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Industry Information
Analysis of LCD display regulator failure
2012-1-28 12:05:39

核心是先进的CPU,设计思想是基于数字化、智能化、网络化的更深层次开发,它的主要特点是: The core of the liquid crystal display regulator is an advanced CPU. The design idea is based on the deeper development of digitalization, intelligence, and networking. Its main features are:

1. Supplemented by the design of large-scale integrated circuits;

2, FLASH large-capacity storage, intelligent conditioning signals;

3. SmartBus bus and high-resolution graphic LCD display;

4. Exquisite structure design, the product has ultra-thin design, small volume and many channels;

5. Not only low power consumption, high accuracy, and strong versatility; the operation is also quite stable, which greatly improves the reliable performance;

6. The communication interface used is RS-232C / RS-485, which supports the standard MODBUS-RTU protocol to achieve remote monitoring; it has realized signal acquisition, recording, display, processing, communication, alarm and other functions.

已广泛应用于电力、制药、酿造、石化、石油、造纸、印染、冶金、建材、化工、热处理和水处理等各种工业现 The liquid crystal display regulator produced by Jinhu Dongxiang Instrument Co., Ltd. has been widely used in various industries such as electric power, pharmaceutical, brewing, petrochemical, petroleum, papermaking, printing and dyeing, metallurgy, building materials, chemical industry, heat treatment and water treatment.

Field, as well as science and education and other industries; and has also developed a dedicated controller for the equipment customized on the LCD display regulator platform.

The liquid crystal display is referred to as LCD, which is mainly composed of a liquid crystal display screen, a signal processing circuit, and a backlight circuit. The display screen is a module, and the signal processing is mainly composed of image signal A / D conversion circuit and control circuit. Backlight

The circuit is an inverter circuit, which is used to light the lamps in the LCD screen.


(1) Structure of LCD screen. The liquid crystal material is poured between two pieces of glass, each of which is connected to a set of electrode pieces, and the electrode pieces are transparent and can transmit light. Horizontal (row) pads are connected to a piece of glass,

The vertical (column) pad is connected to another piece of glass. The pixels are generated at the intersection of a row electrode sheet and a column electrode sheet of the liquid crystal material. At each pixel, thin-film technology is used to make silicon transistor electrodes for controlling

Pixels on and off. Two polarizing films are attached to the outer layer of each glass sheet. The two glass sheets are sandwiched with a color filter, an alignment film, and a liquid crystal, and then sealed into a liquid crystal cell. Two sets of IC modules are made from about 11 driver ICs.

The X-axis and Y-axis of the control panel (ie, the row and column electrodes on the LCD screen), plus the hard plastic frame, control circuit, and backlight module, constitute a complete LCD panel assembly. .


In addition to the LCD screen, the LCD also has the following accessories: (1) Driver board: completes the A / D conversion of the image signal and generates synchronous scanning driving pulses. (2) Drive screen: Connect the drive board and LCD screen. (3) High-voltage board: production

Backlight driving voltage. (4) High-voltage board wire: Connect high-voltage board and backlight. (5) Power adapter (external and built-in): Provides the power supply of the LCD monitor.


(B) the principle of image display. The light emitted from the backlight of the liquid crystal display (cold cathode fluorescent tube) passes through the reflective plate and the light guide plate to spread the light flatly, and then enters the liquid crystal material through the polarizing film, glass substrate, and alignment film.

TFT-LCD (Thin-Film Transistor Driven Liquid Crystal Display) is a silicon transistor electrode made of thin-film technology. It uses a scanning method to select any display point (ie, pixel point) on and off. When the light from the backlight hits the liquid crystal,

Under the action of an external electric field, the arrangement of the liquid crystal molecules will change, which will change the angle of the light that penetrates the liquid crystal; these lights must also pass through the front color filter (including the alignment film) and another polarizing film .

As long as the voltage value of the excitation liquid crystal is changed, the intensity and color of the light finally emitted can be controlled, and pixels with different shades of color can be displayed on the LCD screen.

Second, maintenance of common faults (1) The monitor is completely without power. Usually a power failure. The LCD monitor has two sets of power, one is 5V for the signal processor; the other is 12V for the backlight of the high-voltage board. Use a multimeter to check the insurance tubes one by one,

Rectifier bridges, 300V filter capacitors, power switch tubes, rectifier output diodes, filter capacitors, etc., are generally easy to troubleshoot.
(2) The display screen will not turn on once, 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 an image displayed on the LCD screen, which cannot be seen in front view

See, but you can see the image with "strabismus". Maintenance method: a single high-voltage board can be connected to a lamp test. Because the current design of high-voltage LCD panels is generally symmetrical, the possibility of damage to both sides is

Ben didn't. Generally, the power supply tube, booster tube, booster transformer, and lamp of a certain circuit are short-circuited or unloaded, causing the power management IC protection circuit to operate. The high-voltage board interface has many wires. In fact, only 4 signals need to be connected to the high-voltage board:

① Power, ② Ground, ③ Switch control ON / OFF, ④ ADJ brightness adjustment.


