Take a multimeter, and with the black probe connected to the - battery input probe these points with the red probe in the order listed below:
If any of those voltages aren't right then you can narrow down the problem:
- 1: Cathode of D2 - if this isn't only slightly below your battery voltage then you either have a short circuit, or diode D2 has blown.
- 2: 5V - This is the next stage of the power from the battery. If this isn't 5V then either a short exists after the 5V regulator (on the other side of the board) or the regulator has blown.
- 3: 3.3V on small ICSP header - This is the output of the main 3.3V regulator. If this isn't 3.3v then either the regulator has blown or there's a short further down the line.
- 4: 1.2V - This is regulated inside the main chip. If it's not 1.2V then the main chip isn't functioning.
- 5: 3.3V on Expansion header - This is a separate 3.3V regulator. If both this and the ICSP's 3.3V are bad then it points to the 5V regulator.