I have added the HDMI option board to my Phantom 4 controller.     Using a HeadPlay display, the device works excellent, however, some HDMI devices refuse to display.   

1.  HeadPlay goggles using the HDMI cable work great on both my Phantom 4 and my Parrot Bebop.
2.  Plugging a TX-5D HDMI transmitter into the Parrot Bebop controller works perfect - great picture.
3.  Plugging the TX-5D HDMI transmitter into the Phantom 4 HDMI board on the controller equals no video.

Any ideas?    What are the output specs of the HDMI board (to compare to Bebop to see if this is the problem)?

The goal is to "lose the tether" on the goggles.     The HeadPlay goggles also has a 5.8 receiver built-in and it would be great to have the HDMI output pass through the TX-5D transmitter to be used by the goggles and a remote reciever/display.    I have done this with the Bebop and it works great.

DJI has encryption enabled that may not be compatible with the TX-5D transmitter.