Typical malfunctions of LCD monitors (exchange of experience). Basic malfunctions of LCD monitors Failure of the monitor matrix
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 ones burn out electrical circuits, 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, when illuminated under table lamp- 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 may cause fatal outcome for 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, monitors from BenQ, as well as Samsung and LG from “unfavorable 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 electrical diagram managing 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 heat up and electronic elements, and if there are soldering defects or microcracks, and 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 the modern technical process when soldering, “active flux” is used, which mandatory 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
LCD monitors have been used for more than 10 years. Naturally, they fail. Moreover, if in old CRT monitors the most vulnerable units were a line scan unit and a switching power supply (UPS), then for LCD monitors it is a UPS and an inverter, which provides power to the backlight of the LCD panel (matrix). One characteristic malfunction of the LCD monitor inverter and how to eliminate it are described in this article.
Computer LCD monitor Samsung Syn cMaster 940 M produced in 2007, when turned on, it worked for about two seconds, after which the display went out.
To disassemble the monitor case, you need to unscrew 3 screws and remove the stand from it. After this, use a wide screwdriver to separate the halves of the plastic monitor housing. Then you need to unscrew the two screws securing the VGA connector. After that, having unfastened all the detachable inter-board connectors, you should unscrew both printed circuit boards of the monitor from the metal chassis. We are interested in the power supply and high-voltage inverter board (model SIP-U5F(M)) (photo 1).
The T602S step-up transformer with two high-voltage windings marked TMS91429CT immediately came under suspicion, which photo 1 located at the top.
It was not possible to find such transformers in retail sales.
In order to finally verify the malfunction of this transformer, you should temporarily disable the protection. To do this, a resistor with a resistance of 330...390 kOhm is soldered between pin 1 of the IC1 microcircuit of the FAN7314 type and the common wire. (photo 2). On photo 2 This is the only pad resistor, located on the left side of the photo next to R6. After this, all inter-board connections were restored, and the monitor was turned on to the network. It started working, and a window appeared on the screen with information that the cable was not connected to the PC. The matrix was illuminated evenly, but both “halves” of the T602S step-up transformer heated up. 
After turning off the monitor, T602S was unsoldered, and the resistance of its secondary windings was checked. They turned out to be unequal. The resistance of one of them was about 1.25 kOhm (this is normal), and the other was about 7.5 kOhm, which is not normal. In addition, at the physical location of the faulty winding under the soldered transformer, significant darkening of the printed circuit board was discovered, which indicates that this winding was very hot.
Since there was no original transformer available, a similar dual transformer labeled S320SL from 2008 was remade from a faulty 32-inch SONY LCD TV whose LCD matrix was manufactured by Samsung. In total, one such TV had six dual high-voltage transformers designed to power the backlight lamps of the LCD matrix. The transformer from the LCD TV had to be partially rewound for use in a 19-inch LCD computer monitor. Both high-voltage windings, wound with a thin single-core wire, are left intact. The low-voltage primary winding, wound with stranded wire in two sections of the transformer, was carefully unwound without disassembling the core. Then, the stranded wire from the primary winding was folded in half and twisted. An insulating tube was put on one end of this wire and soldered to one of the freed “legs” of the transformer frame, winding 11 turns around this frame. Excess winding wire is cut off. An insulating tube is put on the second end of the wire, the wire is tinned with solder, using an aspirin tablet (acetylsalicylic acid) as a flux, and soldered to the previously released terminal of the transformer. The wound primary winding is impregnated several times with colorless tsaponlac. The inductance of each high-voltage winding of such a transformer is about 1.5 mH, the resistance is about 1.3 kOhm. The inductance of the rewound low-voltage primary winding is about 170 μH.
At the installation site of the new transformer, 4 additional ventilation holes with a diameter of 5 mm were drilled (photo 3), and an additional heat sink is soldered to the heatsink of the Q601 transistor assembly (on photo 3 left). At the installation location of Q601, on the side of the printed conductors, between the petals of the metal radiator and the heat sink flange of the transistor assembly, a large amount of solder is filled. To further increase the reliability of the device, capacitors C611 and C617 470 μF x 25 V were installed; the space for these capacitors was empty. 
The new transformer is installed on the printed circuit board with its leads facing up. Before installing it, pieces of matches 3 cm long are glued to the ferrite core (to the short lateral sides) using Quintol glue. The transformer is attached to the circuit board using these matches using Quintol glue.
