It's hard to tell what the burnt smell could be. Open it up and check the underside of the PCB (the board with the USB connector). Look for discolored or cracked components. If there is one or two, the BEC could be the source of the smell. If nothing looks bad, it could be any of the FETs. I would send it in regardless.
FYI: I am 99.9% sure the two rectangular devices to the right of the USB port in the pic below is the BEC circuit. Check those and the area around them carefully:
