CD and VCD laser heads are vulnerable parts, and the core component of the vulnerable parts is the laser tube. Many repairmen directly replace the laser head after the laser tube is damaged. This is not cost-effective for customers. In fact, it only costs more than ten yuan to replace the laser tube. The replacement adjustment method is described below.
The defects of the laser tube are usually no laser beam emitted and the laser beam is very weak. No laser beam emission is mostly caused by excessive maintenance and adjustment, which makes the laser tube current exceed 150mA 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 use time of the laser tube. When the laser tube has the above problems, the machine often does not read the disc, the CD does not rotate or the CD does not rotate, but the reading time is long. If other parts are normal, the laser tube should be replaced at this moment.
The laser tube consists of a laser emitting diode and a laser detection diode. The two tubes are packaged in a metal casing. Therefore, the laser tube generally has three pins, one of which (usually with the corresponding symbol on the tube shell) is the common grounding pin, and the other two pins are the input pin of the laser emitting diode and the input terminal of the laser detection diode. The actual damage is often the laser emitting diode, so long as the laser tube is replaced, the problem can be eliminated.
1、 Detection
Before replacement, the following methods can be used to judge whether the laser tube is actually damaged or aged.
1. Visual inspection: test the machine with an empty disk and press the open / close key to investigate whether there are dark red laser spots in the objective lens at the moment when the box is in place. If so, compare the strength of the laser points and the activation speed of the laser diode with those of the same type of normal machine. The laser diodes of some machines are aging seriously, sometimes they can emit light, sometimes they can't turn on, so they should be tested repeatedly during the investigation.
2. Resistance method: maintain the excellent grounding between the human body and the working table, remove the laser diode, and measure whether the forward and reverse resistance of the laser emitting diode is normal with a multimeter R × 1K. Under normal conditions, the forward resistance is 18 ~ 36K Ω, and the reverse resistance is ∞. If the forward resistance exceeds this scale, it shows that the laser emitting diode is broken down or has poor function, and the light intensity potentiometer needs to be replaced or adjusted.
3. Current method: use a multimeter to measure the voltage drop on the load resistance in the laser diode circuit, and then calculate the current flowing through the laser diode according to I = u / R. The label on the laser head usually has a line of numbers (some with letters) to indicate the rated current flowing through the laser diode. For example, 50529 is 50.529ma, and b725 is 72.5ma. The upper limit of the measured value is 10% of the rated value. If the measured value is greater than or significantly lower than this value or the current of the regulated light intensity potentiometer does not change, it can be concluded that the laser diode has been damaged.
4. Power method: use the probe of laser power meter to directly aim at the objective lens of laser head, and then turn on the machine. The laser power reading should be above 0.1MW. If the amplitude of RF signal waveform is small and the power meter reading is less than 0.1MW, the laser diode is aging or damaged.
5. Circuit method: the self-made detection circuit is used for detection. Figure 1 is the common detection circuit. Connect the components and instruments according to the diagram, and adjust VR1 slowly, 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 does not emit light, it indicates that the laser emitting diode is aging or damaged.
2、 Replacement
When removing the laser tube, the three pins of the laser tube should be quickly welded, and it is better not to exceed 3 seconds at any time, otherwise the plastic tape base of the cable will be easily damaged. After welding, screw out the fixing screw of the laser tube to exit the laser tube. Before welding the new laser tube, check whether the forward and reverse resistance of the laser emitting diode is within the normal range. If it is normal, it can be welded. During welding, the common pin of the laser tube should be aligned with the mark of the cable plastic tape base (there is a small bar on the plastic tape base to indicate the orientation of the common pin). After rapid welding, the fixing screw of the laser tube can be installed.
3、 Adjustment
If the laser tube is replaced, the orientation of the laser beam will deviate. At this moment, take out the three fixing screws (two from the bottom up) that fix the upper half of the laser head, so that the laser head can be divided into two parts: laser tube, diffraction grating, beam splitting prism, quarter wavelength plate, reflector, cylindrical lens, photosensitive receiver, etc Bits are adjustable, others are fixed. After replacing the laser tube, it is necessary to calibrate the diffraction grating and cylindrical lens by placing a 60W incandescent lamp on one side of the photosensitive receiver to irradiate its substrate, adding 0-2.8v DC voltage to the pin of the laser transmitting tube, adjusting the voltage to 2V, and then adjusting the orientation of three bright laser light spots in the reflector with a single hole high magnification magnifier, and repeatedly adjusting the diffraction grating plate and cylindrical transmission The lug of the mirror until the three light spots respectively fall on the center of the three square black spots (single beam CD player is one light point) and form a straight line.
After installing the laser head, it is necessary to adjust it to make the machine work in the best condition. The method is to replay test disk or standardize VCD disk, use oscilloscope to investigate RF signal waveform, adjust laser head tangential adjustment screw and light intensity potentiometer to make RF signal waveform clearest and maximum amplitude, and the whole process is completed.
4、 Precautions
1. The whole operation process should be carried out under the condition that human body, things, shell and workbench are well grounded to prevent electrostatic breakdown of laser diode.
2. When welding a new laser tube, clamp the pin end with a metal clip to short circuit it,