Anyone? Perhaps someone with a PixHawk can put a volt meter on the Main OUT (RC) center pins and see if there is any voltage there?
Yes, an external BEC is required. You should also put a 5.6 volt zener across the gnd and positive pins of the PixHawk if you do that so that the PixHawk is protected if the BEC has a problem. It's documented on the setup page for the Pixhawk here: http://plane.ardupilot.com/wiki/common-pixhawk-wiring-and-quick-start/
Thanks for the reply and confirming that a BEC is required.
I'd also like to know if there is any voltage found on the RC power rail when no BEC is connected.
Without a BEC connected here, I can measure about 1.2 VDC on the RC power rail (center pins).
I do have a 5.6 V Zener ready for when I connect BEC there. I'm not using servos, but will use the Zener for protection.
I'm measuring 25Mv on the power rail at the Channel 7 output and 32Mv at the Position 2 of the Aux outputs. So it's essentially zero. I don't connect the BEC leads coming from the ESC, I've clipped off the red wires to avoid having another disaster (I had an ESC fry my FrSky receiver when I was doing some separate testing).
Are you getting 1.2 volts on that power rail with nothing plugged into the Pixhawk other than the USB to your computer? If you are, I'd go back to 3DR and see if that isn't the problem.