Problem when powering with external ps

I built a MIDI controller which sends some TX data (standard MIDI arduino wiring).
On USB power everything works OK but on external (9V, 1000mA) power supply everything works ... besides TX data! - the most important thing in this project. It simply does not send that commands.

Do You know what I'm maybe doing wrong? :confused:

What board are you using? I'm having the same problem with a Mega.

Im using UNO R3 (original). ... i tried doing so many things with this problem and it doesnt help... Maybe You have some ideas?