Abstract: An ac mains test apparatus and method. An audio frequency signal measurement instrument is provided with a circuit for testing the ac mains to which the instrument is connected. The ac mains is internally connected to a coupler which presents an attenuated portion of the ac mains to the low level input of the instrument. A switch is provided for selectively connecting the input either to an audio signal to be measured or to the ac mains portion. The coupler both attenuates the ac mains for input to the instrument and isolates the relatively high ac mains voltage from the low level input.