The location of the pins and the wiring of the windings of the converted transformer does not correspond to the one being replaced. Therefore, the transformer is connected to the circuit with a mounting wire. Moreover, the high-voltage windings are soldered with wires in double PVC insulation. Connected to the transformer terminals PVC tubes. Appearance a fragment of the circuit board after repairs and upgrades is shown in photo 4. Before assembly, eight layers of adhesive tape “Scotch” 50 mm wide are glued to the part of the metal plate that is close to the installed transformer. Despite the smaller size of the rewound transformer compared to the original faulty one, the heating of this transformer during long-term operation turned out to be insignificant. 
After all inter-board connections were restored, the functionality of the monitor was checked and it was assembled. It was then turned on for a 15-minute run, and after a break of 12 hours, it successfully completed a working run at 12 hours.
The described malfunction of Samsung LCD monitors, which have a pulse transformer with two high-voltage windings, manufactured in the second half of the first decade of the 2000s, is considered typical. In a similar way, the author redesigned and repaired two Samsung SyncMaster 940M monitors, the first of which has been working flawlessly for about two years after repair. In the second monitor, in contrast to the case described above, the faulty winding had a reduced resistance (interturn short circuits), as a result of which the transformer became very hot.
I would like to note that some Internet resources suggest disconnecting and/or removing the faulty high-voltage winding of the dual transformer and using a monitor with only two working backlights instead of four. The author considers such repairs to be hackwork and deception of the client. It is acceptable, perhaps, as a temporary measure with the obligatory replacement of the transformer in the future.
In conclusion, we note that computer LCD monitors, unlike televisions, usually do not have built-in speakers and operate silently. Therefore, it is imperative to turn off the 220 V power supply when disassembling, assembling and soldering any elements inside the device.
In order to repair an LCD monitor with your own hands, you must first understand what basic electronic components and blocks it consists of. this device and what each element of the electronic circuit is responsible for. Beginning radio mechanics at the beginning of their practice believe that success in repairing any device lies in the availability of a circuit diagram of a specific device. But in fact, this is a misconception and a circuit diagram is not always needed.
So, let’s open the cover of the first LCD monitor that comes to hand and in practice we will understand its structure.
LCD monitor. Main functional blocks.
The LCD monitor consists of several functional blocks, namely:
LCD panel
The liquid crystal panel is a complete device. As a rule, the assembly of an LCD panel is carried out by a specific manufacturer, who, in addition to the liquid crystal matrix itself, integrates fluorescent backlight lamps, frosted glass, polarizing color filters and electronic board decoders that generate voltages from digital RGB signals to control the gates of thin-film transistors (TFTs).
Consider the composition of the LCD panel of a computer monitor ACER AL1716. The LCD panel is a complete functional device and, as a rule, there is no need to disassemble it during repairs, with the exception of replacing failed backlight lamps.
LCD panel marking: CHUNGHWA CLAA170EA
On the back of the LCD panel there is a fairly large printed circuit board, to which a multi-pin cable is connected from the main control board. The printed circuit board itself is hidden under a metal strip.
LCD panel of Acer AL1716 computer monitor
The printed circuit board contains a multi-pin NT7168F-00010 chip. This microcircuit is connected to the TFT matrix and participates in the formation of the image on the display. From the NT7168F-00010 microcircuit there are many pins, which are formed into ten loops under the designation S1-S10. These cables are quite thin and appear to be glued to the printed circuit board on which the NT7168F chip is located.
LCD panel printed circuit board and its elements
Control board
The control board is also called the main board ( Main board). The main board houses two microprocessors. One of them is a control 8-bit microcontroller SM5964 with an 8052 core and 64 kB of programmable Flash memory.
The SM5964 microprocessor performs a fairly small number of functions. A button panel and monitor operation indicator are connected to it. This processor controls turning the monitor on/off and starting the backlight inverter. To save user settings, a memory chip is connected to the microcontroller via the I 2 C bus. Typically, these are eight-pin non-volatile memory chips of the series 24LCxx.
Main board of LCD monitor
The second microprocessor on the control board is the so-called monitor scaler (LCD controller) TSU16AK. This microcircuit has many tasks. It performs most of the functions related to converting and processing the analog video signal and preparing it for submission to the LCD panel.
With regard to an LCD monitor, you need to understand that it is inherently digital device, in which all control of the LCD pixels occurs digitally. The signal coming from the computer's video card is analog and for its correct display on the LCD matrix it is necessary to carry out many transformations. This is what a graphics controller is designed for, or otherwise a monitor scaler or an LCD controller.
The tasks of the LCD controller include such as recalculation (scaling) of images for different resolutions, formation of the OSD menu, processing of analog RGB signals and sync pulses. In the controller analog signals RGB is converted to digital by 3-channel 8-bit ADCs that operate at 80 MHz.
The TSU16AK monitor scaler interacts with the SM5964 microcontroller via a digital bus. To operate the LCD panel, the graphics controller generates synchronization signals, clock frequency and matrix initialization signals.
The TSU16AK microcontroller is connected via a cable to the NT7168F-00010 chip on the LCD panel board.
If the graphics controller of the monitor malfunctions, as a rule, defects appear related to the correct display of the image on the display (stripes may appear on the screen, etc.). In some cases, the defect can be eliminated by soldering the scaler leads. This is especially true for monitors that operate around the clock in harsh conditions.
During prolonged operation, heating occurs, which has a bad effect on the quality of soldering. This may cause malfunctions. Defects related to the quality of soldering are not uncommon and are also found in other devices, for example, DVD players. The cause of the malfunction is degradation or poor-quality soldering of multi-pin planar microcircuits.
Power supply and backlight inverter
The most interesting thing to study is the monitor's power supply, since the purpose of the elements and circuitry are easier to understand. In addition, according to statistics, malfunctions of power supplies, especially switching ones, occupy a leading position among all others. That's why practical knowledge devices, element base and circuit design of power supplies will certainly be useful in the practice of repairing radio equipment.
The power supply for the LCD monitor consists of two. The first one is AC/DC adapter or in other words, a network switching power supply (pulse unit). Second - DC/AC inverter . Essentially these are two converters. AC/DC adapter is used to convert alternating voltage 220 V to constant voltage small size. Typically, voltages from 3.3 to 12 volts are generated at the output of a switching power supply.
The DC/AC inverter, on the contrary, converts direct voltage (DC) into alternating voltage (AC) with a value of about 600 - 700 V and a frequency of about 50 kHz. Alternating voltage is supplied to the electrodes fluorescent lamps, built into the LCD panel.
First, let's look at the AC/DC adapter. Most switching power supplies are built on the basis of specialized controller microcircuits (with the exception of cheap mobile chargers, for example).
In the documentation for the TOP245Y chip you can find typical examples of circuit diagrams of power supplies. This can be used when repairing power supplies for LCD monitors, since the circuits largely correspond to the standard ones indicated in the description of the microcircuit.
Here are some examples of circuit diagrams of power supplies based on TOP242-249 series microcircuits.
Fig 1. Example of a power supply circuit diagram
The following circuit uses dual Schottky barrier diodes (MBR20100). Similar diode assemblies (SRF5-04) are used in the Acer AL1716 monitor unit we are considering.
Fig 2. Schematic diagram of a power supply based on a microcircuit from the TOP242-249 series
Note that the above circuit diagrams are examples. Actual circuits of pulse blocks may differ slightly.
The TOP245Y microcircuit is a complete functional device, the housing of which contains a PWM controller and a powerful field-effect transistor, which switches with a huge frequency from tens to hundreds of kilohertz. Hence the name - switching power supply.
LCD monitor power supply (AC/DC adapter)
The operating diagram of a switching power supply is as follows:
Rectification of alternating mains voltage 220V.
This operation is performed by a diode bridge and a filter capacitor. After rectification, the voltage across the capacitor is slightly higher than the mains voltage. The photo shows a diode bridge, and next to it is a filtering electrolytic capacitor (82 µF 450 V) - a blue barrel.
Voltage conversion and reduction using a transformer.
Switching with a frequency of several tens - hundreds of kilohertz of direct voltage (>220 V) through the winding of a high-frequency pulse transformer. This operation is performed by the TOP245Y chip. A pulse transformer performs the same role as a transformer in conventional network adapters, with one exception. It works for more high frequencies, many times greater than 50 hertz.
Therefore, the manufacture of its windings requires a smaller number of turns, and, consequently, less copper. But a core of ferrite is required, and not of transformer steel as in 50 hertz transformers. Those who do not know what a transformer is and why it is used, first read the article about the transformer.
