The starting box used with the DC driven four-unit set is similar in operation to that described in section 34 for starting the 1 1/2 k-va. DC to AC motor generator set but instead of a hand operated line switch, an electrically operated line switch or contactor is used (see Figure 35) . This contactor is operated by means of push-button switches provided on the amplifier rack panel. A pilot light next to the switch indicates when the motor-generator set is operating. In the case of the PG-1 equipments two motor-generator sets are used, one being a stand-by equipment for emergency service. Since a pilot light is provided at the push-button operating each motor-generator set, the two pilot lamps also indicate which one of the two sets is running.

The starting switch used with the AC driven four-unit set (Figure 34) is similar in operation to that used with the AC to DC motor-generator set for DC projector drive motors. The description of this last named switch is given in section 33.

These four-unit motor-generator sets are not equipped with individual control panels. All voltages are measured at the amplifier rack, and the details of this will be given in Chapter XIII, "RCA Photophone Equipments." The four-unit motor-generator set with the DC driving motor is oiled as follows:-All four units have oil ring bearings, and the oil cups should be kept filled to within one-eighth of an inch of the top. In the case of the 250 volt machines and the 1000 volt machine, the oil level is checked by lifting up the oil-well covers and observing the height of the oil in the oil vent ,just below the main opening into the oil well.

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