Understanding the Causes of Horizontal Lines on LCD Screens

Understanding the Causes of Horizontal Lines on LCD Screens

This type of fault includes the following specific phenomena:

(1) Full-screen colorful stripes, as shown in Figure 13; (2) Vertical lines, vertical bands, or horizontal lines, horizontal bands on the image, as shown in Figure 14, or in a local area of the screen, as shown in Figure 15;

(3) Interference stripes or interference points on the image, as shown in Figure 16. 1. Fault Cause Analysis (1) LCD Screen Damage In LCD TVs, the timing signals output from the logic board are sent to the internal row and column driving circuits of the LCD screen via TAB cables, converted into control signals for thin-film transistors (TFT), and then sent to the TFT tubes through COF cables, as shown in Figure 17. Note: The flexible soft cable where the logic board meets the column driving circuit is called the TAB line. The flexible circuit board connecting the row and column driving circuits to the internal electrodes of the LCD panel is called the COF line. TAB stands for “Tape Automated Bonding,” which refers to a connection method using anisotropic conductive adhesive to connect the LCD and PCB, offering advantages such as ease of installation and good reliability.

COF stands for “Chip On FPC,” which means fixing the IC on the flexible circuit board. This connection method has a high level of integration, allowing foreign components to be installed on the flexible PCB along with the IC, representing an emerging technology.

If the row or column driving circuits are partially abnormal, or if the TAB or COF cables have poor contact, the corresponding row or column TFTs will not operate, resulting in bright lines, bright bands, black lines, black bands, or some colored lines or bands. For example: a BRAVIA R400B LCD TV (produced by SONY) showed an image as shown in Figure 18 due to poor contact between the COF cable and the LCD panel.

The method for judging the above-mentioned poor contact is as follows: After powering on, gently press the edge of the LCD screen near the logic board with your hand. If the stripes change, it indicates that there is indeed a poor contact in the internal connection cable, which is likely to occur at the ends of the TAB or COF cables, as shown in Figure 19. Note: The horizontal lines, horizontal bands, or vertical lines, vertical bands caused by LCD screen damage, or black screen areas, have fixed positions, with neat boundaries, regardless of whether there is an image or what the image content is.

(2) Poor Contact of the Upper Screen Cable or Abnormal Logic Board Operation If irregular colorful stripes appear on the image, and their position and content are constantly changing, this basically rules out the possibility of LCD screen failure; the problem should be with the abnormal signals sent from the logic board to the internal LCD screen. The reasons are: firstly, the logic board itself is malfunctioning, and secondly, the LVDS signals received by the logic board are abnormal.

For the former, measure the voltages at key points such as VGH and VGL on the logic board, combined with the temperature of the timing signal processing chip on the logic board; for the latter, first check whether the connection between the upper screen cable and the logic board and motherboard is good, and then check the motherboard.

Note:

(1) If the upper screen cable has poor contact, the LVDS signals sent to the logic board will be distorted, easily causing irregular vertical stripes or flower screen phenomena on the screen, usually accompanied by a reddish or greenish image, especially noticeable when the image background is dark.

(2) Normally, the VGL voltage is about -6V, and the VGH voltage is about +26V. In some LCD screens, when the VGL voltage is 0V (or close to 0V), or when the VGH voltage drops to a few volts, vertical stripes will appear on the screen.

(3) Motherboard Software Data Error

The motherboard processes the input signals of TV, AV, HDMI, etc., under program control, and finally outputs LVDS signals to the logic board. If the program data on the motherboard is incorrect, it will cause the output LVDS signal format to be incompatible with the LCD screen, resulting in full-screen vertical stripes.

(4) Poor Performance of the Voltage Regulator Circuit on the Motherboard If the performance of the voltage regulator circuit on the motherboard is poor, the ripple coefficient in the power supply may be too large, which could lead to interference pulses mixed in the upper screen signals, usually manifesting as interference lines or interference points on the image, as shown in Figure 20. (5) Strong Electromagnetic Interference to the Motherboard Strong electromagnetic interference usually causes intermittent short horizontal lines or black dots on the image, accompanied by noise in the sound, and may even cause the remote control to malfunction or respond slowly.

Electromagnetic interference can come from within the machine, such as switching power supplies and backlight driver circuits. A typical example is an LCD screen with a plastic casing, such as the LC320WXE (SBV2) model, which generates strong interference when the lamp is working, affecting the motherboard’s operation; it can also come from external sources, such as induction cookers, hair dryers, vacuum cleaners, etc.

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