The RCP Signal Tracer, Model 777A has been designed to be a timesaving device facilitating the servicing of AM-FM receivers, audio amplifiers, and television sound systems. It is an aid to the serviceman in locating faul ty components, stages, stage gain, distortion, hum, etc . The unit contains an AC operated, three-stage amplifier with traveling probe. The probe allows direct insertion into stages carrying radio frequency currents. In addition to the RF probe, a special microphone-phonograph connector is provided so that the functioning of either high impedance microphones or phonograph pickups may be determined. The step and vernier attenuator is accurately calibrated so that direct reading stage gain measurements may be made. Due to the extremely high sensitivity of the newly developed 777A, a slight hum may be noticed when used "wide open". If this is the case, the line cord should be reversed in the AC socket. The same effect may be had by grounding the case through a .05 mfd. condenser to a water pipe, metal conduit, etc.