first of all, I'm new in the forum so excuse me if i break a rule or smth. also sorry for the bad English.
So, I've been working with an Arduino UNO board for a project for a while now, and all of the sudden my board just doesn't work anymore. It does light up when connected from the USB terminal with 5v, but the jack one doesn't work and wont turn on the board. also, the "L" light (which as far as I know lights when the Arduino is connected to a power supply) lights up when connected, but brightness falls if you touch any conductive area of the board, mainly the 13th output pin. I tested the Atmega in another board and is working fine, so it gotta be something else.
If any of you had a problem like this or have any idea of how to solve it, please let me know.
What was the project that you were working on when it started to fail?
From the title, it sounds like at least the serial-to-USB converter (U3) has died.
Faulty power circuit would be the first suspect; involved are the voltage regulator (U1), the LM358 (U5) and the FET (T1).
Based on the above, I would say that it's beyond repair unless you have the soldering skills and are prepared to invest in components that at the end might work out more expensive than buying a new Uno.
The L-LED is controlled by the microcontroller and not by the power supply. So if the last sketch did not use pin 13 one can not draw a conclusion from that.
Use the good board to upload blink to the Atmega of the faulty board using the swap; next move the Atmega to the "faulty" board and check if the built-in LED blinks.