LCD monitors have a fairly firmly established reputation as products in which there is nothing to break. However, not a single electronic device has yet appeared in the world that does not break down at all, and therefore LCD monitors still have to be repaired. However, the economics of repairs dictate that repairs are profitable where they can be carried out quickly and with the least amount of time. At the same time, the basis of any repair is diagnostics, which often takes up to 90% of a service technician’s time. For quick and effective repair work, you need to know the structure of the monitor, have an understanding of its components and the functions they perform. In addition, the specialist must understand how a malfunction of a particular monitor module manifests itself.

Of course, each LCD monitor has its own characteristics and certain circuit design solutions, the use of which is determined by the characteristics, functions and design of the monitor, but the vast majority of this class of electronic equipment is extremely monotonous. There are several reasons leading to monotony in the construction of LCD monitors.

Firstly, the main element of the monitor is the liquid crystal panel, which, in turn, is a complete functional device with a well-defined set of input control signals, i.e. The functional diagram of the entire monitor is determined precisely by the architecture of the LCD panel. And since almost all panels have a very similar construction, this leads to the fact that various circuits that control the LCD panel must generate the same signals, that is, they must be constructed approximately the same way.

Secondly, there are not so many manufacturers of LCD panels in the world (they can be counted on the fingers of one hand) and therefore the same LCD panels are used in monitors of different companies and brands.

Third, almost all modern interfaces between the monitor and the computer, as well as between the control circuit and the LCD panel, are standardized, i.e. Manufacturers of monitors and panels are within the rather narrow framework of these standards.

Thus, for LCD monitors, communication like: “Yes, they are all the same...” is really suitable.

As a typical diagram for constructing an LCD monitor, we can suggest the diagram shown in Fig. 1. In this diagram you can find all the main functional components of the monitor.

Let's look at the functions of each module of a “typical” LCD monitor, which will allow us to correctly diagnose it.

LCD panel

Undoubtedly, this is the most basic element of the monitor, determining both its quality and its cost. As already mentioned, the LCD panel is a functionally complete device, i.e. In addition to the liquid crystal matrix itself, the panel contains a number of components that allow it to function with a minimum number of external control signals. The following elements are usually integrated into the LCD panel:

1. A matrix of liquid crystals, the position of each of which is determined by two coordinates - row number and column number.

2. A set of transistors (usually TFT) that provide “switching on” and “switching off” of the corresponding liquid crystal cell. These transistors can actually be called switching switches, and each cell of the crystal matrix has its own transistor, i.e. there will be as many of them as there are dots on the screen. These transistors, in turn, are an integral part of the liquid crystal matrix. To switch a transistor, you must “specify” the row number and the column number in which it is located, i.e. The transistor is switched using two signals.

3. Sampling scheme that allows you to include the desired matrix cell. The sampling circuit generates signals to turn on the TFT transistor. The sampling scheme consists of two elements: row drivers and column drivers.

4. TCON control controller. This controller, which is an integrated circuit, processes the input signals coming from the control board. Commands for column and row drivers are formed from these control signals.

5. Interface circuit, which in modern panels is usually a differential signal receiver. The interface circuit receives signals from the control board in serial form and converts them into parallel code.

6. Regulator and converter of input supply voltage.

Backlight

The backlight lamp generates a light flux, which, passing through a matrix of liquid crystals, creates a visible “picture”. Fluorescent lamps are most often used as backlight lamps. These can be either traditional filament lamps or cold cathode lamps (CCFL), which are most often used in modern monitors. The number of backlight lamps ranges from 2 before 6 . On some monitors, the brightness of the lamps can be controlled to provide image contrast and grayscale adjustments. Backlight lamps are usually made in the form of a separate module that can be changed.

Inverter

The functions of the inverter include creating the supply and starting voltage for the backlight lamps. An inverter is a voltage converter, also called a power supply or electronic ballast. An inverter is a pulse converter operating at high frequencies.

At the initial moment of start-up, a voltage is formed at the inverter output 1.5 – 2 kV, which should “light” the lamp. After the breakdown of the discharge gas gap by such a high voltage voltage, generation of a pulse converter begins at frequencies 30 – 150 kHz. In operating mode, the amplitude of the alternating voltage is in the range from 150 before 800 Volt. The lamp, when turned on, is an inductive load for the voltage generator. The function of the inverter is to obtain these high voltage pulse voltages from a low voltage DC voltage, usually rated +12V. The function of the inverter is also to ensure stability of the voltage applied to the lamps, which allows you to create an even, flicker-free light. In addition, the inverter contains current protection that blocks the operation of the circuit in emergency modes.

