Service Interphone
A total of twenty-two hand set jacks are located in various parts of the airframe in order that ground
crew can communicate with one another using the service interphone system. The system is rather simpler than those considered above. Mic. audio from all handsets with press to talk depressed are combined in and amplified by the service interphone amplifier in the interphone audio accessory box. The amplified signal is fed to all handset tels. Volume control adjustment is provided by a preset potentiometer. With the flight engineers interphone switch selected to ON the input summing networks for both service and flight interphone systems are combined. All mic inputs from either system are amplified and fed to both systems
