The dial of the speed regulator can be calibrated by counting the number of revolutions per minute of the turn table. When properly set, the turntable should make thirty-three and one-third revolutions per minute. It can be set approximately by counting the revolutions for one minute, but for accurate setting the revolutions should be counted for at least three minutes. The table should make one hundred revolutions in three minutes.

To operate the motor for sound-on-disc or sound-on-film, see that the snap-switch on the motor terminal and resistor box is in the "Normal" position and that the speed regulator dial is set at the proper reading. The motor is then operated by means of a three-position drum switch or controller. See Figure 16.

To start the motor, the controller handle is moved from the center or "Off" position to the extreme left. To stop the motor, the controller handle is simply returned to the "Off" position. The controller handle should never be placed on the extreme right, or "Brake," position, except when it is necessary to make an emergency stop. Using the "Brake" position every time the projector stops causes excessive wear on the mechanism, and this position should not be used unless absolutely necessary. When the switch is in the "Brake" position, the motor field is connected to the line while the motor armature is disconnected from the line and is short-circuited. The coasting motor then acts as a generator with a heavy load, and comes to a sudden stop. If it is necessary to use the brake, be sure to return the controller handle to the "Off" position, since in the "Brake" position the motor field is connected to the power line and will get excessively hot in a short time. During normal operation the fan on the end of the armature cools the field windings. When using this control switch, turn the handle to the desired position quickly to reduce arcing within the switch.

Improper operation of the centrifugal speed-control device will result in "wows," and may be caused by a dirty slip ring, a pitted brush, or a burned switch contact. The brush may be removed for inspection by removing the brush-holder cap. When replacing the brush make sure that the curvature of the brush fits the slip ring.

The switch contacts and the slip ring are accessible by removing the dial. This is done by removing three screws which hold it to the motor housing. These screws are under the dial, and are accessible by rotating the dial so that the notch uncovers the screw-heads. The fly-weight and slip ring mechanism is held on the motor shaft by a set screw which is accessible by removing the large plug screw on the left-hand side of the motor housing.

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