Hey everyone
I was in the believe that the Uno had "auto-switching" on ext power and usb power, so while it was on usb power I plugged in a 9V battery to the 2.1mm jack. That rendered my ext power input useless. It will not accept power from the plug, and as soon as I plug the battery in, the voltage drops from 9V to 2.5V across the battery.
It did work on battery prior to me plugging it in simultaneously.
Did I fry the voltage regulator?
regards Troels