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i92 SOUND MOTION PICTURES
properly centred; by poor jack contacts in the amplifier; or by "pick-up". of interfering noises, such as radio signals or motor sounds.
J. Centring
Centring is accomplished by three linen strings spaced 120 degrees apart. These strings are so arranged in the cone collar that stress is not exerted upon it, but is taken up by the three strings which are tied together in the centre. Each suspension string is fastened to a small bracket, whose position may be changed by the rotation of a machine screw. A slight rotation of the screw will cause the cone to move perceptibly in the gap. Care should be taken to see that the tension on the strings is just sufficient to hold the coil centrally in the air gap.
4, Jack contacts
Keep ail amplifier jack contacts clean.
5. Pick-up
Check all ground connections of the system.
K. Dial Settings for D. C. Motor Control The R. C. A.-Photophone, Incorporated, furnishes a chart with projectors using the D. C. motor drive, giving the speed control and dial settings for different film speeds. The dial settings are subject to periodical changes in accordance with instructions given by the R. C. A.-Photophone Service representative.
L. Speed Control Switch Markings for D, C. Motor Drive
"Low" is for low speed picture operation: 75 to 90 feet per minute film speed. (Sound reproduction should not be attempted with the switch in this position.)
"Normal" is for sound reproduction: 90 feet per minute film speed. (Variable speed operation should not be attempted with the switch in this position.)
"High" is for high speed picture operation: 90 to no feet per minute film speed. (Sound reproduction should not be attempted with the switch in this position.)
It is very important that switches be thrown, as in