the focus
motor
is raised to about 60
um
below
the proper focusing position, tracking
error
will
commence
to
appear.
This
tracking
error is generated in А and B forms, and appears as
an output from the КРАМ
board.
These signals are
added and applied to the Z3 (1/2) dise sensor com-
parator input.
Phe output from the Z3 (2/2) zero-
Cross
detector
is an
H level output,
and increases
the disc sensor comparator
threshold
level in the
(+) direction.
When
the tracking error level is in-
creased
to a value in excess of the threshold
level,
the output of Z3 (1/2) is switched to H level. The
reason
[юг
increasing
the
disc
sensor
comparator
threshold
level
at this
time
is to eliminate
the
effect of the reflected beam from the dise surface
(protective plastie surface).
When the objective lens is raised further, focus
error is generated, resulting in À and B focus error
signals from the
RF AM
board being applied to the
Zl
(1:2)
differential
amplifier
input.
The
focus
error appearing at the output of the Z1 (1/2) am-
plifier
is applied
to
the
zero-cross
detector
(—)
input. If the objective lens is raised still further, the
focus error change will reach the non-linear section
of the S-shaped curve, and at the point where the
threshold level determined by the (+) input voltage
is exceeded,
the detector output will be switched
to L level. The loop flip-flop outputs Q7 and Q6
will thus be switched to L and H levels respectively,
Fig. 11 Focus servo timing chart
thereby closing the Q1 loop switch, and resulting
in the closure of the focus servo.
At the same time, the zero-cross detector thres-
hold level is raised in the (+) direction, and the disc
sensor
comparator
threshold
level is reduced
by
about *0.2V, followed by the FOCUS LOCK com-
mand
signal being sent to the CONT
board.
Апа
thereafter, the focus servo mechanism
is in normal
operating mode.
Normal Operation
Focus error A, B is amplified by a differential
amp
consisting
of Z1
(2/2).
УҢ]
serves
as the
overall voltage adjustment
for focus offset caused
by the RFAM
section
DC
offset voltage,
photo
detector
sensitivity
corrections,
slight deviations
from the optical focus point, and other factors.
The focus error that appears in the output of
Z1
(1/2)
passes through
VR2
to set the loop gain
and then enters the AGC
circuit.
When
the loop
gain is raised
above
the level normally
required,
noise generated
in the system is caused by the in-
creasing amount
of static generated
by the focus
motor.
Also,
the system
becomes
susceptible
to
17
PR-8210-A
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