Troubleshooting of LCD flower screen

Symptom: A friend in the beginning of the year replaced a 15-inch LCD display, the model is AOC LM-500, has been used very normal. But just after half a year, this LCD is a failure, the specific performance: As long as the start or restart the computer, there will be an approximate "flower screen" fault phenomenon, giving people the feeling as if there is high-frequency electromagnetic interference, the screen of the handwriting is very vague and jagged. When entering the WIN98SE system, this phenomenon occasionally occurs, but the duration is very short and less obvious, most of the time screen display is normal.

Failure check: The author first replaced a video card, found that the fault is still. A working 17-inch CRT flat panel was then used as a replacement to remove the test, and no similar failure occurred.

LCD Failure Principle Analysis: Digital Interface (DVI) is the LCD display of the best interface. However, graphics card manufacturers for cost considerations, graphics cards generally do not have a digital interface, and through the internal digital/analog conversion circuit and video card VGA interface connection. Although this kind of connection form solves the problem of signal matching, it brings the problem that the distortion is easy to be disturbed. The reason, mainly because the LCD itself clock frequency and input analog signal clock frequency to maintain 100% of the synchronization, especially in the analog synchronous signal frequency constantly changing, if at this time the LCD synchronous circuit, or with the video card synchronization signal connection of the transmission line appeared a short circuit, Bad contact, and can not adjust the follow-up in time to maintain the necessary synchronization relationship, it will appear in this article encountered in the startup display exception, and into the system and show normal strange fault phenomenon.

Accordingly, the author judges that the fault point is likely to be in the liquid crystal display internal synchronous circuit, or connect the interface pins and transmission cables above.

Troubleshooting: In view of the LCD internal synchronous control circuit is generally set in an integrated circuit inside, and the possibility of damage to the integrated circuit is negligible. So the author will check the focus on the interface pins, transmission cables and synchronous circuit of the adjustable elements above.

First, the shell of the liquid crystal display to open, exposing the internal connection cable, with the digital multimeter Pass, check the buzzer block, one by one to check the connection between the needle and the corresponding cable of the conduction situation, did not find any open circuit, short circuit and other physical connectivity problems. Then along the cable connection line, find the input control circuit unit on the printed circuit board, and find that there are two miniature WR604 potentiometer on the edge of the 14-pin IC block, one on the printed circuit board and the other is WR1201. The author based on the previous experience in the maintenance of CRT monitors preliminary judgment, the two micro-tunable potentiometer, is likely to be used for synchronous fine-tuning, but in the end which one has the problem? In order to secure and reduce the number of LCD shell removal, the author with anhydrous alcohol will be two and half adjustable potentiometer all cleaned, wiped again, After all volatile alcohol, the liquid crystal display after the recovery of the test machine, found that although the failure has been reduced, but several consecutive boot process failures sometimes will be repeated. But once again opened the shell of liquid crystal display, with a miniature Phillips screwdriver will WR604 clockwise adjust a little, and then restore the shell and once again power test, the liquid crystal display display is normal, so far, troubleshooting.

Although a little adjustment can make the liquid crystal display temporarily return to normal work, but with the use of time, I think the fault is likely to be repeated. Therefore, the author considers the fundamental solution or the replacement of a half adjustable potentiometer. Unfortunately, this is not much bigger than mung bean grain of the half adjustable potentiometer in the ordinary electronic market is difficult to buy, the author has not been the wish.