The inverter is turned on and off by control signals from the control board. If adjusting the image contrast and adjusting the grayscale are carried out by backlight lamps, then the corresponding control signals must come from the control board to the inverter, and not just the signal to turn the lamp on/off.

Control board

The control board provides the generation of control signals for the LCD panel and for the inverter. The control board is exactly that module, the construction of which differs in different monitors, because This is exactly what monitor manufacturers design by purchasing standard LCD modules. The control board usually contains the following elements:

- interface circuits for processing signals from a personal computer (PC);

- analog-to-digital converter (ADC);

- display controller (microprocessor);

- signal transmitter for LCD module.

In addition to these almost mandatory elements, quite often on the control board you can find RAM chips, specialized controllers, ROM and FLASH memory chips.

The control board is connected to the LCD module via a cable, the serviceability of which largely determines the correct operation of the monitor.

Control Panel

The control panel provides communication between the user and the monitor. The control panel contains buttons that allow you to enter modes for setting monitor parameters. In addition, the control panel also has a light indicator, which displays the operating mode of the monitor and can display messages about monitor malfunctions. Messages are displayed in the form of a specific algorithm for turning on and off the LED.

Voltage transformer

The voltage converter actually acts as a power supply, providing all elements of the monitor with the appropriate voltages. There are two options for providing the LCD monitor with supply voltages:

1. With external power adapter and internal voltage regulator and stabilizer.

2. With internal switching power supply.

The first option is used more often. In this case, a constant voltage with a nominal value of 12 ....24 V. The internal converter ensures that voltages are received 5V, 3.3V, 2.5V and others from the adapter output voltage. Linear integrated stabilizers or pulse converters can be used as part of the monitor for these purposes. This power supply option undoubtedly reduces the size and weight of the monitor and, perhaps, leads to increased reliability.

The second option implies that the monitor contains the most common switching power supply, which provides some advantages - the absence of external units, and the ability to connect the monitor directly to the network. On the other hand, increasing the size of the monitor and reducing the reliability of the product may not be to the liking of some users.

Most often, the voltage converter is located on the control board, so malfunctions of the converter are very often attributed specifically to malfunctions of the control circuit.

The main problems that arise during the operation of monitors are described in Table 1.

Table 1. Typical malfunctions of LCD monitors, their causes and solutions

Manifestation of malfunction	Possible causes of malfunction	Repair and diagnostic methods
The monitor does not turn on. The power indicator is not lit.	The internal power supply is faulty (if the monitor does not use an AC adapter)	1. Replace the power supply if it is made as a separate module. 2. Replace the control board if the power supply is installed on this board. 3. Check the input circuits, key transistor, PWM controller chip and, if necessary, replace faulty elements. 4. Check the output circuits of the power supply for short circuits.
	The network adapter is faulty (for monitors with a network adapter)	1. Check the presence of 12-24 V voltage at the output of the adapter. If there is no voltage, replace the adapter. 2. Check the wires between the power plug and the adapter and between the adapter and the monitor. In case of a break, repair it. 3. Replace the network adapter.
	The power button is faulty	Check the button with a tester and, if necessary, replace it.
	The voltage converter on the control board is faulty
	Control board faulty	Replace the control board.
Image brightness is inappropriate (too high or too low).	Inverter faulty	2. Replace the control board.
	Control board faulty	Replace the control board.
Flashing screen (entire screen or one edge)	Inverter faulty	Check the presence of pulsed high-frequency voltage at the inverter output (300V -1500V - the exact rating depends on the specific monitor). In case of absence: 1. Replace the inverter (or repair it). 2. Replace the control board. 3. Check the connecting cable between the inverter and the control board (if the inverter is designed as a separate module).
	The backlight lamp is faulty	Replace the backlight unit.
The screen is dark. The power indicator lights up.	LCD panel is faulty	Replace the LCD panel.
	Inverter faulty	Check the presence of pulsed high-frequency voltage at the inverter output (300V -1500V - the exact rating depends on the specific monitor). In case of absence: 1. Replace the inverter (or repair it). 2. Replace the control board. 3. Check the connecting cable between the inverter and the control board (if the inverter is designed as a separate module).
	Control board faulty	Replace the control board.
	The voltage converter on the control board is faulty.	1. Use a tester to check the presence of output voltages on all stabilizers. If there is no voltage, replace the stabilizers or the board. 2. Replace the control board.
		Turn on the monitor in standalone mode without connecting it to a PC. If any message appears on the screen, the interface cable must be replaced.
The screen turns off after a while.	The inverter is faulty (current protection is triggered)	Replace the inverter unit or check its components.
	The backlight lamp is faulty	Replace the backlight unit.
One of the colors is missing.	The interface cable between the monitor and computer is faulty
	The interface cable between the control board and LCD panel	Replace the cable or check it with a continuity test using a tester.
		Replace the control board.
	LCD panel is faulty	Replace the LCD panel.
The setup menu does not appear on the screen.	Control board elements are faulty	Replace the control board.
The control panel buttons do not function.	Buttons are faulty	Check the buttons using a tester.
	Break in the connecting cable between the control panel and the control board	Check the cable with a tester.
	Control board faulty	Replace the control board.

