Unit won't turn on, everything is in place.

I have soldered on a new charging port onto a customers 3DS and everything is in place. When I start it up it turns blue, no light on the screens then after 3 seconds it makes a popping sound through the speakers and turns off. I have tested the fuses and they are all good and then put it back together again to try and it does the same thing. No idea what it is that is doing it. Anyone got some suggestions?