Something happened to my mega2650 as I was experimenting the other day. It no longer turned on when powering from the 2.1mm jack or from Vin, regardless of the voltage applied. A plastic bubble had formed on the 5V regulator, I assumed I had shorted something and fried it. If I applied 5V to the 5V pin everything worked fine. It also works fine when powered from USB. So I replaced the 5V regulator with a NCP1117 version. But I have the same situation. If I apply voltage directly to the 5V regulator input pin I get 5V on the output pin and everything is fine. I tested up to 12V and it is dead on at 5V. I also checked the PNP switching transistor, and found it was working correctly. As far as I can tell, the only thing remaining is that the trace is severed somewhere between just after the Vin pin and the 5V regulator, maybe the regulator input is not soldered well to the trace? But this was one of the better solder joints I made, so I thought. Any other ideas as to the root cause?