I have an Arduino Mega 2560 board running a ramps 1.4 shield for a 3D printer. For some reason, I can only power the Mega with USB. It does not get power from the ramps shield and it also will not power on when plugged in to a 12v wall adapter. I know all power sources are working because I have a second Mega that works fine on all. What could I check on the Mega board to help figure this out. Other than being powered only by USB, everything else works as expected with no problems.
Looking at this Arduino Mega 2560 schematic, if IC1 or IC2 failed (assuming only one or the other is installed for a given board), then you would only be able to power the board using the USB port.
Looking at this ramps 1.4 shield schematic it is assumed that the RAMPS 1.4 shield is providing 12 volts through D1 to the Arduino Mega 2560. And that testing included using a known good Arduino Mega 2560 board with the RAMPS 1.4 shield board with good results. Thereby eliminating the possibility that D1 on the RAMPS 1.4 shield is damaged.
If these assumptions are correct, this reinforces the likelihood that either IC1 or IC2 has failed on the Arduino Mega 2560 board.