Basic principles and maintenance examples of the h

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Basic principle and maintenance examples of LCD TV

liquid crystal display (LCD) for short. LCD is a large family, and TFT (thin film transistor) LCD type is only one of them. It seals liquid crystal between two glass plates, configures upper scanning line and addressing line (i.e. row and column line) on the lower glass plate to form a matrix, and then manufactures TFT active devices and pixel electrodes at their intersection. In case of color display, it is also necessary to support the sliding bracket with the following matrix pixels in the production of micro processing, and the active frame makes the color filter film of R (red), G (green) and B (blue) corresponding to the horizontal application on the steel column. Finally, it is made into liquid crystal chips by a series of processes such as aligning the upper and lower glass substrates, sealing boxes, pouring, plugging holes and so on. Because the liquid crystal itself does not emit light, it must rely on the modulation of external light to achieve the purpose of display, so there is a light-emitting device in the LCD display module - cold cathode fluorescent lamp CCF, which is a light source that relies on the discharge of cold cathode gas to excite phosphor and emit light. The thin gas mixed with a small amount of Mercury will ionize under high voltage. The secondary electron emission of the ionized gas bombards the mercury vapor, which excites the mercury vapor and emits ultraviolet light. The ultraviolet light excites the phosphor layer coated on the tube wall to make it glow. The light-emitting CCF lamp passes through a special light guide plate and a uniform light plate to make it consistent with the size of the liquid crystal, close to the liquid crystal display panel, and is used as the background light, so as to achieve the purpose of displaying images. Adjust the brightness and contrast of the image by adjusting the brightness of the backlight or the conductivity of the thin film transistor in the liquid crystal chip. LCD TV is mainly composed of display screen, signal processing circuit and backlight circuit. The display screen is a module, and the signal processing is mainly composed of high-frequency circuit, image processing a/d circuit, accompanying sound circuit, control circuit, etc. The backlight circuit is an inverter circuit, which is used to light the function of the lamp tube in the display screen

maintenance examples:

1, white grating, with accompanying sound (15aab/8tt1 movement)

maintenance: power on and start up, and it is found that the screen is white, but there is accompanying sound. It is analyzed that this fault is caused by the LCD screen not working.

check the +5v power supply and line and field signals of the display screen, and it is found that there is no +5v power supply. Check the line is the mainboard L21, +5v power supply

the inductance is open circuit, and it is OK after replacement

2. No grating, with accompanying sound (20aaa/8tt1 movement)

maintenance: after starting up, it is found that the image is faintly visible under strong light. The analysis shows that the machine is caused by the non operation of the backlight. After the machine is disassembled and powered on, it is found that there is no high voltage on the backlight board. Check the power supply and backlight control level of the backlight board, and measure the voltage at J6 of the motherboard with a multimeter. The 12V power supply of pin 1 is normal, but the on of pin 5 should be +5v high level, but it is always 0V at this time. Check the control circuit along the line. The fifth pin of J6 is connected to the 22nd pin of cpu-ks88c4504 through r52/1k patch resistance. To enhance fuel efficiency, use the meter to measure that the 22nd pin of CPU is +5v voltage. One end of r52/1k resistance is +5v, and the other end is 0V. After power failure, measure that the resistance has been open circuit, and everything is normal after replacement

3. Crash: (15aab/8tt1 movement)

maintenance: plug in the power indicator light is not on, measure that the motherboard has +5v voltage output, check the CPU circuit, measure that the power supply of the 12th, 5th and 53rd pins of cpu-ks88c4504 is normal, measure that the CPU crystal oscillator YM has also vibrated, and then measure the voltage of the 19th pin of the reset pin, which should be high level normally, but at this time it is 0V, check the reset circuit and its periphery, and the reset circuit is generated by an IC (q3-ds1813), Check that the power supply of q3-ds1813 is normal, but the reset output is 0V. It is normal after replacing q3-ds1813

4. Black screen: (30aaa/8tp2 movement)

maintenance: after starting up, observe that the backlight has been lit, but there is no Skyworth logo or OSD when starting up. Therefore, it is judged that the mainboard does not send a signal to the display screen. The power supply at the j27 interface of the main board is normal, but there is no LVDS signal. It is judged that the main board does not give the screen drive signal. The 108th pin vs of the main board u15-pw113, the 109th pin HS and the 106th pin DCLK are output, RPH, RPI, RPF?  the 47 ohm drain resistors of PG, RPD and RPE have R, G and B data output, so it can be determined that the LVDS chip u20-ds90c385 is not working. The power supply of +3.3v at pins 1, 9 and 26 of U20

is normal, and the on/off control pin of pin 32 is also high level. It is normal after replacing U20

5. It can be turned on, but sometimes it turns white and then becomes normal, When starting up "Please wait character changes to the left force value of the screen. Generally, use the upper corner of the spoke sensor to repair: because the character sometimes shifts, the analysis should be that there is a problem in the CPU part. Measure the bus voltage, SCL is 3.5V, and SDA is only about 0.6V. It is suspected that the plug-in IC is bad. Disconnect U202 tda7440 pin 21 and pin 22, and the bus returns to normal after unplugging the connection of the tuner. Try to change a tuner as before. After power on, measure the power supply of the tuner and find that fb200 power supply rises to 12V, which is normal This is 5V. When the fb200 is removed, it is 5V again. The analysis shows that the rise to 12V can only be caused by u804 (apxv) voltage stabilizing IC. U804 inputs 12V, outputs 5V after its voltage stabilization, and supplies power to the tuner after c827 and fb200. After replacing u804, the whole machine is normal

summary: according to Hooke's law, the fault of this machine is that u804 (apxv) voltage stabilizing IC is normal without load, but it is damaged after load

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