LCD screen failure troubleshooting
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Automation is the trend of industrial development nowadays. The application of liquid crystal displays can also be seen everywhere. For example, automatic printing machines, with computer reflow soldering, etc., however, the more applications, the more problems there are. Although the internal and external environments are different, the reasons for the problems may be similar.
The following articles are collected for reference and hope to help users with similar problems.
Symptom: At the beginning of the year, a friend replaced a 15-inch LCD monitor. The model is AOCLM-500, which has been used normally. But just after half a year, this LCD monitor has a fault, the specific performance is: as long as the computer is started or restarted, there will be a phenomenon of "flower screen", which gives people the feeling that there is high frequency electromagnetic interference. The writing on the screen is very blurry and jagged. When entering the Win98SE system, this kind of failure phenomenon occasionally occurs, but the duration is short and not obvious, and most of the time the screen display is normal.
Fault check: I first replaced a graphics card and found that the fault persists. Then a normal 17-inch CRT flat-panel display was used as a replacement test, and no similar failure occurred.
Analysis of the failure principle of liquid crystal display: Digital interface (DVI) is originally the best interface for LCD display, but for the cost consideration of graphics card manufacturers, there is generally no digital interface on the graphics card, and the internal digital/analog conversion circuit and the VGA interface of the graphics card. Connected. Although this connection form solves the problem of signal matching, it brings about the problem that it is susceptible to interference and distortion. The reason is mainly because the clock frequency of the liquid crystal display itself is difficult to maintain 100% synchronization with the clock frequency of the input analog signal, especially when the frequency of the analog synchronization signal is constantly changing, if the synchronization circuit of the liquid crystal display is at this time, or If the transmission line connected to the synchronization signal of the graphics card has a short circuit or poor contact, and cannot adjust the follow-up in time to maintain the necessary synchronization relationship, the startup display abnormality encountered in this article will appear, and the normal display will be displayed after entering the system. Strange fault phenomenon.
According to this, the author judges that the fault point is likely to be a synchronous circuit inside the liquid crystal display, or a connection interface pin and a transmission cable.
Trouble-shooting: Considering that the internal synchronous control circuit of the liquid crystal display is generally disposed inside an integrated circuit, the possibility of damage to the integrated circuit is negligible. So the author will focus on the interface pins, transmission cables and adjustable components of the synchronization circuit.
Firstly, the outer casing of the liquid crystal display is disassembled to expose the internal connecting cable. The buzzer is checked by the on/off of the digital multimeter, and the conduction between the connecting pin and the corresponding cable is checked one by one, and no physical connection such as disconnection or short circuit is found. Sexual problems. Then along the direction of the cable connection, find the input control circuit unit on the printed circuit board and find that two miniature semi-adjustable potentiometers are mounted on the side of the 14-pin package integrated circuit block, one labeled on the printed circuit board. WR604, the other is WR1201. Based on the previous experience of repairing CRT monitors, the authors initially judged that these two miniature semi-adjustable potentiometers are likely to be used for synchronous fine-tuning, but which one has problems? In order to ensure the safety and reduce the number of times to remove the LCD display case, the author cleaned and wiped the two semi-adjustable potentiometers with absolute alcohol. After the alcohol was completely evaporated, the LCD display case was restored and the test machine was turned on. It has been alleviated, but the fault sometimes repeats during several consecutive starts. Once again, unpack the LCD monitor, use a micro Phillips screwdriver to adjust the WR604 clockwise a little, then restore the casing and test it again. The LCD display shows normal. At this point, the fault is eliminated. Although a little adjustment can make the LCD monitor temporarily resume normal operation, but with the extension of the use time, the author believes that the fault is likely to be repeated. Therefore, the author considers the fundamental solution or replace a semi-adjustable potentiometer.