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Troubleshooting LCD monitor power supply pack is easier than the crt monitor power supply. It is small and compact and usually has only one output which is 12 volt and range from 2 ampere to over 4 ampere. Some designed have 18 and 24 volts also. Nowadays most LCD power supplies are build into the main board such as the Dell E151FP LCD Monitor while the older type were separated as power pack module.
The power pack module type is easy to diagnose because you can replace another working unit just to make sure is the power pack that giving problem. You may also use an external dc power supply to power up the LCD to confirm if the power pack is faulty. If you do not have the external dc power supply, you can always use this method to isolate the problem especially the symptoms are no power, low power and power blink. Using the rear light bulb (12 volt) from a car and connect it to the power pack output jack. If it light brightly then the power pack is okay. If it blink or dim the power pack have problem. Why have to use this method to test it? Because, if there are components shorted or leaky at the LCD main board, it will affect the power supply performance.
From experienced, most of the LCD power supply problems whether in the power pack or built into main board were due to the failure of electrolytic capacitors (most probably due to heat) in the primary and secondary section. Most of the capacitors have a high ESR which causes the power supply to produce low output, blink or no power at all. Typical value electrolytic capacitors in the primary side are 47 and 100 micro farad 50 Volt and the secondary sides are 1000 to 2200 micro farad 25 volt. Sometimes you can see the capacitor top casing became bulge and the sleeve that covered the capacitor casing turned into darker color like brownish.
The power section in some of the LCD screen are still using the UC3842 pulse width modulation (pwm) ic. This type of ic is quite easy to find in the market compare to some power ic which is very difficult to get. Samsung 153V LCD monitor is using the TOP247F power ic which is quite easy to troubleshoot if power fail in this model. Some power pack that came in only have minor burnt and after cleaning it and replaced the fuse, it will work as good as the new one.
Troubleshooting LCD power supply is not that difficult provided if you know how a power supply work. If you are unsure of how to troubleshoot power supply, I urge you to read all my power supply articles at the power supply page. I will continue to post more power supplies articles at that page and hope you could drop by more often to check out my website.
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