Repair method for backlight of liquid crystal display of video camera
Li Xiaowen South China Normal University Guangzhou, Guangdong 510631
Abstract: The aging and damage of electronic components are inevitable. This article introduces the method of replacing high-brightness light-emitting diodes when the cold-cathode tube as the backlight of the liquid crystal display of the camcorder is damaged.
Keywords: liquid crystal display (LCD); backlight; light emitting diode (LED); replacement
There are usually two viewfinders for camcorders (including digital camcorders DV). The larger one is called MONITOR. It is often set on the left side of the camcorder and can be turned to the left. Because of its large size, it can be used at a slightly longer distance. It is comfortable to watch and use. People generally like to use it, so it is frequently used, but it also has the disadvantage that under direct light, the image is blurred and even the image cannot be seen; as a supplement there is a small, called When using EVF, the eyes need to be close to watch, which is convenient for users to use when the outdoor strong light is directly irradiated. Most video camera products in recent years are set up like this, and most of the devices used by MONITOR and EVF are liquid crystal displays (LCDs). The liquid crystal display itself does not emit light. It needs a light source on the back of it. This light source is what we call a backlight. The light of the light source is transmitted through the liquid crystal display, and we can see the image. With the increase of use time and various reasons, the light-emitting device will gradually age. Some video recorders that use cold cathode tube (CCFL) as the backlight, the phenomenon that we can observe when aging is that the image will gradually dim, Later, it will be brighter on one side and darker on the other side in the horizontal direction, and finally will be completely off. At this time, the image is basically not visible on the LCD. But this process is not observable by every machine, and some images will suddenly disappear, such as when the cold cathode lamp is suddenly damaged. Figure 1 is its working principle diagram. Q7503 and Q7504 are driving circuits for LCD backlights, and the operating current is relatively large; T7501 is a step-up transformer with a high voltage, which is 1000 ～ 2000V, because different models use different sizes of liquid crystals The display screen varies, the larger the size of the general LCD screen, the longer the lamp tube used, the higher the required voltage, and damage to its driving and booster circuits is also very common.
1. Confirm whether the components of the backlight are not working properly
Turn on the power of the camcorder, do not turn on the MONITOR, and observe the image of the EVF. If it is normal, you can confirm that the main board of the camcorder is working properly. This confirmation can exclude the misleading of no image due to the lens cover and the iris not opening, and the circuit of the main board. . When the motherboard works normally, turn on MONITOR, keep the camera in a dark place, aim the lens at a place with sufficient light, obvious lines, and good contrast, and continue to swing the machine slightly to observe MONITOR carefully. If you can observe the looming picture, it is basically It can be confirmed that the LCD is normal, but the components of the backlight do not work properly.
Second, confirm whether the backlight tube is damaged
In the state of shutdown, open the shell of MONITOR, you can see there is a circuit board, Figure 1 is a certain type of backlight drive and booster circuit. Refer to the figure to check the operation of the circuit during startup. In the figure, BACKLIGHT is a light-emitting device-cold cathode lamp; Q7501, Q7502 and T7501 are self-excited oscillation, drive amplification and boost parts. Q7504 and Q7503 are the control part of the LCD BACKLIGHT circuit. BL CONT terminal is the entrance of the control signal. When the MONITOR is closed, the control terminal signal is low, Q7504 and Q7503 are not closed, and Q7501 and Q7502 are not normal. The working bias current does not vibrate, the backlight circuit does not work, and pins 6 and 10 of the T7501 have no high-voltage output (approximate sine wave); when MONITOR is turned on, the control terminal signal is high, Q7504 and Q7503 are turned on, and the BACKLIGHT circuit works. Use the AC file of the oscilloscope or digital electronic watch at the moment of booting (ordinary multimeter is not available, because its internal resistance is not large enough and the frequency is not high enough, which will affect the normal operation of the circuit). Check pins 6 and 10 of T7501 or TL7501 and TL7502 to confirm If there is a sufficiently large waveform (peak-to-peak value) and operating voltage, if it is insufficient or not, please refer to the circuit diagram given in Figure 1 and check the fault location of the operating voltage. This article will no longer cover this aspect of the inspection; if the voltage is normal, you can Confirm that the backlight lamp is damaged. The reason why it is emphasized to check at the moment of booting is that some circuits are designed with a protection function (not shown in Figure 1). When the circuit works abnormally, the circuit is forced to stop vibration, and the measured value at this time may cause erroneous judgment.
Third, the replacement method to repair the components of the backlight
In general, it is best to repair the original circuit, but due to the specialization of some components, the manufacturer's supply also has a certain time limit. After this time limit, it is difficult to find if the manufacturer does not supply, as shown in Figure 1. T7501 and BACKLGHT lamps. When you can't find the corresponding components, you can try to use high-brightness light-emitting diode (LED) circuit components to replace and repair.
Referring to Figure 2, a small piece of universal or self-made printed circuit board is used to make a driver circuit board except for the high-brightness light-emitting diode in the dotted frame according to the circuit shown in the figure. Disconnect R7504 in Figure 1 (L7502 in this figure, * No. means unused), the three wires are connected to the corresponding points of the original backlight circuit, find a place with a larger space to fix the circuit board, and pay attention to insulation isolation, not short-circuit; refer to the position shown in Figure 3 Remove the damaged BACKLIGHT tube from the light guide plate (the tube is mostly fixed with self-adhesive cloth, it is not difficult to remove), and fix the light emission of the high-brightness light-emitting diode in the dotted line to the light guide plate evenly fixed to the original tube In terms of location, it is sufficient to connect the pins of the light-emitting diodes and the corresponding points of the self-made driving circuit with wires.
To be careful of:
(1) The internal space of the liquid crystal display is very small, all components must use the patch type, and the circuit board should also be as small as possible;
(2) The number of light-emitting diodes depends on the size of the LCD screen. When the LCD screen is large, the number should be increased so that the interval is not too large. Otherwise, the brightness of the LCD display may be uneven, affecting the image display effect ;
(3) The power supply voltage of the original backlight drive circuit is not necessarily 5V. The current of the light-emitting diode can be adjusted by adjusting the resistance of R8001 and R8002. Generally, the working current of the light-emitting diode is about 20mA, and the brightness of the light can be satisfied. Claim;
(4) With reference to Figure 4, increasing the current will increase the luminous intensity of the LED, but the increase will not be too much. If the current is too large, it may increase the amount of heat generated by the LED and affect the service life.
If the requirements are not very high, it is also possible to simplify the circuit of FIG. 3 to the circuit shown in FIG. 5, at this time, the operating current of each light-emitting diode must be adjusted by adjusting R8041, R8046, R8049, and R8052. The resistance was changed to 47Ω, which may be more appropriate. In order to improve reliability, at this time, the power of Q8022 should be considered to increase.
Dinining Chair ,Nice Dining Chairs,Cute Dining Table Set,Dinner Room Chairs
Feat Top International(China) CO.,LTD , https://www.ft-project.com