Analysis of common failures of led LCD TV_LCD TV repair idea

1. There is no sound and no sound when starting up, the power light flashes once and turns into a constant light, and the screen flashes white light when it is turned on. This fault is mostly caused by damage to the backlight driver board, or it may be caused by damage to the lamp tube in the screen.

2. There is a blurry screen (mosaic) after turning on the computer, and the sound is normal. This phenomenon is caused by a bad digital board (unconnected vias or poor IC contact), and the second is a bad internal connection.

3. There are three failures when the machine is turned on, the power light is not on, the power board of the younger brother is broken, and the CPU part of the younger brother is not working properly.

Four, the light flashes and cannot be turned on

The CPU bus is not working properly or the boot program IC (BIOS) is bad, and the contact between the "BIOS" IC and the CPU is bad.

5. If there is a black band or bright line on the TV screen, please ask a professional technician for repair. Alibaba focuses on maintenance, and APP easily solves your maintenance problems.

6. Failures such as full screen dots, regular vertical bars, etc., are caused by the damage of the buffer board of the screen assembly. You can replace a pair of buffer boards (even if one side is broken, you will not buy a pair and sell a single one to you) ) Or measure which IC is bad and just replace it.

7. Fuzzy screen, full screen color dots, power supply protection caused by short circuit, VS or VA voltage momentary failures, etc., are caused by the damage of the Y board in the PDP machine.

8. Failures such as boot protection and dimming brightness are caused by the broken X board in the PDP machine.

9. There is light on the screen, but there is no character, no image, or irregular color, lack of color, negative image, failure to turn on, etc. The fault is caused by a problem with the logic board in the PDPLCD.

10. Dark bands, lines and other faults are the most common. Basically, they can be summarized as screen problems. Some require maintenance conditions to repair, and some are caused by high temperature, which causes the ACF of the COF and the screen connection point to separate. This phenomenon can be used with a flat head. Repair the underlayment layer of the soldering iron with the heat-insulating medium.

11. There is light when the machine is turned on, there will be no light or sound for a while, except for (SHARP), etc., it is the inverter circuit (high voltage board) of the screen assembly that there is a problem, and the aging and damage of the light tube will lead to the protection of the high voltage board. The way that the light pipe is still bad for the high-voltage board itself is to cancel the mean value comparison feedback circuit on the high-voltage board.

1. The backlight is turned off when the LCD screen is on when the AC is turned on. At this time, the audio, remote control, and panel button control functions are all normal. This phenomenon is caused by the backlight circuit protection. The reason is mostly the abnormal power supply of the backlight booster board. CCFL backlight circuit, if a certain backlight tube is open (usually the circuit protection caused by the opening of the tube socket on the backlight booster board or the socket is not plugged tightly) or a certain tube break can cause the above failure

2. There is no change in the backlight switch machine, the sound, remote control, and panel button control are all normal. This fault needs to check the following working conditions: 1) The power supply of the backlight booster circuit, the common large screen is 24 volts, very few use 120 volts, small screens are generally 12 volts, 2), the switching control signal of the backlight boost board oscillator output by the CPU control circuit is usually high-level start, mostly 3V-5V lamp lighting control signal. If all the above working conditions are met, the backlight booster board can be replaced. If the backlight booster board is replaced, the failure remains the same, mostly because the backlight tube in the LCD screen assembly is damaged.

3. When the backlight is on and not bright, it is usually caused by poor contact between the lamp socket of the backlight booster board and the lamp, and the power supply of the backlight is high or low. In actual maintenance, the 24V power supply digital meter should not have a 1V error (power supply). The 24V voltage output on the board should be stable whether it is no-load or loaded)

1. Failure phenomenon

There are three common failures of the LCD color TV backlight power supply board: one is no voltage output, resulting in a black screen, and the sound is normal; the other is a low output voltage, resulting in a failure of low screen brightness; the third is the output voltage is abnormal, which will cause the backlight after booting. The light goes out once it lights up.

2. Fault judgment method

(1) Checking method Check whether the resistance and chip appearance on the backlight power supply board are normal. If there is discoloration or explosion, it means that it is damaged.

Check whether the appearance of the high-voltage transformer (or energy storage inductor) is deformed, whether the magnetic core is broken, whether the pins are unsoldered, and whether there are traces of sparking near the pins.

Check the switch tube (or inverter tube) on the backlight power supply board, whether the socket pins are disconnected, etc.

Check whether the chip components such as resistors, capacitors, and integrated circuits on the backlight power supply board are off or their pins are unsoldered.

(2) Voltage measurement method LED backlight power supply board output voltage: the power supply voltage of the LED light bar is DC voltage, and its voltage value is generally between more than one hundred volts and more than two hundred volts. Therefore, you can use a multimeter with a DC voltage of 250V. It can be confirmed whether it is normal by measuring the voltage value output by the board.

The output voltage of the CCFL backlight power supply board: Since the output of the CCFL backlight power supply board is high-frequency alternating current, the voltage is as high as about 1000V, so it is introduced in the past newspapers and books. It is not possible to use an ordinary multimeter to directly measure its output voltage. Use methods such as drawing arc and detecting induced voltage for estimation. In fact, it can be measured with a digital multimeter. The detection method of the output voltage of the high-voltage inverter is introduced below with the Changhong iTV40650X LCD color TV measurement.

