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Laser tube faults usually show that no laser beam is emitted and the laser beam is very weak

Views:2     Author:MC      Publish Time: 2021-08-31      Origin:MC

CD, VCD laser head is a vulnerable parts, the core component of the vulnerable parts is the laser tube, many maintenance in the laser tube after damage to spend dozens of yuan directly replace the laser head, which is not cost-effective for consumers, in fact, as long as the cost of replacing the laser tube can be more than ten yuan. The following describes the method of replacement and adjustment.

Laser tube faults usually show that no laser beam is emitted and the laser beam is very weak. No laser beam is emitted because the current of laser tube exceeds 150mA or electrostatic breakdown laser tube due to excessive maintenance and adjustment. The laser beam is very weak and the output power of laser is gradually reduced because the laser tube is used for too long. When the laser tube has the above faults, the machine often does not read the disk, the optical disc does not turn or the optical disc rotates but reads the directory for a long time. In the case of normal other parts, the laser tube should be replaced at this time.

Laser tube containing a laser emitting diode and a diode laser detection, two tube enclosed in a metal casing, so the laser tube usually have three pins, one of the pins (generally in the tube and shell have corresponding tags) for public grounding pin, the other two pins for laser diode input input end of feet and laser detection diode. The actual damage is often the laser emitting diode, as long as the replacement of the laser tube can be eliminated.

A, detection,

Before replacement, determine whether the laser tube is really damaged or aging. The following methods can be used.


1. Visual inspection method: Empty disk test machine, press OPEN/CLOSE button, observe whether there is dark red laser point in the objective lens at the moment when the box is in place. If so, compare the strength of laser point and the speed of laser diode activation with similar normal machine. Some of the laser diode aging is serious, sometimes can light, sometimes do not light on the observation should be repeated test machine.

2. Resistance method: Keep good grounding body and workbench, remove the laser diode, measured with a multimeter R x 1 k block direction of laser diode resistance is normal, normal circumstances forward resistance in 18 ~ 36 k Ω, reverse resistance is up, if the forward resistance than this range of laser diode breakdown or performance is bad, need to change or adjust intensity potentiometer.

3. Current method: use a multimeter to measure the voltage drop of the load resistance in the laser emitting diode circuit, and then calculate the current flowing through the laser diode according to I=U/R. The label on the laser head generally has a line of numbers (some with letters), used to indicate the rated current flowing through the laser emitting diode, such as 50529 is 50.529mA, B725 is 72.5mA rated current. The measured value is up to 10% of the rated value as the upper limit, if the measured value is greater than or significantly lower than this value or adjust the light intensity potentiometer current no change, can determine the laser diode has been damaged.

4. Power method: the laser power meter probe is directly aimed at the laser head objective lens and then switched on, and the laser power reading should be above 0.1MW. If the RF signal waveform amplitude is small and the power meter reading is less than 0.1mW, it indicates that the laser diode is aging or damaged.

5. Circuit method: that is, self-made detection circuit for detection. Figure 1 is a commonly used detection circuit. According to the picture, connect the components and instruments, slowly adjust VR1, so that the current flowing through the laser diode gradually increases in 35 ~ 60mA, the voltage is about 2V, at this time the emission window should be very bright red light, if the light or no light indicates that the laser emitting diode aging or damage.

Second, the replacement of

When removing the laser tube, quickly weld the three pins of the laser tube, the time had better not exceed 3 seconds, otherwise it is easy to damage the plastic band base of the cable. After welding, unscrew the fixing screw of the laser tube to exit the laser tube. Before welding the new laser tube, check whether the positive and negative 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 plastic base of the cable (there is a small horizontal bar on the plastic base to indicate the position of the common pin). After quick welding, the fixing screw of the laser tube can be installed.

Third, adjust the

Replace the laser tube, the position of the laser beam will produce deviation, at this point out three fixed in the fixed laser head part of the screw (seen from down to up, some for 2), the laser is divided into fluctuation two parts, the lower part of the laser tube, diffraction grating and dispersion prism, a quarter wavelength, reflector, cylindrical lens, optical receiver, etc., The position of diffraction grating and cylindrical lens can be adjusted, the other are fixed. Replaced after laser tube to make a correction of diffraction grating and the cylindrical lens, its method is to put the 60 w incandescent light sensitive receivers irradiation on only one side of the substrate, the laser tube pin with 0 ~ 2.8 V dc voltage, the voltage will be dispatched to 2 V, with a single high power magnifying glass to observe three bright reflection in the mirror of the laser light spot position, Adjust the lugs of the diffraction grating plate and the cylindrical lens repeatedly until the three spots fall in the center of the three square spots (single beam CD machine is one spot) and form a straight line.

After installing the laser head, it must be adjusted to make the machine work in the best condition. The method is to replay the test disk or standard VCD disk, observe RF signal waveform with oscilloscope, adjust the laser head tangential adjustment screw and light intensity potentiometer, RF signal waveform is the most clear, the maximum amplitude, the whole process is completed.

Four, notes

1. The whole operation process should be carried out under the condition that the human body, tools, chassis and workbench are well grounded to prevent electrostatic breakdown of laser diode.

2. When welding the new laser tube, the application of metal clip to clamp the pin end will be short circuit, can avoid electrostatic breakdown of laser diode.

3. During welding, it is best to cut off power for fast welding and choose low melting point welding wire to prevent damage to plastic base band.

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