I am sure someone here has the answer.
I have this old Mega 2560 board R3. It was once the heat of my 3d printer.
One day a wire crossed paths from the 12v 20amp power supply and instantly smoked the regulator on the arduino board making it unusable.
I replaced with another and life went on.
Many years later I started using this board again and discovered it worked just fine using the USB cable for power and communication.
So I thought, it must be the 5v regulator that blew (I remember it being really smelly too), and replaced the regulator.
Now the board works fine with external power, which flows through the new regulator. Whereas it did not before.
However, when powered by USB, the new regulator gets super hot. Which is not as designed. Kind of the opposite of what I was experiencing when it blew up at 12V (it only worked via USB power after the incident). After replacing the regulator, now it only works with 12v (external power). Well, it works with USB power, too, but the regulator gets super hot, which is odd because it is not even being used.
Is there some sort of transistor that senses the power source and switches things off/on or something? Maybe that has been blown to ...