The result is a very compact transformer. It is also worth noting that switching power supplies are very economical and have high efficiency.
Rectification of alternating voltage reduced by a transformer.
This function is performed by powerful rectifier diodes. In this case, diode assemblies labeled SRF5-04 are used.
To rectify high-frequency currents, Schottky diodes and conventional power diodes with p-n junctions are used. Conventional low-frequency diodes for rectifying high-frequency currents are less preferable, but are used for rectifying high voltages (20 - 50 volts). This must be taken into account when replacing defective diodes.
Schottky diodes have some features that you need to know. Firstly, these diodes have a low transition capacitance and are able to quickly switch - go from open to closed state. This property is used to operate at high frequencies. Schottky diodes have a low voltage drop of about 0.2-0.4 volts, versus 0.6 - 0.7 volts for conventional diodes. This property increases their efficiency.
Schottky barrier diodes also have undesirable properties that hinder their wider use in electronics. They are very sensitive to excess reverse voltage. If the reverse voltage is exceeded, the Schottky diode irreversibly fails.
A conventional diode goes into reversible breakdown mode and can recover after exceeding the permissible reverse voltage value. It is this circumstance that is the Achilles heel, which causes the burnout of Schottky diodes in rectifier circuits of all kinds pulse blocks nutrition. This should be taken into account when carrying out diagnostics and repairs.
To eliminate voltage surges that are dangerous for Schottky diodes and are formed in the transformer windings at the pulse fronts, so-called damping circuits are used. In the diagram it is designated as R15C14 (see Fig. 1).
When analyzing the circuitry of the Acer AL1716 LCD monitor power supply, damping circuits were also found on the printed circuit board, consisting of a 10 Ohm SMD resistor (R802, R806) and a capacitor (C802, C811). They protect Schottky diodes (D803, D805).
Damping circuits on the power supply board
It is also worth noting that Schottky diodes are used in low-voltage circuits with a reverse voltage limited to a few tens of volts. Therefore, if you need to obtain a voltage of several tens of volts (20-50), then diodes are used p-n basis transition. This can be seen if you look at the datasheet for the TOP245 chip, which lists several standard schemes power supplies with different output voltages (3.3 V; 5 V; 12 V; 19 V; 48 V).
Schottky diodes are sensitive to overheating. In this regard, they are usually installed on an aluminum radiator to dissipate heat.
Distinguish diode based p-n junction from the diode on the Schottky barrier can be found according to the conventional graphic designation in the diagram.
Symbol for a diode with a Schottky barrier.
After the rectifier diodes, electrolytic capacitors are installed to smooth out voltage ripples. Next, using the resulting voltages 12 V; 5 V; 3.3 V powers all LCD monitor units.
DC/AC inverter
In terms of its purpose, the inverter is similar to electronic ballasts (EPGs), which have been found wide application in lighting technology for powering household lighting fluorescent lamps. But, there are significant differences between the electronic ballast and the LCD monitor inverter.
An LCD monitor inverter is usually built on a specialized chip, which expands the range of functions and increases reliability. For example, the backlight inverter of the Acer AL1716 LCD monitor is built on the basis of a PWM controller OZ9910G. The controller chip is mounted on a printed circuit board using planar mounting.
The inverter converts direct voltage, the value of which is 12 volts (depending on the circuit design), into alternating voltage of 600-700 volts and a frequency of 50 kHz.
The inverter controller is capable of changing the brightness of fluorescent lamps. Signals for changing the brightness of the lamps come from the LCD controller. Field-effect transistors or their assemblies are connected to the controller microcircuit. In this case, two complementary assemblies are connected to the OZ9910G controller field effect transistors AP4501SD(Only 4501S is indicated on the chip body).
Assembly of field-effect transistors AP4501SD and its pinout
Also, two high-frequency transformers are installed on the power supply board, which serve to increase the alternating voltage and supply it to the electrodes of the fluorescent lamps. In addition to the main elements, the board contains all kinds of radio elements that serve to protect against short circuit and lamp malfunctions.
Information on repairing LCD monitors can be found in specialized repair magazines. For example, in the magazine “Repair and Service of Electronic Equipment” No. 1, 2005 (pp. 35 – 40), the device and circuit diagram of the LCD monitor “Rover Scan Optima 153” are discussed in detail.
