Using Arduino > Installation & Troubleshooting

Mega not showing on Device Manager, Possible fix re-flashing 8u2?

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Hi guys! I've been working for a while now with this mega 2560 R3, and while working with an LED matrix (I made myself) somehow I feeded the mega with 12V (not through the Vreg, but through the pins 5V and GND). It started reseting itself and when I realized what I've done I pulled every cable. No smoke or spark showed up so I'm discarding any burned chips.
Luckily when I connected the arduino with usb the program it was loaded with kept running (the program controlled a 4017 decade counter, btw). The only and major problem I had since then is that my pc (after restarting, also tried with a laptop. Same OS though) will not recognize the arduino in any way, not even the sound it makes when a usb device is plugged in.
Reading for a few days now I realized that the 8u2 might be bricked and I need to re-program it. Found the .hex on github (also on my Arduino folder). I borrowed a  friend's mega ADK which works flawlessly in the same pc-usb port-cable and loaded it with ArduinoISP sketch, connected everything (note that I connected all 6 cables to the ISP header near the 8u2, not to the 2560) and after debugging some sintax errors in the CMD prompt I got the correct commando for AVRdude to load the hex into my arduino. Now this won't work since it says

--- Code: ---avrdude.exe: stk500_getsync(): not in sync: resp=0x00
--- End code ---

The error LED on my friend's mega ADK flashes while the heartbeat glows.
I used this blog and a few arduino articles to figure all this up but still can't run it.
Am I going the wrong way? Maybe it did burned the 8u2 meaning I have a useless mega.
Any help will be appreciated. And I'm really sorry for such a long explanation but didn't want to leave any important stuff out (I'm sure I did)

NOTE: I just tried the DFU mode to flash the 8u2 with the flip program but I can't get it to show up as a DFU device. Maybe I'm doing it wrong, maybe the 8u2 is actually burned.

You can program via the ICSP header if the 8U2 is no good.

that's what all the explanation is there for. I used a friends mega ADK as ISP to program the 8u2 and the result was the error in the getSync() method on avrdude.

Doesn't the Mega 2560 Rev 3 use 16u2 as USB to serial instead of 8u2?

in that case my mega may not be a rev 3, actually I have no way to confirm it. But I can assure you it has an 8u2 on it (my friend's mega ADK also has an 8u2 and when I plug it in it says mega adk rev3) anyways, it doesn't really matter which rev it is, does it? I know it is an 8u2 and it's not working as it should


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