a suitable power supply preferably 208 volts ac in operation fig4 operates generally similarly to fig2 however in this embodiment timer 61 extends input pulses corresponding to magnetron moding to enable counter 118 to register each as a discrete count isolators 100 102 provide voltage isolation between the mode detector and the output circuit and provide one gate trigger signal on lead 110 for each input pulse received through resistor 98 the mode detector of fig2 may be used to detect incipient failure of a microwave oven magnetron by connecting the mode detector 12 to an oven magnetron circuit 10 as shown in fig1 and then energizing circuit 10 in a commercial setting observation of three or more modes minute indicates incipient failure and the magnetron tube should be replaced immediately to avoid interruption of the commercial enterprise an indication of less than three modes per minute indicates the tube still has some useful life in a consumer or residential oven application a higher limit may be set since such an oven typically is used far less than in a commercial application hence for a consumer oven ten or more modes minute is an appropriate indication of incipient failure in the mode indicator of fig4 the total number of modes is recorded and displayed this embodiment is intended to be monitored at relatively infrequent intervals until an indication is given that the tube has begun to mode from then on the counter 118 is observed at more frequent intervals for example at the beginning and end of one minute at the start or end of each working day the difference in readings between the end and the start of the interval divided by.