Among monitor malfunctions, there are quite often those that can be easily fixed with your own hands in a few minutes. For example, the already mentioned Acer AL1716 LCD monitor came to the repair table due to a broken contact of the socket outlet for connecting the power cord. As a result, the monitor turned off spontaneously.
After disassembling the LCD monitor, it was discovered that a powerful spark was formed at the site of the poor contact, traces of which were easy to detect on the printed circuit board of the power supply. A powerful spark was also formed because at the moment of contact the electrolytic capacitor in the rectifier filter is charged. The cause of the malfunction is solder degradation.
Solder degradation causing monitor failure
It is also worth noting that sometimes the cause of a malfunction can be a breakdown of the diodes of the rectifier diode bridge.
Monitor problems
If, when you turn on the monitor, the LED on its panel does not light up - the power indicator - most likely the monitor's power supply has failed. But before passing a final “sentence” on him, check network cable and the presence of voltage in the network.
Most often, the source of monitor malfunction is: poor-quality power supply, aging of elements as a result of operation, violation of temperature conditions, mechanical damage and liquid ingress into the case. If liquid gets inside the monitor, turn it off immediately and dry it thoroughly. Don’t rush to turn it on; it’s better to wait a little than to ruin the equipment.
The electrical network sometimes experiences sudden surges in voltage, which negatively affect the performance of the monitor's power supply and can lead to its failure. Often, monitor repair is not limited to repairing the power supply. Image distortion may occur, extraneous sounds during operation, overheating.
If there is noise on the monitor (image distortion, jitter, darkening at the edges, etc.), the reason is either a software problem or a defect in the monitor (video card).
Run several different programs, performing the same function. If the problem appears in only one of them, for example when watching a movie, the problem is in the program itself. If not, there is a hardware problem. If the problem is in the program, check the settings and replace the codecs.
Problems may arise due to voltage fluctuations in the network. Perhaps when several electrical appliances that consume a lot of energy are turned on at once. Try connecting your PC to a different outlet. Pay attention to the integrity of the power cord, check for kinks or creases. If the voltage drops continue, install a high-quality surge protector, voltage stabilizer.
Interference may be caused by powerful radio or electrical equipment. It is necessary to move such devices a sufficient distance from the monitor or shield it.
CRT monitors become magnetized over time, which leads to noise and distortion. If the monitor is magnetized by a single exposure, you need to turn it on and off several times in a row; the built-in demagnetization system will handle it. If the problem is not solved, purchase a special demagnetization coil. The “folk” method is to run an electric razor around the perimeter of the monitor, sometimes it helps.
The presence of “broken” pixels - constantly glowing or shadowed dots - can only be “cured” by replacing the matrix, which is not always justified from a financial point of view.
Sometimes the defect may be caused by non-optimal monitor settings. You should be especially careful if there are small children in the house, they really like to “push” buttons. Some pets love to chew cables, for example, once a domestic cat chewed the cable connecting system unit with a monitor. As a result, color rendition was disrupted.
It is advisable to carry out periodically maintenance monitors - disassemble, clean from dust and solder critical areas, if required.
Typically, monitor repair consists of replacing or repairing a faulty power supply unit, control board, replacing a microcircuit, transistor, fuse, and connectors.
Often, the inoperability of an LCD monitor is caused by a faulty power supply, which can be easily corrected at home. Careful troubleshooting will allow you to successfully repair or at least accurately identify the components that require replacement.
When working, try not to rush, so as not to confuse the outputs, as well as the power supply itself connected to the monitor.
Increased secondary supply voltage leads to damage to the power board or processor board of the LCD monitor, switching unit (depending on the monitor device, the voltage may be supplied to different units).
In any case, only diagnostics will reveal the malfunction. In most cases, all parts are available and faults can be eliminated. Preliminary diagnostics will allow you to identify possible damage, identify faulty elements, eliminate repeated failures and the occurrence of interference when turning on the power source after repair.
Determine the type of power supply, the design of the power converter, the circuit design and purpose of the power supply circuits, then the element base, the type of microcircuits and transistors used.
Both internal and external power supplies are used. The first is located in the monitor body. This is a switching converter that transmits AC mains voltage to multiple DC output buses. The disadvantage of the built-in power supply is the presence of a powerful high-voltage pulse converter inside, which negatively affects the operation of the monitor.
External power supply is network adapter, designed as a separate module for converting AC network voltage into DC voltage. Both are made using a pulse converter circuit. An external power supply eliminates the power stage from the monitor, increasing reliability.