(Three) the display screen is black, no backlight or the power light is always green. The squinting LCD screen has a display image, which mostly belongs to the problem of the power supply circuit of the high-voltage board. The focus is on checking the 12V power supply. The inspection of the high-voltage board can be performed in the following order: power fuse

Wire → Switch control tube → Power management IC → Push-pull amplifier tube → Power switch tube → D / A conversion circuit → Boost circuit → Coupling capacitor → Lamp tube.

(4) Bright lines, bright bands or dark lines on the screen. This failure is usually a problem with the LCD screen. The bright line failure is a problem with the cable connected to the LCD screen body, or the drive IC of a row or column is damaged. Dark line fault is the LCD screen

There is leakage in the body, or the TAB flexible board connection is open. The above two problems have basically no maintenance value, because the price of an LCD screen is too high.


(5) Huaping or white screen. This failure is usually a problem with the driving circuit of the LCD screen. First change the driver board and driver screen cable test, if not check the power supply circuit of the screen backplane. The inspection can be performed in the following order:

Whether the pressure block has a power output → screen body drive board fuse → DC-DC conversion circuit → negative pressure forming IC → row and column control IC.


(6) Dummy or trailing characters. This failure is usually a problem in the signal path or optical path. The inspection can be performed in the following order; check the VGA signal cable, and check whether the ground wire of the RG.B tri-color cable is properly connected → replace the driver screen cable

Or transfer board → change driver board → LCD screen back panel signal interface IC → LCD screen back panel contrast potentiometer adjustment → LCD screen light guide plate misalignment → polarizer misalignment.


(G) Bright spots appear on the LCD screen. For one or two large bright spots, you can try to gently press with your fingertips, the bright spots can disappear, indicating that the switch tube and the electrode of this pixel are virtually connected. Small black and gray points may be internal light guides or

The polarizer is dusty and can be cleaned.
(8) The LCD screen has low brightness. The inspection can be performed in the following order: check the ADJ brightness adjustment circuit of the high-voltage board → change the lamp → change the high-voltage board → adjust or replace the light guide plate.

(9) The LCD screen is disordered and unstable. Under different working modes, the LCD may have some interference, most of which are normal, and a few are caused by the circuit or the environment.


Maintenance example 1: A misbranded 15-inch LCD monitor with disordered and unstable screen.


Failure analysis: After testing according to the Huaping failure method, the failure was found. However, when I accidentally moved a subwoofer speaker on the computer table, I found that the display screen on the LCD screen also changed.

That is, the power of the speaker is cut off, and the speaker is removed from the computer desk. At this time, the display returns to normal display.


Maintenance example 2: A NEC 15-inch LCD monitor, the failure phenomenon is that the power indicator is on, but nothing is displayed on the screen.


Failure analysis: The power indicator of the LCD display is on, indicating that the power of the display is good. The screen has no display. The most common situation is that there is a poor contact between the LCD control module and the segment electrodes of the LCD panel.

malfunction.


Troubleshooting: Open the casing of the monitor, and you can see the circuit boards of the control circuit and power circuit module. After the circuit board is removed, the X and Y electrodes of the display panel are led out of the data line (the X electrode is referred to as the back electrode, and the Y electrode is simply

(Referred to as a segment electrode) to begin the test.


  的段电极与显示模块输出驱动板之间,是由导电橡胶来连接的。 The segment electrodes of the liquid crystal display regulator and the output drive board of the display module are connected by conductive rubber. How to check the display screen: With a common wire, peel off a small piece of insulation on one end.

Then wind the other end around the power cord of the desk lamp (other household appliances can also), hold the back electrode lead of the LCD panel with your left hand, or touch it with a metal probe (the back electrode of each column of pixels) All in one

Starting), then holding the wire with your right hand, and touching each segment electrode (ie, the Y electrode) with the bare wire ends. As long as the pixels corresponding to each segment electrode react, it can be concluded that the display is good.

Based on this, it is determined that the failure occurred on the conductive rubber, which may be caused by poor contact. Wash the conductive rubber with high-purity absolute alcohol, and then put it back in place. Then restore the parts. The power-on test monitor can display normally.

Troubleshooting.

Several parameters to pay attention to when buying an LCD display:


Brightness unit cd / m2 (the squared m2 I can't make superscript and I'm too lazy to stick)

The recommended value is 250 ~ 300. The higher the value, the better.

Contrast X00: 1 200 or more is enough I recommend 300: 1 (I wonder if there is 250?)

Response time: Refers to the response speed of the LCD to the input signal.


It is divided into two parts: rise time and fall time, and the response time is the sum of the two.


The shorter the response time. When users look at the moving picture, there will be no gloom or procrastination.


Of course, the smaller the value, the better :) 40 or less is recommended.