Figures 2 - 5 show quick algorithms for diagnosing and repairing monitors when corresponding malfunctions occur. For greater clarity, repair algorithms are presented in graphical form. These algorithms will help carry out quick repairs at the level of individual modules.

Due to the content of a number of precious metals and environmentally hazardous substances, monitors are subject to strict disposal requirements. The procedure for their disposal in organizations is even more strictly regulated. But before “throwing away” a batch of outdated or failed equipment, a company or budgetary institution must write it off its balance sheet. Violation of the regulations for the write-off and disposal of monitors may entail a large fine, according to current legislation.
The ALAR LLC company specializes in services for write-off and disposal of office equipment in Moscow and the Moscow region. Having 10 years of experience behind us, monitoring the latest trends, changes and the emergence of new requirements for this issue, we provide services at a highly professional level to commercial enterprises and budgetary organizations of the capital.

Monitor faults for write-off

Physical and mental wear and tear is the most common reason for sending a computer monitor for disposal. This is explained by:

• first of all, their service life is relatively short - 3–5 years (depending on the type of monitor) according to the current regulations,
• secondly, equipment becomes obsolete very quickly due to the emergence of new improved technologies,

The list of problems with CRT (CRT) monitors includes the following:

• frame scanning unit as a result of violated requirements for operating temperature indicators;
• monitor power supply with subsequent breakdown of modules due to power surges;
• horizontal scanning unit as a result of breakdown of the transformer booster compressor and high-voltage circuits due to dust accumulation;
• video card elements as a result of obsolescence or non-compliance with operating temperature conditions;
• elements of the kinescope, which results in a decrease in the level of magnetization, distortion of the image, and disruption of color rendering (the cause may be mechanical damage or obsolescence of its elements).

The reason for writing off TFT (LCD) monitors may be the failure of:

• backlight lamp or its inverter due to expired service life or mechanical damage;
• system board at the end of its service life, etc.;
• matrix board as a result of external influences: shock, moisture, etc.;
• power supply due to voltage surges.

The reason for writing off LCD monitors may be damage to:

• power supply with subsequent burnout of primary electrical circuits;
• voltage inverter and backlight lamps as a result of impact or burnout due to improper operation, expiration of service life;
• processor board;
• matrix boards due to moisture, shock;
• electrolytic capacitors, inverter transistors, etc.

If it is determined that the monitor is unsuitable for repair, it must be replaced or written off. The established reason must be indicated in the results of the examination of the monitor for write-off.

Write-off of monitors in budgetary institutions

In budgetary institutions, separate regulations for the write-off of computer equipment are observed in accordance with Government Decree No. 834 of October 14, 2010 and the “Regulations on the write-off of property.” This is due to the fact that, in essence, the institution is not the owner, i.e. it simply does not have the right to dispose of it, but only to use state property (resources) for the purpose of operational management (according to Article 298 of the Civil Code of the Russian Federation).
Almost every budgetary institution has its own Write-off (Accounting) Commission, whose responsibilities include:

• examination of equipment (monitors) subject to write-off and disposal and accompanying technical documentation;
• making a decision indicating the reasons and timing for decommissioning the monitor;
• drawing up an act for writing off office equipment.

