@TacoT Thanks for the explanation. This is from the FAQ:
Honeycomb Alpha & Bravo Users
If you are using either the Alpha and or Bravo you will need to do one of 2 things for the GNS430/530 to work:
1) Make sure that the AVIONICS BUS 1 switch is set to ON
2) Remove the switch command in the setting for SET AVIONICS MASTER 1
IMPORTANT: If using BOTH together then the command is automatically assigned to switch number 17 of the Alpha and switch number 47 of the Bravo.
I had assumed this extends to the 750.
Funny thing though. When I turn off Avionics Bus 1, e.g. after landing, the 750 reboots and comes back on.