I was looking the schematics of serial, usb, ng and diecimila models (BT seems to have power source directly connected to "9v. pin".), and all of them has only a diode from power source to "9v. pin". So, it was supposed that you got power source tension - 0.7 volts (diode) on "9v. pin" always. I don't understand why he must change power selector to usb to have power source to "9v. pin".