Pin 13 issue

Hello, i'm gonna be quick. I have insereted by MISTAKE 12 V into pin 13,noe the led is always turnee on. It las 3 V on output. How can i solve the problem? Did i burned something?

Yup, you sure did.

My betting is you either trashed the chip (often abusing one pin will break that pin - sometimes other stuff too, though - I wouldn’t trust the board after that), the opamp buffer, or both.

If it’s a board with the socketed chip, try replacing it and rebootloading the chip (must get one with optiboot preloaded if you don’t have an ISP programmer or another Arduino to bootload it with), but the LED may still not work. If you can replace the chip, worst case you do so, find that the opamp is messing up the pin’s behavior, and cut trace from pin 13 where it reaches the opamp, and accept the loss of the onboard LED (or replace the opamp - though it’s a little harder, since it’s an SMD). If you can’t replace the chip, get a new board, and don’t expose the GPIO pins to 12v,

The other pins are working good, i have tested some programs on them, and they are working. I have the mega adk board,so i can't replace the cip.

now i noticed what 505f resistor with gold solderings, beside usb connector is getting warm. Is it ok?

It is a polyfuse. Always power the board from external supply, don't depend on the power through USB.