Common failure phenomena of LCD screens and corresponding maintenance ideas
Oct 22, 2021
LCD screen failure phenomena: blurred screen, white screen, dark screen, faint image, vertical bars, color distortion, color cast, moire interference, color inversion, false or trailing characters, bright lines, bright bands or dark lines, etc.
(1) Huaping
1) General description of the phenomenon of flower screen failure: There is an image on the screen, but the image is covered with interference such as dots, flakes, and mosaics.
Corresponding maintenance direction: check the signal source, logic board, LVDS signal, main board, screen parameters, LCD screen. 2) Fuzzy screen failure phenomenon: The user menu is normal, but the image is fuzzed.
Corresponding repair ideas: check the motherboard and scaler processing chip circuit, repair the chip.
3) Fuzzy screen failure phenomenon: Fuzzy screen from time to time, the image in the middle of the screen is mixed with many small color dots.
Corresponding maintenance ideas: poor contact of LVDS plug-in wires.
4) Huaping fault phenomenon: After repeated power-on, the stripes and colors of the flowers change.
Corresponding repair ideas: check the logic board
5) Flower screen failure phenomenon: After repeated power-on, the stripes and colors of the flowers remain unchanged.
Corresponding repair ideas: the screen itself has a high chance of failure.
6) Fuzzy screen failure phenomenon: the image has interference, and there is also interference on the characters.
Corresponding maintenance ideas: logic board, screen signal connection, digital board signal output circuit.
7) Huaping fault phenomenon: the characters are normal, and the image has blocky and dot interference. YPbPr input signal, no interference. Then input from the AV port, the image appears interference.
Corresponding maintenance ideas: false welding of frame memory, image enhancement processing IC control signal line is broken.
8) Fuzzy screen failure phenomenon: the images are all dotted, finely broken screen, and the characters are normal.
Corresponding repair ideas: the fault lies in the video decoder circuit. Because the faults generated by the image enhancement processing circuit are blocky, not finely divided.
9) Fuzzy screen failure phenomenon: there is distortion or netting interference on the image. AV test machine everything is normal.
Corresponding maintenance ideas: medium amplifier and high amplifier circuits.
10) Huaping fault phenomenon: the image is regular blocky Huaping.
Corresponding repair ideas: poor communication between the motherboard cache and the main chip.
11) Huaping fault phenomenon: Huaping ghost image, the whole screen has messy color stripes.
Corresponding maintenance idea: turn off the image contrast to judge. If the contrast is turned off, the image lacks color, it means that the fault is in the format conversion circuit of the motherboard and the LVDS connection line. On the contrary, if the contrast is turned off, the image is basically normal, indicating that it is T-CON The format converter of the board does not work normally. The logic board is mainly checking the screen power supply and chip power supply, the format converter communicates with the frame memory, and the screen memory data is damaged.
12) Fuzzy screen failure phenomenon: Fuzzy screen appears instantaneously on the upper left when starting up.
Corresponding repair ideas: BLON control voltage is abnormal.
13) Fuzzy screen failure phenomenon: the color of the image is reversed, and when the menu display appears, the complementary color formed by the lack of color appears.
Corresponding repair ideas: the motherboard and screen interface timing is inconsistent, re-upgrade the corresponding software data.
14) Fuzzy screen failure phenomenon: color vertical bar, control function is normal, and there is a black screen phenomenon.
Corresponding maintenance ideas: The LVDS signal sent by the main board does not correspond to the logic timing of the logic board driver, check the LVDS line and the logic board.
15) Fuzzy screen failure phenomenon: the image presents uneven brightness, and there are vertical and horizontal intermittent bright lines interference. This phenomenon also exists when inputting other test signals.
Corresponding repair ideas: poor LVDS signal transmission.
16) Fuzzy screen failure phenomenon: Checkered regular blocky fancy screen (full screen vertical bars).
Corresponding maintenance ideas: communication between the main chip and DDR.
17) Fuzzy screen failure phenomenon: the grating flashes in the vertical direction, and the grating is staggered at one-half of the position.
Corresponding maintenance ideas: The problem is caused by the loss of the row and column drive signals and timing control signals. 1) The problem of the row and column drive circuit itself. 2) Poor contact between the logic board and the row and column drive circuit causes the drive signal to be abnormal. 3) The logic board format conversion circuit does not send the RSDS signal.
18) Fuzzy screen failure phenomenon: when there is an image screen, you can see a vertical vertical channel composed of colored dots, and when there is a menu display, you can see a white raindrop-like diagonal pattern. When there is no signal, the whole screen can be faintly seen larger uneven Color particle dots.
Corresponding repair ideas: pixel loss, check the cache.
19) Fuzzy screen failure phenomenon: The image is displayed normally, but there will be many small “snowflake” colored dots in a local area.
Corresponding repair ideas: bad LVDS line.
20) Fuzzy screen failure phenomenon: it is normal when there is no signal and the blue screen is normal, and the image is fuzzy when the signal is added.
Corresponding repair ideas: poor communication between the motherboard DDR memory and the motherboard.
(2) Typical faults of bad logic board
Typical failures of a bad logic board: no image, intermittent color lines in the vertical direction of the screen, and no characters (this is very important). The upper screen voltage is generally 5V or 12V, depending on the screen model. Then test the voltage on the LVDS output interface to see whether the static and dynamic conditions have changed. If it does not change, it can basically be judged that there is a fault on the logic board. . As long as the format is the same as the upper screen voltage, it can be replaced.