The documented results of the examination, the Commission’s decision and the act are submitted to the GRBS for approval. The final decision on write-off is made by:

• a representative (or his leadership) of government bodies authorized to manage budget funds (GRBS), after agreeing on all the nuances with the Federal Property Management Agency
• directly the management of the budgetary institution, vested with relevant powers by higher authorities.

Only after receiving a positive response to the write-off, a budget institution can contact a recycling company. All agreements and terms of cooperation are formalized in a contract. ALAR LLC actively cooperates with budgetary organizations in Moscow, offering favorable conditions for the disposal of office equipment.

Modern LCD monitors- complex devices constructed from an active LCD matrix, a power board, an inverter board and a matrix backlight system. Since the assembly is carried out from parts from different manufacturers (there are no more than 8 of them) and in different factories under different conditions, the quality, accordingly, is very different for models even from the same manufacturer.

There are cases of manufacturing defects, as well as defects in soldering, assembly and design errors in the electronic circuit of the device. Repair of LCD monitors comes down to analyzing the symptoms of the malfunction, localizing the failure and debugging by replacing parts if necessary.

Typical malfunctions of modern LCD monitors

1. The power supply board is faulty. In this case, the primary electrical circuits burn out, while the internal circuits often remain intact.
2. Failure of the voltage inverter. The inverter is responsible for supplying high voltage to the backlight lamps. In this case, the screen will be dark, and when illuminated under a table lamp, it will be faded.
3. Failure of backlight lamps. The display is either dim or completely dark. Lamps can “die” due to mechanical damage, “shrink”, or simply burn out due to their operation at increased brightness.
4. Failure of the control board with the processor. In this case, the image may be geometrically distorted, a message about incorrect resolution, frequency, or parameters may be displayed, and the image may move.
5. Mechanical damage to the matrix, water or other liquids or foreign objects getting into the device. Part of the image, stripes, or streaks may be displayed. Objects falling inside can be fatal to the monitor.

Practice shows that most failures occur in electronics. In particular, inverter transistors, electrolytic capacitors that were incorrectly selected by the developer and faulty and soldering defects fail.

The matrix and backlight lamps themselves never fail, only due to mechanical damage. It happens that the lamps dim or one of the backlight lamps fails.

In this case, the image will be darker and faded. According to statistics, BenQ monitors, as well as Samsung and LG monitors from “unsuccessful” series and batches, are most often repaired.

Backlight system malfunctions

The backlight system is organized in approximately the same way in monitors from different manufacturers.

• An LCD matrix is ​​a set of LCD crystals arranged in a grid matrix.
• Each matrix cell is connected to a thin film transistor (TFT), which controls the opening and closing of the LCD cells. In fact, we get a matrix of conductors.
• Behind it there is a reflective layer and at the edges there is a matrix illumination system, consisting of 2 or 3 pairs of cold cathode lamps.

When one of the lamps stops glowing, the brightness of the monitor decreases significantly and the backlight becomes uneven. If all lamps are turned off, the screen becomes dark.

The screen can go out either due to the fault of the lamps themselves or due to the electrical circuitry for controlling them.

Replacing a burnt-out lamp is quite difficult.

• Firstly, they are very fragile and crack with any careless movement.
• Secondly, a voltage of about 1000 V is supplied to the lamps, which requires removing static electricity from the lamp control circuit.
• Thirdly, it is quite difficult to find similar lamps that have failed.

The most common electronic part of a monitor that fails is inverter board. This board contains control transistors and step-up transformers, which supply high voltage to the backlight lamps.

Lamps may also go out if the factory solder is of poor quality. In particular. This may appear after a certain period of work. Usually both lamps turn off at once, this is due to the peculiarities of their connection according to the circuit. During operation of the monitor, the boards and electronic elements heat up, and if there are soldering defects or microcracks, the contact becomes “floating” due to the different heat capacity of the elements.

Electronics malfunctions

Lately, monitors with broken or oxidized contacts, overheating of elements, as well as poor-quality factory soldering or corroded soldering are increasingly being received at our service center.