When using the 700V AC voltage block of the DT9205 digital multimeter, the voltage is 357V; when using the 200V AC voltage block, the voltage is 37.1V. During the measurement, if the test lead is in poor contact with the output terminal of the high-voltage transformer, a pink arc will appear, which also means that there is a high-voltage output; if there is no arc and no voltage, it means that the high-voltage transformer has no high-voltage output.

[Reminder] Since the high-voltage transformer of the tested color TV outputs high-frequency pulse voltage, and the AC voltage range of the digital multimeter is set for measuring low-frequency AC voltage, the measured value is much lower than the normal value (about 1000V).

Backlight power supply board working voltage: The working voltage of the back light power supply circuit is provided by the power board, mostly 24V. If the backlight power supply board has working voltage input but no high voltage output, it means that the backlight power supply board is abnormal.

Backlight on/off control signal: Most of the backlight power supply boards use high level (3-5V) mode to start, and low level to close. If the backlight power supply board does not have this control signal input, the backlight power supply board does not work and there is no voltage output.

Backlight brightness control signal: When troubleshooting abnormal backlight brightness, the backlight brightness control signal is also the main detection point. The backlight brightness control voltage of the Changhong iTV40650X LCD color TV changes with the brightness changes, but no matter how it changes, the voltage cannot be 5V or 0V. If the voltage value is 5V or 0V, it will definitely cause a malfunction of abnormal brightness.

(3) Dummy load method

Since the backlight power supply board has a complete protection circuit, the protection circuit will operate when the backlight is abnormal, causing the backlight power supply board to stop working after it is started. In order to determine whether the failure is caused by the abnormality of the backlight or the abnormality of the backlight power supply board, a dummy load should be used to determine the cause of the failure.

CCFI, the dummy load of the backlight power supply board: The dummy load of the CCFL backlight power supply board (inverter board) is best to use CCFL backlight tubes, of course, you can also use special maintenance tools, for example, KYD-PWV2 of Changhong Kuaiyidian Electric .0 is the special dummy load tooling for the CCFL backlight power supply board of the LCD color TV. It can meet the maintenance of the CCFL backlight power supply board with 1 to 8 high-voltage output interfaces.

After the dummy load is connected, if the inverter board can work normally, it means that the backlight power supply board is normal, and the fault is caused by the abnormality of the backlight; if the fault persists, it means the backlight power supply board is abnormal.

[Prompt] If there is no such dummy load tooling, you can also make multiple inverter dummy loads with indicator lights, as shown in the figure below. The diode VD1 in the picture is used for rectification, RU2 or RG2 can be selected, and LED1 is a light-emitting diode, which is used as an indicator light. Because LED1 also has a rectifier function, it is not necessary to install VD1 in actual production.

Dummy load of the LED backlight power supply board: The dummy load of the LED backlight power supply board is relatively simple, you can choose a 25W/220V incandescent lamp. Connect it to the pin of the output terminal of the backlight power supply board. If the incandescent lamp emits light and the voltage at both ends is normal, it means that the backlight power supply board is normal, and the fault is caused by the abnormality of the backlight. If the fault persists, the backlight power supply board is abnormal.

3. Substitution method

Check if the backlight power supply board is not soldered or damaged, you can replace it with a normal backlight power supply board of the same model for inspection. If the LCD TV returns to normal after replacement, it means that the replaced backlight power supply board is damaged; If the fault persists, it means that the fault lies in other parts.

1. The monitor has no power

(1) Power failure: This is a very simple failure. The internal power supply and the external power supply of the general LCD extension are common. The external power supply is more common. Regardless of the type of power supply, its structure is much simpler than the power supply of crt display. The vulnerable ones are generally some small components, such as fuse, rectifier bridge. 300V filter capacitor, power switch tube, power management IC, rectifier output diode, filter Capacitors, etc. Commonly used ic for power supply boards: 6841203d06. These commonly used pmw chips are available in professional LCD accessory stores like me.

(2) Driver board failure: The driver board burns insurance or the voltage regulator chip fails. Some machines have built-in switching power supplies and output two sets of power supplies, one of which is 5V for signal processing, and the other is 12V. The high-voltage board is used for backlighting. If the switching power supply part of the circuit fails, it may cause the two sets of power supplies to fail to output. First check whether the 12V voltage is normal, and then check whether the 5V voltage is normal, because the working voltage of the MCU chip of the A/D driver board is 5V, so when you find the fault that cannot be turned on, first use a multimeter to measure the 5V voltage. If there is no 5V voltage or 5V voltage becomes very low, then one possibility is that there is a problem with the input stage of the power supply circuit, that is to say, there is a problem with the power supply part of the 12V conversion to 5V. This kind of failure is very common. Check the 5-terminal voltage regulator block (common model 8050SD -LM2596-AIC15-01 etc.).