The LVDS lines from the main board to the logic board have certain rules. The red line on the side is the positive pole of the power supply, and the black line is generally the negative pole of the power supply. The LVDS signal lines are twisted together (4 pairs/single 6 bits; 5 pairs/single 8 Bits; 8 pairs/double 6 bits; 10 pairs/double 8 bits).
Nowadays, some logic boards and screens are connected together, called X+C all-in-one, which is generally a screen of 32 inches or less, and a screen of 32 inches or more is generally composed of X+C separated type.
The damage of the logic board will cause the screen to not display images normally, and of course there is no menu display, but the buttons and remote control work (this is very important).
The characters are inserted from the last stage of the CPU circuit.
The difference between the false screen of the logic board and the false screen produced by the screen: the false screen of the logic board is shown as a regular top and bottom false screen, we can change the signal source or change the 4:3 mode to confirm.
Logic board repair ideas: 1) Check whether the main chip power supply IC, filter capacitor and other vulnerable components are obviously burnt or have abnormal colors; 2) Test whether the short circuit resistance of each power supply test point is reduced, and whether the insurance is open; 3) Measure whether the power of each power supply test point is normal, that is, the power supply Vcc---5V/12V, the main chip---3.3V, 2.5V, 1.8V, 1.2V, etc., VAA---14V, VGH---about 26V , VGL---about minus 6V, ④VREF---13V, VCOM---6V); 4) Test whether the data exchange between LVDS DDR and the main chip is defective; 5) Check the power strips of each connection line
Fuzzy or white screen, if it is confirmed that the logic board is broken, repair ideas: Whether the driver board 5V to 3.3V voltage regulator block (AIC1084) has power output. → Screen body driver board fuse (F)-3.3V → DC-DC Conversion circuit → negative pressure forming IC (-7V) → row and column drive IC.
Common problems of logic board IC damage:
AAT1101A, M150XN05---white screen, dim boot image, etc.
AAT1164, M170EG01---the screen will be white after using for a period of time, and the screen will be white after being turned off and on for a period of time
AAT1343---boot white screen, flower screen, vertical bar, etc.
AD8567---white screen
AU30707---White screen, blurred screen, light image
AU30803---White screen, blurred screen, light image
AUO-003 M170EG01---White or blurred screen at startup, color distortion at startup, color vertical bars, etc.
MST8116B---fuzzy screen, white screen, no display when connected to the signal line, etc.
MST8131A, MST8131A, MST8111B, MST8111A ---fuzzy screen, white screen, no display connected to the signal line, etc.
SE7889-LF---Flower screen
(3) Mixed category
1) Replace the original motherboard with a universal board, and the images are all normal. Generally, the logic board is normal, but the fault is on the main board. But this will also easily lead to misjudgment. Experience: The main board causes most of the screen to be incorrect.
2) Color cast failure: generally enter the factory adjustment mode for adjustment. If this mode is not available, the repair idea: replace the screen cable and adapter board → rewrite the driver → the driver board is broken (rare) → the control IC of the screen back panel is broken (rare) → unplug the screen line to observe the backlight color (backlight flat color) The lamp is aging)→change the lamp.
3) Image fault:
a) If the fault is related to the signal source (for example, it appears in TV; it does not appear in AV), first suspect the previous part of the main chip;
b) If all images and OSD display are abnormal, suspect the later part of the LVDS signal (including the LVDS line and TCON part);
C) If vertical lines, vertical bands, or left and right half of the screen appear abnormal on the screen, it is basically a problem near the RSDS line of the TCON part.
4) Black or white screen failure:
a) First determine whether the fault is in the switching power supply, the signal processing part or the TCON part.
b) The fault range can be judged by measuring the LVDS signal connecting the signal processing part and the TCON part. If it is normal, suspect the back-end TCON part; if it is not normal, check the previous signal processing part. c) For the TCON part of the inspection, the main inspection: key point voltage, RSDS line connectivity.
5) Screen bright lines, bright bands or dark lines: Generally, it is a fault inside the LCD screen. The bright line failure is generally a problem with the cable connecting the LCD screen or the driver IC of a certain row and column is damaged. The dark line is generally the screen The main body is leaking, or the TAB flexible board connection is open. The above two problems are basically the screen is broken.
6) The white screen is divided into two situations: 1) There is signal input, the whole screen is white, and the image cannot be seen clearly, which is caused by the failure of the control circuit board; 2) There is signal input, the whole screen is white, and it can I see the image, but the image seems to be covered by a layer of fog, which is caused by the defective cell. It cannot be repaired and the LCD screen needs to be replaced.
For a white screen, there are three voltages to check: 1) 10V (or 12V), which is a voltage generated by a simple boost from the 5V (or 3.3V) screen power supply voltage. 2) 25V (or 30V), which is output by the DC---DC circuit. 3) Negative 7 volts, it is also output by the DC---DC circuit. If these three voltages are all normal, then consider whether the main chip is soldered, and whether it is damaged. Another important point to check is the screen cable. Check whether the screen cable is installed in place and in good contact. If necessary, clean the screen cable. In addition, the DC-DC conversion circuit of the screen, the first thing to check is the filter capacitor, and the second is that the IC of the DC-DC circuit is much bad.
7) Characters are vacant or trailing: 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 → change the driver board → LCD screen backplane signal interface IC Bad → LCD screen back panel contrast potentiometer adjustment → LCD screen light guide plate misalignment →-polarizer wrong.
8) The screen displays an error message "out of frequency range": check the signal line → rewrite the MCU driver → replace the EPROM → rewrite the EPROM program → change the driver board.
9) When the power is turned on, the white screen will appear backlit if the switch button is not pressed: 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 reversed. Some of the switch signal output of the drive board MCU is abnormal and the MCU can be rewritten. Program repair. Or change MCU.