In modern soldering processes, “active flux” is used, which must be removed after soldering. At some factories, the technical process is disrupted. If the flux is not removed from the board, then after some time it begins to corrode the soldering elements. As a result, after 1.5-3 years the monitor fails.

There are also cases of malfunctions of the control board processor. These are the most difficult cases of repair. Sometimes, in case of failure, warming up the contacts, processor or board with a soldering hair dryer helps.

In particular, “glitchy” MICOM processors installed in some lines of Samsung and LG monitors are famous for this. If the processor overheats or fails, then it must be replaced with exactly the same one, which can sometimes be quite difficult.

All these electronic malfunctions require the engineer to have extensive experience and the use of special tools, and sometimes entire complexes. All this can be found in our service center.

The standard LCD monitor today has a fairly simple design. This is the case, the protective panel on the screen, the matrix, the matrix backlight device, the power module, the processor and controllers. There are also input interfaces and a power connector. All monitor malfunctions are reduced to the failure of one of the listed parts. A laptop monitor is not fundamentally different from a regular desktop monitor.

Each breakdown has its own symptoms. Typical faults of LCD monitors can be reduced to mechanical and electronic.

Mechanical problems with the monitor

They are very easy to remove. This is a power wire that has jumped out of the connector or dirt that has gotten onto the contacts of the connecting cable. Such defects are treated by disconnecting all wires one by one and connecting them.

In the case of a laptop, this also includes damage to the connecting cable, which is constantly experiencing cyclic loads. The cable can be easily replaced with a new, similar one, and the malfunction will be indicated by interference when the lid with the display moves.

Electronics malfunctions in the monitor and their elimination

If we combine all standard faults into groups, we get list of typical breakdowns.

Monitor won't turn on

The monitor does not turn on after pressing the power button. Most often, the problem lies in the failure of the power supply module. The first thing to fail in a power supply is the capacitors. Very carefully disassemble your monitor, having first disconnected it from the power supply, and check the condition of the cans. Don't forget that capacitors store an electrical charge and can shock you. If you do not understand the basics of electrical engineering, contact a professional service.

If the capacitors are swollen, then the problem is obvious. It is necessary to buy similar parts and replace them. All parts are marked, so an analogue will be easy to find. If there is no obvious damage, then you can simply replace the entire power board with a similar one or a new one. In addition, you need to carefully inspect the soldering on all parts. Sometimes the problem is obvious. It will be quite problematic to do more complex manipulations on your own.

The image appears dim

The picture is present, but there is no backlight. If you light the monitor, you can see that there is a picture. In this case, you need to think about a violation in the backlight lamp circuit or failure of the lamp itself. In addition, the power supply or inverter may also be damaged. All these parts can be replaced yourself if new suitable options are available.

Horizontal or vertical stripe on the monitor

The monitor works, but there is a continuous one-color stripe running through the entire image. The malfunction is often associated with damage to the matrix, which occurred from the long service of the monitor or is simply a consequence of poor manufacturing. The only thing that can be done is to check the connection of the contact connectors to the matrix and try to carefully glue the loose contacts. But most likely, a new matrix will be required.

Dark or colored spot on the screen

It most often occurs as a result of a blow to the monitor. Frequent and strong poking of fingers at the monitor also leads to this refusal. The matrix will need to be replaced.

Bright single dots on the monitor

These are dead pixels - a common problem inherent in inexpensive monitors. Looks like a bright dot of a different color on the monitor. Consequence of a manufacturing defect or long work. It may also indicate mechanical damage to the matrix. Sometimes it can be eliminated using software methods.

Reduced monitor brightness

This is a consequence of the failure of the backlight lamp.

Image jitter and noise

Technically, this problem will be quite difficult to solve for the uneducated user. However, sometimes the problem can be solved very simply by simply replacing the cable. Instead of a regular cable, you need to take a cable with an electromagnetic interference suppressor.

Repairing a modern LCD monitor

Modern monitors are quite reliable and device failure is a fairly rare occurrence. However, such cases do occur. If the malfunction is not associated with a cable that has come loose or a bus that has fallen out of the connector, then it is associated with a failure of the electronics. Unfortunately, it is the electronics that most often break down. A serious problem can no longer be solved by a simple block repair and deep knowledge in the field of electrical engineering is required.

