Hello, I have been working a customers laptop for about a week now.
Here's the problem: It is a Compaq Presario F500. It was brought to me because it would power on, but black screen, no external display. So I figured it was the same problem all F500 and F700 mobos have, the nVidia chip. So I did the penny trick and left the laptop on for several hours. Ok. I come back plug in a external monitor and power it up. Laptops boots into Windows and works fine with external monitor.

Now comes the problem. I go to put the laptop back together and as soon as I plug in the LCD's video cable into the motherboard the laptop looses power. It kinda acts like it's a short somewhere. The fan clicks a little as if it's trying to start, there is a noise coming from the speakers, no lights come on, no beeps, only sign of power is the DC Jack light. Unplug the video cable and laptop powers up normal.

So the symptoms are:
ONLY when LCD video cable is connected laptop will now power on.
Fan makes clicking sounds as if trying to start.
Speakers make noise about every 5 seconds.
No lights, No beeps, No Power

When LCD video cable unplugged:
Laptop boots fine with external monitor

Troubleshooting already performed:
Checked in known working machines; RAM, HDD, LCD, LCD Inverter Board, Video cable, Power Switch, DC Jack,
Changed the whole motherboard and problem still persists.

Any help or suggestions would be greatly appreciated.

I think your surmise about a short circuit is likely correct. Check the cabling and connectors first, but it may well be that either the backlight is bad, or the display itself is fubar. My guess would be the backlight, but that is just a SWAG.

I made it sound as if it only did when the video cable alone was plugged in, keep in mind that the other end of the video cable was already plugged into the inverter board and LCD screen.

I have replaced the LCD with a brand new one, same thing. Same with the inverter board too.

Changed video cable again... works now... very strange!

I've seen poorly made cables short out like that. Glad you got it sorted.

One thing tho. Laptop internal screens are NOT hot swappable. Connecting the internal screen to a running laptop can do damage! The external connector to a screen has protective diodes and capacitors, the internal connector doesn't.

Wow, did not know that! Thanks for the tip Rik! Knowing that makes me think I might have cause the cable to go bad myself, because like I said in the orginal post, this was not the original problem, the common nVidia chip on these mobos was indeed the initial problem. Again, thanks to everyone for the help!

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