Cannot connect to Arduino over USB

My computer will not recognize my Arduino Mega when I connect it using USB. It's not my computer because other Arduinos work fine. Any clue what might be wrong?

[u]Detailed Description[/u] I'm working on a robotics project powered by a computer PSU, with one of the 12V wires connecting to the Arduino through a barrel jack adapter. Everything was fine for a while but all of a sudden I noticed that the power light on the Ardunio was flickering. I unplugged the barrel jack and now if I try to power the Arduino from the PSU the power light starts blinking rapidly.

I think the problem might be with the regulator because when I connected it to USB the power light would turn on then fade out after a few seconds. Now the power light stays on but the board will not appear in my list of available Serial ports in the Arduino IDE. Also, I'm not sure this is relevant but the LED connected to pin 13 comes on when I connect to the board, even though the sketch that was on the Arduino when the problem started did not use pin 13 in any way.

I have quite a bit of experience fixing electronics so if anyone can figure out which component is broken I would very much prefer to replace a single part rather than the entire board.

Have you tried this Mega on another computer? Did you by chance have both the DC-in and USB connected at the same time?

The first thing you can try is reflashing the 16U2 on the Mega with a new firmware, downloaded from here:

https://github.com/arduino/Arduino/tree/master/hardware/arduino/avr/firmwares/atmegaxxu2

(MEGA-dfu_and_usbserial_combined.hex)

You need an AVR ISP (such as avrisp2 or USBasp) or another Arduino (with the ArduinoISP sketch loaded) in order to reflash the 16U2.

It can, however, be several other things.

You can try getting 5V from the PSU and connect that directly to the Vcc (5V) pin on the Arduino.

AlxDroidDev: Have you tried this Mega on another computer?

I have not, although wouldn't that prove the same thing as trying a different Arduino on the same computer?

AlxDroidDev: Did you by chance have both the DC-in and USB connected at the same time?

What would that break exactly? I try not to connect the Mega to both power sources at once but I was tired so it's very possible I wasn't paying attention...

AlxDroidDev: The first thing you can try is reflashing the 16U2 on the Mega with a new firmware...

You need an AVR ISP (such as avrisp2 or USBasp) or another Arduino (with the ArduinoISP sketch loaded) in order to reflash the 16U2.

I don't know if I have access to an AVR ISP (I'll have to check) but I can use ArduinoISP. I'll see if that helps.

AlxDroidDev: It can, however, be several other things.

Like what? If it's something I can test with a multimeter, that would be greatly helpful.

AlxDroidDev: You can try getting 5V from the PSU and connect that directly to the Vcc (5V) pin on the Arduino.

I'm hesitant to use the PSU's 5V line because the 74LS14N (hex Schmitt-trigger inverter) chip I have on that line isn't behaving properly either (supporting my alternate theory that the power supply, which isn't exactly intended for this kind of use, needs some serious regulating hardware).