uglyfish:
If I power the Uno via USB the 5v pin reads 5v, as expected.If I power the board through Vin or the barrel adapter with 12v, the 5v pin reads 3.09v.
If I power the board through Vin with 12v and USB, the 5v pin reads 4.5v.
No other pins connected.
Is there a way of fixing this board?
Welcome to the Arduino forum.
What else is connected to the Arduino? What are you using to read those voltages? Are you reading them on the actual Arduino pins and Arduino ground, or on a bread board?
Paul