When thinking about repairing your monitor, calculate your planned budget. In difficult cases, you will need to purchase an expensive LCD matrix and pay for the work of a professional technician. Most often, this is impractical, since a new modern monitor will cost commensurately with the repair of the old one.

Modern LCD monitors are complex devices constructed from an active LCD matrix, a power board, an inverter board and a matrix backlight system.

Since the assembly is carried out from parts from different manufacturers (there are no more than 8 matrix manufacturers) and in different factories under different conditions, the quality, accordingly, is very different for models even from the same manufacturer.

There are cases of manufacturing defects, as well as defects in soldering, assembly and design errors in the electronic circuit of the device.

LCD Monitor Repair boils down to analyzing the symptoms of a malfunction, localizing the failure and repairing by replacing parts if necessary.

Typical malfunctions of modern LCD monitors:
The power supply board is faulty. In this case, the primary electrical circuits burn out, while the internal circuits often remain intact.
Failure of the voltage inverter. The inverter is responsible for supplying high voltage to the backlight lamps. In this case, the screen will be dark, and when illuminated under a table lamp, it will be faded.
Failure of backlight lamps. The display is either dim or completely dark. Lamps can “die” due to mechanical damage, “shrink”, or simply burn out due to their operation at increased brightness.
Failure of the control board with the processor. In this case, the image may be geometrically distorted, a message about incorrect resolution, frequency, or parameters may be displayed, and the image may move.
Mechanical damage to the matrix, ingress of water or other liquids and foreign objects into the device. Part of the image, stripes, or streaks may be displayed. Objects falling inside can be fatal to the monitor.

Practice shows that Most failures occur in electronics. In particular, inverter transistors, electrolytic capacitors that were incorrectly selected by the developer and faulty and soldering defects fail.

Matrix and backlights They never fail on their own, only due to mechanical damage. It happens that the lamps dim or one of the backlight lamps fails. In this case, the image will be darker and faded.

According to statistics, BenQ, as well as Samsung and LG monitors from “unsuccessful series and batches” are most often repaired.

Backlight system malfunctions

The backlight system is organized in approximately the same way in monitors from different manufacturers. An LCD matrix is ​​a set of LCD crystals arranged in a grid matrix. Each matrix cell is connected to a thin film transistor (TFT), which controls the opening and closing of the LCD cells. In fact, we get a matrix of conductors. Behind it there is a reflective layer and at the edges there is a matrix illumination system, consisting of 2 or 3 pairs of cold cathode lamps. When one of the lamps stops glowing, the brightness of the monitor decreases significantly and the backlight becomes uneven. If all lamps are turned off, the screen becomes dark. The screen can go out either due to the fault of the lamps themselves or due to the electrical circuitry for controlling them.

Replacing a burnt-out lamp is quite difficult. Firstly, they are very fragile and crack with any careless movement. Secondly, a voltage of about 1000 V is supplied to the lamps, which requires removing static electricity from the lamp control circuit. Thirdly, it is quite difficult to find similar lamps that have failed.

The most common electronic part of a monitor that fails is inverter board. This board contains control transistors and step-up transformers, which supply high voltage to the backlight lamps.

The lamps may go out if poor quality factory soldering. In particular. This may appear after a certain period of work. Usually both lamps turn off at once, this is due to the peculiarities of their connection according to the circuit.
During the monitor, the boards and electronic elements heat up, and if there are soldering defects or microcracks, the contact becomes “floating” due to the different heat capacity of the elements.

Electronics malfunctions

Lately, monitors have been coming to our service center more and more often. with broken or oxidized contacts, overheating of elements, as well as poor-quality factory soldering, corroded soldering.
In modern soldering processes, “active flux” is used, which must be removed after soldering. At some factories, the technical process is disrupted - if the flux is not removed from the board, then after some time it begins to corrode the soldering elements. As a result, after 1.5-3 years the monitor fails.

Meet and cases of control board processor malfunctions. These are the most difficult cases of repair. Sometimes, in case of failure, warming up the contacts, processor or board with a soldering hair dryer helps. In particular, the buggy MICOM processors installed in some lines of Samsung and LG monitors are famous for this. If the processor overheats or fails, then it must be replaced with exactly the same one, which can sometimes be quite difficult.

All these electronic malfunctions require the engineer to have extensive experience and the use of special tools, and sometimes entire complexes. All this can be found in our service center.

Principle of LCD monitor