LCD Backlight Repair

There is a simple way to test with a general high-voltage bar. First, test the lamp to see if the problem is caused by the lamp. If it is not, just replace the high-voltage bar. The easiest way is to repair the high-voltage part. There are many reasons, such as high-voltage coils.

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At present, the market is full of LCD monitors, and the maintenance of LCD monitors is more difficult than CRT monitors. This article will explain some knowledge and techniques of LCD monitor maintenance.　　 Liquid crystal is a substance between solid and liquid. It is an organic compound with regular molecular arrangement. If it is heated, it will appear in a transparent liquid state, and if it is cooled, it will appear a turbid solid state of crystalline particles, which has the characteristics of liquid and crystal, so it is called "liquid crystal".

The principle of liquid crystal display is simply to energize the liquid crystal placed between the two electrodes, and the arrangement of the liquid crystal molecules will change when the electrodes are energized, thereby changing the optical path of the transmitted light and realizing the control of the image. The TFT liquid crystal panel is composed of surface protective glass, ternary color filter plate, polarizing plate, FET transistor (thin film transistor) electrode deposited on the glass substrate, liquid crystal, and common electrode also deposited on the glass substrate. The bottom layer is composed of polarizing plate, backlight plate (light guide) and backlight source. The light is transmitted in from the bottom layer, and is controlled by the liquid crystal and the polarizing plate, and the colorful image is generated with the help of the filter plate.

According to the physical structure, common liquid crystal displays can be divided into the following types: TN, STN, DSTN three liquid crystals are passive matrix LCDs, their principles are basically the same, the difference is that the twist angle of each liquid crystal molecule is slightly different. Among them, DSTN (commonly known as "pseudo-color") was widely used in early notebook computer monitors and handheld game consoles, but because it had to borrow external light sources to display images, it had great application limitations, but these early reflective singles Color or color LCD without backlight design can be made thinner, lighter and more power-efficient. If it can be innovated technically, these things are still very useful for handheld computers and game consoles. The TFT thin film transistor active matrix LCD is the mainstream used in our liquid crystal displays today. It has the advantages of fast screen response, good contrast, high brightness, large viewing angle, and rich colors.

Everyone knows that each dot of a TFT liquid crystal display consists of three parts: red, green and blue. Generally, the dot pitch of a 15-inch TFT liquid crystal display with a resolution of 1024X768 is about 0.30mm. The TFT liquid crystal display is different from the CRT display in that it has a fixed resolution. The picture quality is best only under the specified resolution, and the picture can be displayed in the way of expansion or compression under other resolutions.

In addition, it should be noted that the traditional display uses an electron gun to emit electron beams, which will generate radiation sources when hitting the screen. Although its existing products have been greatly improved in technology, the radiation damage is continuously reduced, but it is still impossible. Radical cure; and LCD monitors have very low radiation. The display screen of the traditional display uses phosphors, which display images by striking the phosphors with electron beams. Therefore, the brightness of the display is brighter than the transparent display of the liquid crystal, and the viewing angle is much better than that of the TFT liquid crystal display. In terms of display response speed, the traditional display has a very good response speed due to its technical advantages.

How to judge the backlight is damaged

The liquid crystal itself does not emit light, and the appearance and brightness adjustment of its image depend on the adjustment of the brightness of the backlight. When the liquid crystal display is working, the light emitted by the backlight passes through the liquid crystal screen to reflect the image content displayed on the liquid crystal screen into human eyes. Only then can we see the text and images displayed on the liquid crystal display. If the backlight is damaged, there will be no light, and we can't see anything at this time. However, if we carefully observe the LCD screen, we will see a faint image display on the LCD screen, which means that the backlight related circuit is broken. If the backlight circuit is intact and there is a problem with the display circuit, then we will see bright white light from behind the LCD screen. Most of the failures of liquid crystal displays are backlight circuit problems or power supply problems. The most likely cause of backlight circuit failure is the internal short circuit or open circuit of the boost coil.

First power up the LCD separately, observe the fault phenomenon, whether there is the above-mentioned fault performance. Then connect the signal line to the host, turn on the display, and observe whether the power indicator of the display is always green and whether there is an image on the LCD screen (although the backlight is damaged, you will find a faint image appears through careful identification).

Why does the LCD screen appear to be stained?

There are two possibilities for the appearance of color spots. One is that the LCD screen is partially stressed, resulting in a large area of dead pixels. The other is the poor contact of the drive screen cable.

What is a high voltage board?

The high-voltage power supply board is responsible for supplying power to the lamps of the LCD. It transforms the DC low-voltage power supply into a high-frequency high-voltage power supply to light up the lamps. It is a power conversion device and is prone to heat generation, so it is more prone to damage. The dark screen is often the high-voltage board is broken!

In fact, the high-voltage board is a switching power supply, but compared to the ordinary switching power supply, it lacks the rectification and filtering part of the subsequent stage, and focuses on the conversion of high-frequency and high-voltage. It converts the low-voltage direct current (usually 3~14V) on the main board into a high-frequency alternating current through a switch, and then boosts it through a high-frequency transformer to reach the voltage for lighting the lamp. The power and signals of the high-voltage board come from the main board. Generally, there are several wires connected to the main board: power supply V+, power ground G, switch signal S, and brightness signal F (some are not). When the computer is turned on, the power supply is supplied, the switch signal S starts the switch oscillating circuit, the switch tube works, the transformer performs the voltage increase, and the lamp tube is lit. The vulnerable components on the high-voltage board are mainly oscillating circuit components, switch tubes, and high-voltage packages.

Introduction to the interface classification standard of LCD display

Theoretically speaking, since LCD displays are purely digital devices, digital interfaces will inevitably replace analog interfaces. However, most LCD displays on the market currently use analog signal interfaces. The fundamental reason is the inconsistency of norms and standards.

At present, the technical standards for digital interfaces are gradually being unified. More and more display chips have the ability to support digital video output. Graphics card manufacturers are beginning to integrate digital display interfaces on graphics cards. Below we will introduce three video digital interface standards one by one.

① P&D

The Digital Plug-and-Display (P&D) standard was formulated by the Video Electronics Standards Committee (VESA), but when the standard was released in 1997, it had been greatly out of touch with the actual situation at the time. For example, the display signal interface defined in the P&D standard is a multi-functional interface that can transmit digital and analog signals at the same time. The signal transmission is superfluous, and no graphics card manufacturer is willing to add such an expensive and useless interface to their products. It is precisely because of VESA's delay in failing to produce a decent standard. Many companies have jointly launched their own standards with their partners, which has made the current situation of digital interface standards chaotic.

② DFP

The DFP-Digital Flat Panel Group standard is an industry standard proposed by Compaq. The 20-pin DFP interface can support a maximum resolution of 1280X1024.

A large company that supports the DFP standard is Canada’s ATL, which produced the first graphics card with a DFP interface. Later, VESA also selected the DFP interface as the transition of the P&D standard. In fact, as long as the function definitions of the two interface standards are compared, you will find that there is no big difference between the two. In the definition of electrical performance, the two are completely consistent. The DFP standard removes the expensive and impractical options in the original P&D interface standard, such as USB, IEEE 1394, etc., so the DFP standard must be implemented when it is implemented. Much cheaper. However, the DFP standard only supports a resolution of 1280X1024, and the inherent defect of insufficient resolution makes the DFP interface impossible for too long.