Sharp DV-NC70U Service Manual - Page 14

DV-NC70U/C DV-NC72U 7. OPERATION OF PICKUP 7-1. CIRCUIT CONFIGURATION OF PICKUP The pickup unit reads signals from the disk, and the flexible cable is connected to the board. The following signals flow through the cable. 7-2. EQUIVALENT CIRCUIT OF PICKUP 7-3. POLARITIES OF SIGNAL FO+ TR- TR+ FO- ACTUATOR HIGH-FREQUENCY CONVOLUTION VOS OSC LD PD - VA + - VB + - VC + - VD + Vref Vcc GND HOLOGRAM LASER UNIT Power adjust 1 TR- 2 FO+ 3 TR+ 4 FO- 5 VR 6 PD 7 VCC 8 GND (NC) 9 LD 10 GND 11 VOS 12 NC 13 NC 14 Vref(VCC/2) 15 VA 16 VD 17 NC 18 GND Focus FO+, FO- Tracking TR+, TR- When electric current is flowed from FO+ to FO-, the lens comes to near the disk. When electric current is flowed from TR+ to TR-, the lens goes toward the outer circumference. 19 VB 7-4. SIGNALS OF PICKUP 20 VC 1 7-4-1. Tracking drive signal (TR+, TR-) The signal drives the tracking servo mechanism which projects the beam on the track by moving the objective lens (OL) to the outer or inner circumference (at a right angle against the track) of the disk. 7-4-2. Focus drive signal (FO+, FO-) The signal drives the focus servo mechanism which aligns the focus on the pit of the disk by elevating OL (vertically against the disk surface.) 2-focus objective lens The VR terminal is connected to GND. 7-4-3. Monitor Diode (PD) Actuator assembly Since the laser diode largely varies output of the laser light even if the flowing current is slightly varied, the projection light is detected with the monitor diode to control the laser light to be equally output. Since the current varies on the monitor diode according to the intensity of the received light of the laser diode, the drive current of the laser diode is reduced if the current of the monitor diode increases. On the contrary, the drive current of the laser diode is increased if the current of the monitor diode decreases. As the projection light of the laser diode becomes stronger, the current of the monitor diode increases to increase the current which flows into the monitor diode output (PD). This is input to the pin 44 of IC301 and is compared with the reference voltage to control the drive current of the laser diode. LD The circuit is called ALPC (Automatic Laser Power Control). PD When the quantity of laser light increases, the current shown in figure increases and the PD terminal voltage rises. 7-4-4. Laser diode drive current control (LD) Power supply to drive the laser diode IC301 is used to control. The LD terminal voltage lowers, and the quantity of light reduces. (IC301 is actuated by voltage input.) 7-4-5. High-frequency convolution module power supply (VOSC) The high-frequency convolution imposes the high-frequency signal on the DC current to impose the high frequency on the drive current of the laser. Thus, the interference of outgoing light and reflected light is prevented. 7-4-6. HF Signal (VA, VB, VC, VD) Signals recorded in the disk 14