The SEERSU74A/1/1 Radio Selector Unit is used as the instructor's selector unit in an airborne communications control system. It amplifies microphone and receiver signals and is designed for use with a 300Ω microphone. With the NORMAL/SBY switch in NORMAL, transmit and receive on all channels selected on the pilot's selector unit are available; in SBY, facilities are confined to the UHF/SBY system irrespective of the pilot's selection. All telephone signals except override and warning tone are controlled by a single overall gain control. MUTE keys the pilot selector's mute circuit and CALL overrides the pilot's intercom gain. Mic and telephone amplifiers are duplicated in parallel with two independent 28V DC sources, and a front-panel test switch verifies the duplicated elements. A standby indicator lamp lights when the UHF/SBY transmitter is selected on the pilot's unit.