The ERSU72D/1 Radio Selector Unit amplifies microphone signals and provides selection of intercom or transmit facilities along with control and amplification of receiver signals. Intercom audio level is set by a front-panel potentiometer and a CALL push-button overrides the intercom. Telephone amplifiers are duplicated and connected in parallel, with power supplies drawn from two independent 28V DC sources so the system continues to operate if one amplifier or supply fails. The unit contains microphone and telephone channels, switch assembly and switch-and-potentiometer assembly, with two identical telephone amplifier and mute circuits.