Dead Arduino UNO

Using Arduino IDE 1.x
1

My Arduino Uno died last night.
I connected a 12v external power supply (wall wart type) to the external power connector
According to the specs the regulator should allow for this.

since doing this the Uno will not respond to the IDE
I have tried re-uploading my sketch as well as Blink - no luck the Uno does not connect.
when I do this the RX led blinks slowly a few times up that is it - the TX led never lights up

I have also tried pressing the reset button just as I click upload on the IDE

The Uno's power LED does come on
When I press reset there is no visible response on the uno (no changes in any leds)

I have put a multimeter on the 5v and 3.3v pins:
with the USB connected they both work as expected
When I connect a 6v adapter the pins give 6v and 3.3v
When I connect a 9v adapter the pins give 8.5v and 3.3v

So! It seems to me that is it likely that:

  1. the 5v side of the voltage regulator is probably fried - since the 5v pin is not stable
  2. the ATMega is dead - Since it is not responding to upload commands (when connected/powered via USB)

Is my diagnosis correct?
What can I try to fix it?
The only thing i can think of is to replace the ATMega and stop using the external power connection unless it's with an externally regulated 5v source.

Any one have any other thoughts on this?

2

Looks more like the regulator has died as you say.
If you are comfotable and have both the skills and equipment you could change it out (50/50 chance)

There is a common theme over the last 8 months or so to indicate an issue with above 10 volts killing real arduinos. You can find a lot of posts about 12 volts or higher killing a board.
Clones seem a lot better in this regard.