35
Servo Processing
There are several main servos that can be tested for normal operation.
Sled Motor Drive
When the power On button is pressed, the sled motor is driven inward
until the optical assembly that is connected to it closes the S101 limit
switch. This closure places 5V at IC101/pin 27 so it may turn on the
servos to begin the disc playback sequence.
Laser
The Laser servo is the first to be turned on when the S101 limit switch is
closed and the loading switch is in the (disc) chucked position. IC101/pin
14 outputs a HIGH (5V) to RF Amp IC103/pin 22 to enable the laser servo.
The laser diode and light sampling photodiode are in the optical assembly
and the remainder of the regulating circuit is in IC103 with Q101 as an
external amplifier. IC103/pin 3 drops from 5V to about 3V to turn on the
laser. Q101 turns on, supplying current to the laser diode at pin 10 of the
optical pick-up ribbon cable connector. The laser diode will turn on when
there is sufficient current to drop about 1.8 to 2.2V at the diode.
Precautions
Although not recommended, if you must apply an external voltage to check
the diode, you must current limit this with a 1k-ohm resistor to prevent
laser diode damage. NEVER LOOK AT THE LASER DIODE LIGHT.
Use a laser power meter to measure the intensity of this class 1 laser.
Also, if your ohmmeter outputs more than 2V, do not check the laser
diode for diode continuity.
The laser intensity is monitored by a photo diode in the optical assembly.
The PD output into IC103/pin 4 is used to regulate the LD output from pin
3.
Laser Diode Control Voltages
Laser IC103/pin 3 Q101/Collector IC103/pin 4
OFF 4.76V 0V 0V
ON 3.17V 1.98V 0.116V
Focus
The focus search operation is also initiated when the disc is chucked in
the CD Mechanism (S801 not shown) and the limit switch (S101) is closed.
Servo Control IC101 generates the electrical signal that results in the
visible lens movement in the optical assembly.
The changeover from search to servo lock occurs when RFO signal is
present and the FE “S curve” shaped signal voltage crosses to/through
Vc voltage (half of Vcc. Vc = 2.5V). At this recognition time, FOK at
IC101/pin 24 goes HIGH to mark the transition to servo operation. If this
transition were inhibited so only the search operation took place, the RFO
and FE waveforms would look like this:
ch2
ch1
CH1 1.00 V= PKD
CH2!2.00 V= MTB2.00ms- 3.10dv ch1+
1
2
T
S Curve focus Search (Changer is in ADJust Test Mode)
Channel Signal Name Location
1 RFO test point IC103/pin 16
2 FE test point IC103/pin 14
Time base = 2msec/div
Scope ground is taken at Vc (IC103/pin 12)
Normally as the FE (Ch 2) signal would cross through Vc, IC101 would
start focus servo and the top half of the FE waveform (above the Vc base
line) would never take place.
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