Overhaul method
1. When repairing the backlight board, you should master the characteristics and luminous principle of the backlight tube, the composition and working principle of the backlight board. Although the shape of the backlight plate and the component model composition are different, the basic principle is the same. As long as there is a theoretical basis, we can analyze the faults of the backlight board, and there are only 3-4 kinds of faults. The principle of high voltage output and power amplification circuit of backlight board is simple, and full bridge/half bridge drive is mostly used, and MOS tube or power module is generally used.
2. The maintenance of the backlight board must first ensure that the power supply part works normally. That is, the standby power supply is 3.3V or 5V, 12v and 24v, the PFC circuit works normally, and the backlight system can be overhauled.
3. When overhauling, you can remove the wire from the power board to the motherboard and connect the standby voltage of 5V to the three pins of STB (standby control), BRI (dimming) and BL-ON (backlight control) respectively. When it is normal, the backlight board should be able to emit light normally. This step can determine whether it is a motherboard problem or a power board problem.
4. If the backlight is still off, it means that no pulse signal is applied to the backlight inverter. When we overhaul the backlight circuit, we can regard the backlight system as a line scanning circuit.
5. The backlight control chip has a feature that it is not controlled and protected by any feedback pin for about 2 seconds when the chip is powered on, and it has an output pulse, which will enter the normal working state after getting the normal feedback signal from its detection pin after starting. If there is no feedback signal or the feedback signal is abnormal after startup, the chip will enter the protection state and stop outputting the excitation signal.
6. Using this characteristic, firstly, when the backlight control IC is powered on, the voltage output of the pulse output pin is detected. If possible, you can use an oscilloscope to measure the signal waveform of the pulse output pin, or use the AC voltage of a multimeter to detect the pulse output.
According to the voltage value here, we can judge whether the fault point is in the circuit with the backlight control chip as the core or in the subsequent circuit. (Note that after chip protection, it will be output about 2 seconds after power failure and restart. )
7. If there is a signal output, it can be judged that the fault is in the rear drive part.
Otherwise, the fault lies in the control part of the backlight chip.
We can measure whether there is AC excitation signal output at the output end of the excitation transformer at the next stage (because the working frequency of the backlight circuit is about 56KHZ, which exceeds the frequency response of our common multimeter, and there is no accurate voltage value. Generally, the digital meter is about 30 volts, and the mechanical meter is about ~ 10 volts. It should be emphasized that no matter what the voltage value is, the voltage values obtained by the two windings must be the same, otherwise there will be problems.
8. When the output pulse of the excitation transformer is normal, we will continue to measure step by step until we find the short circuit point of the pulse.
The above-mentioned maintenance process is a quick and effective maintenance method to repair the backlight circuit that is not lit after starting.
Extended data
The biggest difference between liquid crystal and plasma is that liquid crystal must rely on passive light source, and plasma TV belongs to active luminous display equipment. At present, the mainstream LCD backlight technologies on the market are LED (Light Emitting Diode) and CCFL (Cold Cathode Fluorescent Lamp).
CCFL (cold cathode fluorescent lamp) backlight is the most important backlight product of LCD TV at present. Cold cathode fluorescent lamp, namely CCFL (cold cathode fluorescent lamp) or CCFT (cold cathode cathode fluorescent lamp).
Its working principle is that when high voltage is applied at both ends of the lamp tube, a few electrons in the lamp tube collide with the electrode at high speed to produce secondary electron emission and start to discharge. After the mercury or inert gas in the tube is impacted by electrons, it excites and radiates 253.7nm ultraviolet light, and the generated ultraviolet light excites the fluorescent powder coated on the inner wall of the tube to generate visible light.
The service life of CCFL lamp is generally defined as: when the ambient temperature is 25℃, the lamp is driven with rated current, and the working time when the brightness drops to 50% of the initial brightness is regarded as the service life of the lamp. At present, the life of LCD TV back cursor can reach 60000 hours. CCFL (cold cathode fluorescent lamp) backlight is characterized by low cost, but its color performance is not as good as LED backlight.
LED backlight uses light-emitting diode as backlight, which is the most promising technology to replace the traditional cold cathode fluorescent lamp in the future. Light emitting diodes (LEDs) are made of several thin layers of doped semiconductor materials. One layer contains excessive electrons, and the other layer lacks electrons to form positively charged holes. When working, current passes, electrons and holes combine with each other, and excess energy is released in the form of optical radiation.
Light emitting diodes with different light emitting characteristics can be obtained by using different semiconductor materials. At present, light-emitting diodes that have been put into commercial application can provide red, green, blue, cyan, orange, amber, white and other colors. White LED backlight is mainly used in mobile phones, while LED backlight used in LCD TVs can be white, red, green and blue. Multicolor LED backlight can also be used in high-end products to further improve color expression, such as six primary color LED backlight.
The advantages of LED backlight are thinner, about 5 cm, wide color gamut, which can reach 105% of NTSC color gamut, and the luminous flux of black can be reduced to 0.05 lumen, thus making the contrast of LCD TV as high as 10000: 1. At the same time, LED backlight also has a life span of 6.5438+10,000 hours. ?
At present, the main problem that restricts the development of LED backlight is cost. Because the price is much higher than that of cold fluorescent tube light source, LED backlight light source can only appear in high-end LCD TV abroad.
Between the high cost of LED backlight, a manufacturer introduced an improved CCFL (cold cathode fluorescent lamp) backlight LCD TV product, represented by Sony and Sharp. Sony's wide color gamut cold cathode backlight tube (WCGCFL) improves the purity of green color and makes the color gamut wider by adding phosphorus to the silver light of ordinary cold cathode backlight tube. The most prominent thing is that the green degree of TV is quite dazzling.
But the disadvantage is that the cost is high and ordinary families can't accept it. At present, some Samsung products also have this technology. Sharp chose the idea of low cost. Sharp's eighth-generation panel products use four-wavelength backlight technology (previously used for 65-inch products).
The principle of Sharp's four-wavelength backlight is to enhance the expressive force of red by adding red LED between tubes. Moreover, the cost of red LED is very low, and it is smarter than Bisogni. It is also obvious that Sharp's panel will be red when it is completely black!
Baidu encyclopedia-backlight board
Baidu encyclopedia-backlight