When all connections have been made and adjustments checked, turn on the amplifier. Test the exciter lamp for proper position by holding a white card in front of the sound gate. When the filament image is centered in the round spot the lamp is in position. Move it up or down until correctly set, and tighten the thumb screw.

Allow the amplifier to warm up for one minute, as certain of the tubes used require that time before they will function. Give each projector a "click test" by flicking a card in and out of the sound gate with the volume turned up and the projector "faded in." A loud click or thump in the loudspeaker will result. If no sound is heard, consult the trouble chart, section 282.

Next test the amplifier for hum, and adjust so as to minimize it. Since the tube filaments are AC operated, it is necessary to include a hum adjustment. Figure 136 shows the location of the hum adjusters, which consist of potentiometers connected across the filament circuits, and adjustable by means of a slotted screw head. These are turned with a screw driver, either to the right or left, until the hum is at a minimum. The procedure is as follows:-Keep the volume control on zero, and remove the UY-227 tube. Adjust the UX-256 hum adjuster for minimum hum. Replace the UY-227 and remove the UY-224. Adjust the UY-227 hum adjuster for minimum hum. Replace the UY-224 and adjust the UY-224 adjuster.

279. Operation of the Equipment. -Although the portable Photophone equipment is quite different in construction from the theatre equipments, as has been pointed out, the operation is not very different. Since the projectors are quite similar to standard types in the construction of the mechanism, the film is threaded through them in the same way.

Figure 131 shows the Type PP-6A portable projector mechanism with film threaded. Figure 134, showing the sound projecting mechanism, and the film threaded through the sprockets, sound gate and rollers illustrates how this is done on the Type PP-6B projector. The projection mechanism threading is the same as for the other model. Figure 132 shows the sound projecting mechanism of the PP-6A projector. A black line is painted on the center plate as a threading guide.

The operation of the portable Photophone equipment when using sound film is somewhat different from the standard routine of running sound projectors. The main reasons for this are that the motor comes up to speed almost immediately, and the projection lamp is turned on at each change-over, and does not need to be started beforehand as with an are. In addition to this the fader switch changes the exciter lamp supply as well as the sound output. Because of these things it can be seen that the operation of changing over, starting and stopping must take into account these facts. The procedure is therefore as follows:--

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