The CD and VCD laser heads are vulnerable parts. The core component of the consumable parts is the laser tube. Many repairers spend tens of dollars directly replacing the laser head after the laser tube is damaged. This is not cost-effective for the customer. Actually, Just spend more than ten yuan to replace the laser tube. The replacement adjustment method is described below.

Laser tube defects often appear as no laser beam emission and the laser beam is very weak. The absence of laser beam emission is caused by excessive maintenance adjustments that cause the laser tube current to exceed 150 mA or electrostatic breakdown of the laser tube. The laser beam is very weak, and the laser output power is gradually reduced due to the long time of use of the laser tube. When the laser tube has the above problems, the machine often does not read the disc, the disc does not rotate or the disc rotates, but the reading directory is long. In the case that other components are normal, the laser tube should be replaced at this moment.

The laser tube includes a laser emitting diode and a laser detecting diode. The two tubes are packaged in a metal case, so the laser tube generally has three pins, one of which has a corresponding symbol (generally corresponding on the tube). The common ground pin, the other two pins are the input of the laser emitting diode and the input of the laser detecting diode. The actual damage is often a laser emitting diode, as long as the replacement of the laser tube can eliminate the problem.

First, testing

Before the replacement, first determine whether the laser tube is actually damaged or aged, and the following methods can be used.

1. Visual inspection method: empty disk test machine, press OPEN/CLOSE button to investigate whether there is dark red laser spot in the objective lens when the box is in place. If yes, compare the strength of laser spot and the speed of laser diode activation with similar normal machines. . Some of the laser diodes of the machine are more aging, sometimes emitting light, sometimes not starting up, and the test should be repeated during the investigation.

2. Resistance method: keep the human body and the workbench outstanding grounding, remove the laser diode, and measure whether the forward and reverse resistance of the laser emitting diode is normal with the multimeter R×1K block. Under normal conditions, the forward resistance is 18~36kΩ, and the reverse resistance is ∞ If the forward resistance exceeds this scale, the laser emitting diode breakdown or malfunction is clarified, and the light intensity potentiometer needs to be replaced or adjusted.

3. Current method: The voltage drop across the load resistor in the laser emitting diode circuit is measured with a multimeter, and then the current flowing through the laser diode is calculated according to I=U/R. There is usually a row of numbers (some with letters) on the label on the laser head to indicate the rated current flowing through the laser emitting diode, such as 50529, which is 50.529 mA, and B725, which indicates a rated current of 72.5 mA. The measured value is increased by 10% of the rated value. If the measured value is greater than or significantly lower than this value or the current of the conditioning light potentiometer is unchanged, it can be concluded that the laser diode is damaged.

4. Power method: directly align the laser head objective lens with the laser power meter probe and then turn it on. The laser power reading should be above 0.1mW. If the amplitude of the RF signal waveform is small and the power meter reading is less than 0.1 mW, the laser diode is aged or damaged.

5. Circuit method: the self-made detection circuit is used for detection, and Figure 1 is a common detection circuit. Connect the components and instruments as shown in the figure, slowly adjust the VR1, so that the current flowing through the laser diode is gradually increased within 35~60mA, and the voltage is about 2V. At this moment, the emission window should have very bright red light, if it is slightly bright or not. It clarifies that the laser emitting diode is aged or damaged.

Second, replace

When disassembling the laser tube, it is necessary to quickly solder the three pins of the laser tube. It is best not to exceed 3 seconds at any time, otherwise the plastic strip base of the cable may be damaged. After welding, unscrew the fixing screw that fixes the laser tube to exit the laser tube. Before the new laser tube is soldered, check whether the forward and reverse resistance of the laser emitting diode is within the normal value. If it is normal, it can be soldered. When welding, the common pin of the laser tube should be aligned with the mark of the cable plastic tape base (a small horizontal strip on the plastic tape base indicates the orientation of the shared pin). After the quick soldering, the laser tube fixing screw can be installed.

Third, adjust

Replace the laser tube, the orientation of the laser beam will be biased. At this moment, take out the three fixing screws that fix the upper part of the laser head (from the bottom up, some are 2), so that the laser head will be divided into two parts, the lower part. The half part is a laser tube, a diffraction grating, a dichroic prism, a quarter-wave plate, a mirror, a cylindrical lens, a photosensitive receiver, etc., wherein the diffraction grating and the cylindrical lens are adjustable in orientation, and the others are fixed. After replacing the laser tube, the diffraction grating and the cylindrical lens should be proofed by placing a 60W incandescent lamp on one side of the photosensitive receiver to illuminate the substrate, and adding 0 to 2.8V to the pin of the laser emission tube. DC voltage, adjust the voltage to 2V, and then use a single-hole high magnification magnifying glass to investigate the orientation of three very bright laser light points in the mirror, and repeat the adjustment of the diffraction grating and the lug of the cylindrical lens until the three spots are respectively It falls on the center of the three square black spots (the single-beam CD machine is a spot) and is in a straight line.

After the laser head is installed, it must be adjusted to make the machine work at its best. The method is to replay the test disk or standard VCD disk, use the oscilloscope to investigate the RF signal waveform, adjust the laser head tangential conditioning screw and the light intensity potentiometer, so that the RF signal waveform is the clearest and the largest, and the whole process is completed.

Fourth, matters needing attention

1. The whole operation process should be carried out under the condition that the human body, the thing, the casing and the worktable are all grounded to prevent the static electricity from penetrating the laser diode.

2. When soldering a new laser tube, use a metal clip to clamp the pin end to short-circuit it to avoid electrostatic breakdown of the laser diode.

3. It is best to use electric power to quickly weld when welding, and use low melting point welding wire to prevent damage to the plastic base belt.

For more information about laser tubes and laser tubes of laser tube manufacturers of Shanghai Shanpu Laser Technology Co., Ltd., please contact Xiaobian.