Gaming DS Phat turns on with green light but black screens

[ALT703]

I recently bought a DS Phat for very cheap that didnt turn on at all, so i sent it to a friend whos an electrical engineer, and he worked on it but couldnt fix it, now it turns on, as in theres a green light but no sound or video. Any ideas?
Thanks!!

 

[IC_]

The same thing happens on a DS lite when there's no wireless card or it's making a bad connection but the wireless card is not removable on the original DS so I don't know if you can fix this without replacing the motherboard
You could also try the obvious things like reinserting the battery, trying to charge it for some time and removing any inserted game cards but you or your friend probably already did that