Both options usually form +3.3 V, +5 V, +12 V, +3.3 V buses at the output to power digital chips; +5 V for standby voltage and power supply for digital, analog circuits, LCD panels, etc.; +12 V – for powering the inverter of backlight lamps and LCD panels.
In an external power supply, all voltages are generated from a single 12–24 V input bus using DC-DC converters in D.C.. The transformation is carried out linearly or pulse regulator. The former are used in low-current circuits, and pulse converters are used in those channels where the current reaches significant values. The DC-DC converter is almost always located on the main control board of the monitor.
The circuitry of the converters is the same, the difference is in the number of output buses at the output and element base. The converters are made on the basis of pulsed step-down voltage converters and contain a multi-channel PWM microcircuit that controls the output power signal in a cascade.
Adjustment and stabilization of output buses is carried out using PWM technology via feedback circuits.
A visual inspection of the parts and condition of the printed circuit board will reveal external defects in the elements. Faults in fuse, varistor, thermistor, resistors, transistors, capacitors, chokes and transformers are determined.
A blown fuse in a glass case is easy to identify - a burnt-out wire, deposits on the glass, damage to the glass. The fuse tripping current is approximately 3A. Replacing with a fuse with a high current will damage other elements of the power supply or the LCD monitor itself.
Varistors, thermistors, and capacitors in the input circuits of the power supply often suffer mechanical damage when they fail. They turn out to be split, cracks are visible, the coating flies off, soot is on the body. Faulty electrolytic capacitors “swell” or have damaged housings, in which the electrolyte can splash onto adjacent radio components. When resistors burn, the color of the housing changes and traces of soot may appear. Sometimes cracks and chips appear on the resistor body.
Pay attention to violations of the integrity of the case, changes in the color of elements, traces of soot, the presence of foreign objects, the slightest damage to printed conductors and places with suspicious soldering quality.
If the fuse is blown, be sure to check the rectifier bridge diodes, thermistor, varistor, output filter capacitor, key transistor, current resistor. This way you will detect a short circuit at the power supply input, if it is present. Be sure to check the control chip (PWM controller).
Be careful when selecting replacements for powerful key transistors and elements of secondary output stages (diodes, capacitors, chokes). Install a powerful switching transistor (or a powerful hybrid chip) onto the radiator carefully. Frame powerful transistor usually connected together with its collector (drain), so it must be isolated from the radiator. For insulation, mica gaskets and special heat-conducting rubber are inserted between the installed radiator and the transistor body, and if the body is completely plastic, then only heat-conducting paste is used. After installing and soldering the transistor, make sure that its collector (drain) does not come into contact with the radiator.
A test run of the power supply is carried out with a load. Instead of a monitor, it can be loaded with equivalent external circuits, for example, a + 12 V and +24 V light bulb with a power of 10–60 W. To measure voltages, it is advisable to connect a voltmeter to the power supply output before turning it on.
At the testing stage before turning on, you can also replace the mains fuse with a 220 V light bulb with a power of 100–150 W; it will give a visual representation of the current consumed by the source as a whole. If, when you turn on the power supply, the lamp glows strongly, the power consumption is high and there may be a short circuit in the primary circuit of the power supply; with normal current consumption, the glow of the lamp is dim. This method is a safety violation, so be careful.
At the moment of switching on, it is necessary to observe all safety measures, observe the operation of the power supply with safety glasses, since when switched on, failure of the electrolytic capacitors is possible. During the initial startup and operation of the power supply, pay attention to the appearance of possible sounds (whistles, clicks). The appearance of smoke or a burning smell will indicate an unresolved problem and the presence of a malfunction. Sparks and flashes are usually observed when fuses, power switches and diodes fail.
Provide opportunity quick shutdown power supply 220 V from the power supply.
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 regular monitor desktop.
Each breakdown has its own symptoms. Typical faults 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 a malfunction will indicate 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 are stored electric charge and may give you an electric shock. 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 monitor's long service life or is simply a consequence of poor quality 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
This dead pixels– a common problem typical of inexpensive monitors. Looks like a bright dot of a different color on the monitor. Consequence of a manufacturing defect or long work. Also, it may indicate mechanical damage matrices. 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 simple block repairs and it is necessary deep knowledge in the field of electrical engineering.
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.
