I recently built a MIDI controller, which is sorta like a keyboard played with your feet, using an Arduino Uno.
When I power the Uno through USB (like when I'm sending the sketch to it) from my computer, everything works exactly as it should. All of the note and function keys put out the correct values to the MIDI jack when read in MIDI-OX.
When I try to power it ANY other way, all I get is a constant stream of random data at the MIDI jack.
I've tried everything from 5V to 12V switching power supplies going into the Uno's power connector. I've even tried a MB102 breadboard power supply going into the USB connector on the Uno. Always the same result - a constant stream of gibberish.