The 302 and 442 mixer's tone oscillators have several modes of operation. From the Setup Menu, the tone frequency and level can be set.
Additionally, when the tone oscillator is active, pressing the battery check button will begin a mode where the left channel tone will alternatively go on and off. This feature is a helpful method to identify left channel from right channel. It is possible to get the tone oscillator in a state where the left channel tone is off and the right channel tone remains on. This can occur if the battery check button is pressed when the alternating left tone level is "low". Cycle the tone oscillator switch to return to normal tone mode.