Today’s article talks about troubleshooting the 19 “CMV LCD monitor model CM-922D. The complaint was that there was no power as the customer told me. This power supply was not built into the circuit so it was You need an external power adapter to turn it on. The input to the LCD monitor is 19V with 3.1 amps from the adapter. Since the complaint was without power, we have to test it to check if the power adapter is failing or the LCD monitor .
The AC source was connected to the power adapter and I could see that the power LED was on. Checking the output voltage of the power adapter, I got about 19 volts, which is good. Even if the LED is lit and has the correct output voltage, this does not mean that the power adapter can sustain itself when there is a load (the LCD monitor) connected to it. I once came across a Compaq 17 “LCD monitor that has a precise output voltage from the power adapter, but when I connected it to the monitor, the screen appeared for a fraction of a second before shutting down. Usually in this case , your expert will tell you that it could be that the inverter circuit is faulty or the lamps are having problems. You would be surprised to know that the actual cause of the shutdown was due to the power adapter. It cannot sustain the output voltage when there is a load on it. .
This type of failure was normally due to a bad filter of the capacitors located at the output of the secondary circuit. If you have an ESR meter, you can quickly locate faulty electrolytic capacitors. If you don’t have the meter, just replace the caps directly. As I mentioned in my previous repair articles, to confirm whether or not the power adapter really works, you need a light bulb to act as a load. Make sure the power LED light does not blink as long as the bulb is plugged in. Even if it blinks once, this shows that the regulation is not good and needs to be checked completely. On the above China made LCD monitor, the power adapter works perfectly fine.
Since I have already confirmed that the power adapter is good, my attention was now on the LCD boards. Removing this cover was a breeze and you can easily access the board when the metal casing was removed. The first thing I checked was the SMD fuse located on the main board. If the fuse opens, there will be no supply to the ADC / SCALAR and CPU IC, so the power LED will not light. Checking this SMD fuse was quite simple as the method to check it is the same as a normal fuse. Use an analog or digital meter configured for ohm range or continuity check. You can check the fuse while it is still in circuit. Yes, the fuse was found to be open circuit.
Now there is a question that we must answer. What causes the fuse to open in an open circuit? Well, from what we know about electronic repair, there are three possibilities why a fuse opens the circuit. First, the fuse may become open circuit due to its service life. Some fuses can last for many years, while others only last a couple of years. In other words, the life of a fuse depends on the qualities of the materials used. Second, the external factor, such as a surge, causes the fuse to open in circuit to protect the main board. Third, the main board itself has a problem (such as a shorted IC and other electronic components) that caused the fuse to open in circuit.
If he LCD monitor problems falls into the first and second category and then replacing only the fuse will definitely solve the problem. However, for this LCD monitor, replacing only the fuse did not work at all and the fuse opened again. You have only two options to solve this type of problem. First, you can use an external regulated power supply as explained in my previous repair articles and connect the negative output of the power supply to ground and the positive output to the fuse (the line that leads to other circuits) to detect the short circuit. Slowly vary the output voltage and monitor the current drawn. Once the current has been drawn around one or two amps, now place your finger on the electronics on the main board. If you find a component that gets very hot, that’s the cause of the short.
In this article I am going to do it another way to detect the short circuit. Are you ready? Now instead of replacing the fuse with the exact value which is 3 amps, I replaced it with a 5 amp fuse. The purpose for me to put the 5 amp fuse in place of the 3 amp fuse was because “if the fuse doesn’t blow, the component that has a short will blow.” Do you get the idea? Three things can happen when you use this form to expel the culprit. First, the power adapter may turn off due to high current draw from the main board. Second, shorting electronic components (be it an integrated circuit, diodes, capacitors, transistors, etc.) will get hot and you can quickly locate them with ease by touching them. Remember not to use it for a long time, otherwise if the shorted components get too hot it will open the circuit and now you will have a hard time locating the fault. Third, you may hear a click and spark from the shorted component. Trace the source of the spark and you can easily pinpoint the location and sometimes even the actual component generating the spark.
For the old LCD monitor, I saw a little spark on the top of the main board once the power input connector was connected. It took me to the exact location and when I checked the SMD ceramic capacitor (which generated the spark underneath it) with the analog meter set to X 10K ohms range; in fact, it developed a small short circuit. Replacing only the fuse and the SMD ceramic capacitor resolved the power failure issue on this 19 “LCD monitor. This monitor uses inverter IC (OZ960G), A1084-25CM (2.5 volt voltage regulator), AMC1117-3.3 ( 3.3 volt voltage regulator), ADC IC (MST8136B) and CPU IC (MYSON MTV312MV64).
Conclusion: There are always options as to what methods you want to use to detect the short. If you prefer the latter, I suggest that you take some photos first before something actually burns out beyond the description. With the photos, it will at least give you some clues as to what the components or even the part numbers might be. Well, there is always an interesting way to solve electronic problems and this is one of the reasons why I like electronic repair so much because you can always act as a detective to discover the culprit – true satisfaction! Oh! By the way, you can use the above method for any other electronic equipment and it is not just limited to troubleshooting. Have a great hunt, my friend!