Good evening,
For the tests, I fed the arduino mega with an external 12v power supply by the power jack, and as soon as I plugged the USB connector the power supply on the power jack, the power supply collapsed at 2 , 5v, now if I plug the 12v on the jack no LED will light, and no 3.3v and 5v, on the pins out!
Operation ok with the USB on the other hand!
Either the arduino was dead before, or you should not connect the usb and a power supply at the same time?
Pierre