Ensure that the correct input is set up with enough gain if a microphone is used as the modulator source. Call up one of the vocoder presets (OS2 program H124, OS1.5 program D124). Set the vocoder balance to be value 063 "vocoder only".
Either activate the constant gate or play notes on the Nova II at the same time as talking or singing into the microphone. The vocoded output should be audible. If the modulator signal can still be heard over the top of the vocoded output this is due to the amount of sibilance that has been added. The sibilance can be extracted by high pass filtering the modulator source or by synthesising noise. Either set the sibilance type to noise or reduce the sibilance level to reduce the audibility of the modulator signal.