The diode modulator of the trainer is shown in Figure.4.The modulating signal is applied to one end of R17,while the carrier is applied to the corresponding end of R18.The signal at the junction of R17 and R18 is the sum of the carrier and audio.Note that the carrier is not amplitude modulated at this point,but it is simply added to the audio signal.
Diode D1 conducts through R19, during the positive swings of the anode signal,however when the signal swings negative,D1 cuts off.Thus the signal developed across R19 will consist of positive going pulses,as shown.
Note that the positive pulses vary in amplitude in accordance with the audio signal amplitude.