If the Detector carriage is now slid along the waveguide, the meter should move up and down. Adjust the source Attenuator so that the maximum reading of the meter is about full scale. If the source Attenuator is now set outside the range 20 to 60 open, the Detector probe may need adjusting. Slacken its locknut, and screw it in until a satisfactory reading is obtained. (It may be necessary to disconnect its lead if many turns are required).