Types of faults and solutions for LCD screens:
1. The monitor has no power
(1) Power supply problem:
This is a very simple problem. There are two types of internal power supply and external power supply for the general LCD extension. The external power supply is more common. Regardless of the type of power supply, its structure is much simpler than the power supply of crt display. The vulnerable parts are generally some small components, such as fuse, rectifier bridge, 300V filter capacitor, power switch tube, power management IC rectifier output diode filter capacitor, etc. .
(2) Driver board problem:
There is a problem with the driver board burning insurance or the voltage regulator chip. Some machines have a built-in switching power supply and output two sets of power supplies, one of which is 5V for signal processing, and the other is 12V for high-voltage board point backlighting. If there is a problem with some circuits of the switching power supply, it may cause the two sets of power supplies to not output. First check whether the 12V voltage is normal, and then check whether the 5V voltage is normal, because the working voltage of the MCU chip of the A/D driver board is 5V, so look for the switch If there is no problem with the machine, first use a multimeter to measure the 5V voltage. If there is no 5V voltage or the 5V voltage becomes very low, then one possibility is that there is a problem with the power supply circuit input stage, which means that there is a problem with the power supply part of the 12V conversion to 5V. .
LCD
This kind of problem is very common to check the terminal voltage regulator block (common model 8050SD—LM2596—AIC15-01, etc.). Another possibility is that the 5V load is heavier and the 5V voltage is pulled very low. In other words, after There is a problem with the signal processing circuit of the first stage. Some of the circuits are damaged, causing the load to increase. Pull the 5V voltage very low. Check the problematic components of the later stage one by one. After replacing the problematic components, the 5V can return to normal and the problem is normal. This is done, and I often encounter problems that cannot be turned on normally after the 5V voltage returns to normal.
There are also many reasons for this situation. On the one hand, the MCU program may be flushed and it may not turn on, and the MCU itself is damaged. For example, the I/O port of the MCU is damaged, so that the MCU cannot scan the buttons. The problem caused by the MCU, it is useless to find the hardware problem, even if you change the MCU, the problem will not be solved, because the MCU needs to be programmed and coded. If there is no way to find the original AD driver board to replace, We can only replace it with general-purpose A/D driver boards such as 151D or 161B.
2, the display will not light up once it is on, but the green light of the power indicator 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 usually a display on the LCD screen, and the way to look is "squint". The maintenance method can be tested by connecting a single lamp high-voltage board to a lamp tube because the current high-voltage board planning of liquid crystal displays is generally a symmetrical plan, and there may be basically no damage on both sides. Generally, the old machine is prone to problems with the power management IC load balancing protection caused by a short circuit or no-load of the power tube, the booster tube, the booster transformer and the lamp tube. Many beginners think that there are so many lines in the high-voltage board interface. The replacement is very complicated. It is actually very simple. Only 4 signals need to be connected to the high-voltage board: 1 power supply. 2 ground. 3 switch control ON/OFF. 4ADI brightness adjustment.
First determine the positive and negative poles of the power cord. Generally speaking, the positive pole is connected with the fuse. The negative pole is mostly connected to the negative pole of the capacitor. Then determine the voltage. The best way to determine the voltage is to look at the label of the capacitor. If it is about 6V, then it is 3.3. For V, if the capacitor is marked about 12V, then the input voltage must be 5V, if it is about 24V or above, then it is 12V, and so on. Divide the number of volts marked on the capacitor by two and the nearest few volts is a few volts. Some people say that it still doesn't light up after this connection, or it just flashes and then goes out. Yes, there are many high-voltage boards that are mostly like this, so what should I do?
Find out the control pin and see which pin is connected to a small transistor. Generally, it is directly connected to the transistor. At most, there is a small capacitor in the middle. It should be easy to identify. The control pin is generally 3.3V and 5V. It is grounded, so if we don't know, we first try to ground it. Then connect it to 3.3V and then connect to 5V. If the input voltage and control voltage are mostly 3.3V, they can be directly combined.
3, the display screen is black without backlight. The power light is always green
The images displayed on the squint LCD screen are mostly high-voltage board power supply circuit problems. Focus on checking whether the 12V power supply (fuse F) and the 3V or 5V switching voltage are normal. If there is no output switching control voltage due to MCU problems, you can directly extract the 3-terminal regulator block (AIC1084) 3.3V instead. The idea of repairing the high-voltage board (power fuse-switch control tube-power management IC-push-pull hair tube-power switch tube- DA conversion circuit (energy storage inductance rectifier) --- LC boost circuit (boost transformer boost capacitor)-coupling capacitor-lamp.
4, the screen has bright lines, bright bands or dark lines
This kind of problem is generally a problem with the LCD screen. The bright line problem is generally caused by the problem of the cable connecting the LCD screen body or the driver IC of a certain row and column is damaged. The dark line is generally the screen body has leakage, or the TAB flexible board connection is open. The above two problems are basically for The machine was sentenced to death, and there is no repair value, because the price of a screen is too high.
5, flower screen or white screen
This kind of problem is generally caused by a problem with the drive voltage of the screen. First, change the drive board and drive screen line test. If it fails, check the power supply circuit of the back panel of the screen. Repair ideas: Whether the drive board 5V to 3.3V voltage regulator block (AIC1084) has power output .---Screen driver board fuse (F)-3.3V -----DC-DC conversion circuit-negative voltage forming IC (-7V)-row and column driver IC.
6, color cast problem
Generally, you can enter the factory adjustment mode for adjustment. If there is no repair idea in this mode: replace the screen cable and adapter board ----- rewrite the driver program ----- the driver board is broken (uncommon) ----- the control IC of the back panel of the screen is broken (uncommon) )-------Unplug the screen cable to observe the backlight color (the flat color of the backlight is the aging of the lamp)-------Change the lamp.
7, characters are empty or trailing
Check the VGA signal line, and focus on whether the ground wire of the RGB three-color line is connected properly -----replace the screen line or adapter board-----rewrite the driver program-----change the drive board---- LCD screen backplane signal interface IC is broken --- LCD screen backplane contrast potentiometer adjustment - LCD screen light guide plate misalignment-polarizer misalignment
8. There are stains inside the LCD screen
Wipe or replace the protective film --- Disassemble the screen and clean the outer polarizer and plexiglass (treated with cotton ball purified water) --- Dry by air duct
9, LCD screen light leakage or uneven light
Reinstall the lamp tube-----adjust the light guide plate
10, LCD screen highlights
One or two big bright spots can be tried to lightly press with your fingertips. The bright spots can disappear, which means that the switch tube and the electrode of the pixel are mostly virtual. The small black and gray spots may be dust on the internal light guide plate or polarizer. Can be cleaned. The above methods must first obtain the customer's consent during the test before they can be operated, otherwise the consequences shall be paid by the author.
11, LCD screen brightness is low
Check the ADJ brightness adjustment circuit of the high-voltage board --- change the lamp tube --- change the high-pressure board --- adjust or replace the light guide plate
12, error message "out of frequency range"
Check the signal line--------Rewrite MCU driver-------Replace EPROM------Rewrite EPROM program-----Replace the driver board
13. After power on, do not press the switch button, the white screen will display the backlit button and the image can be displayed normally
The switch signal of the high-voltage board interface and the ADJ signal are reversely connected, which causes the output of some of the switch signals belonging to the drive board MCU to be abnormal. You can rewrite the MCU program to repair it.-------Replace the MCU
