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

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

avrdude.exe: stk500_getsync(): not in sync: resp=0x00

The error LED on my friend's mega ADK flashes while the heartbeat glows.
I used this blog http://blog.ethanfrei.com/2011/10/arduino-uno-revived.html 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

I realized that the 8u2 might be bricked and I need to re-program it

If it is completely bricked (ie. by putting 12V into it) reprogramming it might not help.

I'm skeptical about it burning down on me because the atmega 2560 was fed with the same 12V and it still works (the program it had keeps running now if I power it)

If you have a spare Uno or similar lying around, install the "check signature" sketch described here:

Then you can plug that into the ICSP header for the Mega, and then the 8U2 and see if they both respond. Sounds like the Mega should, and maybe the 8U2 won't.

Took me a while to understand how to use this tool (used a Mega ADK to check my Mega, but no big deal). The 2560 seems to answer just fine:

Atmega chip detector.
Entered programming mode OK.
Signature = 1E 98 01
Processor = ATmega2560
Flash memory size = 262144
etc etc etc

While connected to the 8u2 only the first line appears:

Atmega chip detector.

Guess it's dead after all. I can't stress this enough though, both the 2560 and the 8u2 were powered with 12V. Why is the 2560 still alive?

rmaero:
While connected to the 8u2 only the first line appears:

Atmega chip detector.

Guess it's dead after all. I can't stress this enough though, both the 2560 and the 8u2 were powered with 12V.

That line appears before it tries to talk to the chip. So it isn't responding.

Why is the 2560 still alive?

Luck.

Well you can still program it over ICSP header.
You can still use the Arduino IDE.
Plug Either an arduino or an AVR programmer(supported by arduino IDE) and upload your sketches.
Don't forget to select the correct programmer in the programmer list.

StereoBucket:
Well you can still program it over ICSP header.
You can still use the Arduino IDE.
Plug Either an arduino or an AVR programmer(supported by arduino IDE) and upload your sketches.
Don't forget to select the correct programmer in the programmer list.

I'm thinking of using the RxTx pins that the 8u2 was using to program the 2560 with an USB to RS-232 converter like so http://www.guatapo.com/images/stories/PRODUCTOS/Cables/usb_a_serial.jpg and a 232 to TTL level converter such as MAX232. Both I have right here in front of me.

That should work. You may need to manage setting reset low at the right moment.

Could I use the rts pin on the RS232 for that? Actually now that I think about it that is handled by the IDE.
How about I manually reset the 2560? Would that be enough? I could live with that.

Connect the RTS line in series with a 0.1 uF capacitor, to the Reset pin. You may also want to put a 10K resistor from Reset to +5V as a pull-up.

The classic circuit for a Reset pin, I've seen it many times...
I really must thank you for all the help you've given me. You are one of the only people who knew what I was dealing with and was able to help me.
I sincerely appreciate your help.
Best regards, Rodrigo.