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Today I did some cleaning in my laptop as GPU and CPU were overheating (100C). I have done it many times so I was confident in myself. After putting all together the internal LCD screen was not working(totally switched off). I connected an external monitor and it was fine.

The CPU is still overheating so I guess I will buy thermopaste tomorrow.

But I am concerned about the internal monitor. What could be the reason for that? I checked connections and everything seems to befine. I thought maybe only the backlight is not working, but after illuminating the monitor I still couldn't see any pixels working.

How can I know what kind of problem can it be?

EDIT: Just checked again, cannot see any faint image. Tried to switch the monitors on Windows, but it doesn't do anything. It's not recognized at all. The webcam on monitor works fine.

It's HP Compaq CQ60

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Have you eliminated the possibility of backlight issues? Cupping your hand over the laptop screen or viewing it in a dark room do you see any faint images? –  music2myear Dec 29 '11 at 21:17

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My initial reaction to this would be to make sure ALL connections are secure, I don't know how far down you had to tear the machine to clean it, but something may have been strained and loosed in the LCD assembly. One of the inverter cables, or more likely the data cable running up the back of the display that plugs into the back of the panel itself. I'm sure you checked the motherboard end as you probably had to separate that one to take the palm rest off to access the fans/cpu. Make sure ALL are clean and secure. If they are, something else has gone out or was damaged in the process, maybe a pin on the internal cable connector, or the like. Check everything.

Four connections make an LCD panel function typically. The motherboard connection, the back of the LCD, the DC input to the inverter and the AC output from the inverter.

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What voltage is in the inverter input? So I can check if it even gets any voltage. –  Arturs Dec 29 '11 at 21:36
    
Inputs depend on brand of the laptop, and of the inverter and panel used, but it should be between 10 and 20V DC. So using the "20DCV" setting on a volt meter should work. As an aside, the output is usually around 500 - 700 ACV, so if you have a 750ACV setting or 1000ACV setting, you can check output as well. I have seen the inverter itself go bad on many occasions. Though you said you didnt see any image at all on the screen so this is a low possibility, as that would only effect the backlight (as would Charlie's post below, I thought of that as well but didnt mention it because of this fact) –  Paperlantern Dec 29 '11 at 22:07

We could only hope it's something simple like this...if it was working before you disassembled it, you may want to make sure the monitor switch (which turns off the LCD when you close the laptop) isn't stuck. I have seen and done this before.

You didn't say what brand or model it is so I can't tell you where it's located or if it even has one.

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As I said before, the PC doesn't even recognize the monitor. Moreover, the pc goes on sleep when the laptop is closed (not just screen goes off). Added the brand to the original post. –  Arturs Dec 29 '11 at 22:05
    
Two things; 1. if the LCD is not recognized at all it is most likely a loose connection. And 2. when it goes to sleep when you close it just proves the switch is working. When you disassemble again check all the cables for pinches and breaks too. –  CharlieRB Dec 29 '11 at 23:26

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