One possibility is that the 5V load is heavier and the 5V voltage is pulled very low. In other words, there is a problem with the signal processing circuit of the subsequent stage, and some of the circuits are damaged, causing the load to increase, and the 5V voltage is pulled very low. Low, check the components that have problems one by one. After replacing the failed components, 5V can be restored to normal. The failure is usually solved here. It is also often encountered that the 5V voltage cannot be turned on normally after the 5V voltage returns to normal. There are also many cases in this situation. The reason is that, on the one hand, the program of the MCU may be washed out, which may cause it to not boot, and the MCU itself is damaged. For example, the I/O port of the MCU is damaged, so that the MCU cannot scan the buttons. When encountering this kind of fault caused by the MCU, The problem of finding the hardware is useless. Even if you change the MCU, it will not solve the problem, because the MCU needs to be programmed and coded. If there is no way to find the original AD driver board to replace, we can only use the universal A /D driver board substitution such as: 151D or 161B, etc.

2. The display does not light up once it is on, but the power indicator light is always on.

This kind of problem is usually caused by abnormal high voltage, and the protection circuit is activated. In this case, there is usually a display on the LCD screen, and the way to look is "squint".

3. The display is black without backlight. The green light of the power light is always on.

The squint LCD screen has a display image, which is mostly a problem with the power supply circuit of the high-voltage board. Focus on checking whether the 12V power supply (fuse F) and the switching voltage of 3V or 5V are normal. If there is no output switch control voltage due to MCU problems, you can directly extract the 3.3V of the 3-terminal regulator block (AIC1084) instead. The idea of repairing high voltage board (power fuse----switch control tube----power management IC----push-pull hair tube----power switch tube----DA conversion circuit (energy storage inductor, Rectifier)-LC boost circuit (boost transformer, boost capacitor)-coupling capacitor-lamp.

4. The screen has bright lines, bright bands or dark lines

This kind of problem is generally a malfunction of the LCD screen. The bright line failure is generally caused by a problem with the cable connected to the LCD screen or the driver IC of a certain row and column is damaged. The dark line is generally the main body of the screen has leakage, or the TAB flexible board connection is open. The above two problems are basically the death penalty for the machine, and there is no repair value, because the price of a screen is too high.

5. Fuzzy or white screen

This kind of problem is generally caused by a problem with the drive voltage of the screen. First, change the drive board and drive screen line to test. If it fails, check the power supply circuit of the screen back panel. Repair ideas: whether there is a voltage regulator block (AIC1084) from 5V to 3.3V on the drive board. Power output. ---Screen driver board fuse (F)-3.3V-----DC-DC conversion circuit----negative voltage forming IC (-7V)----row and column driver IC.AAT1101AM150XN05 screen power management When the chip is damaged, it appears as a white screen and a dim start-up image. It is a common problem with the new AAT1164M170EG01 screen, which is mainly used on the BenQ FP71G+ LCD. The screen will be blank after using for a period of time, and the screen will appear again after shutting down and using for a period of time. This chip is a high-voltage generation and management chip for LCD screens. BenQ LCD monitors are often broken and the fault is: white screen.

6. Color cast failure

Generally, you can enter the factory adjustment mode to make adjustments. If this mode is not available, the repair idea: replace the screen cable and adapter board ----- rewrite the driver program ----- the driver board is broken (uncommon) ----- the control IC of the back panel of the screen is broken (not Common) -------- Unplug the screen cable and observe the backlight color (the flat backlight is the aging of the lamp) -------- Change the lamp.

7. Characters are empty or trailing

Repair ideas: check the VGA signal line, and focus on whether the ground wire of the RGB three-color line is properly connected-----replace the screen line or adapter board-----rewrite the driver program-----change the drive board- ---LCD screen backplane signal interface IC is broken---LCD screen backplane contrast potentiometer adjustment---LCD screen light guide plate misalignment---polarizer misalignment

8. There are stains inside the LCD screen

Maintenance idea: Wipe or replace the protective film-----disassemble the screen body to clean the outer polarizer and plexiglass (treated with cotton balls, pure water)---dry by air duct

9. LCD screen light leakage or uneven light

Maintenance idea: reinstall the lamp tube-----adjust the light guide plate

10: LCD screen highlights

One or two big bright spots, you can try to gently press the bright spots with your fingertips, and they can disappear, indicating that the switch tube and the electrodes of this pixel are mostly virtual connection. The small black spots and gray spots may be caused by dust on the internal light guide plate or polarizer. Can be cleaned.

11: LCD screen brightness is low

Check the ADJ brightness adjustment circuit of the high-voltage board, replace the lamp tube--------change the high-voltage board--------adjust or replace the light guide plate

12: Error prompt "out of frequency range"

Check the signal line--------Rewrite MCU driver-------Replace EPROM------Rewrite EPROM program-----Replace the driver board

13: After power on, do not press the switch button, the white screen will appear backlit, and the image can be displayed normally after the button is pressed

The switch signal of the high-voltage board interface and the ADJ signal are reversely connected, and part of the switch signal output of the drive board MCU is abnormal, and the MCU program can be rewritten to repair it. -------Change MCU

14: add one more point

When replacing the aging screen of the lamp tube, find out the size of the lamp tube.