changing the operation of the film feed clutch
assembly while the drive system is operated in
reverse. Some of the older models require that
the reels be exchanged on the arms for re-
winding. That is, the film on the takeup
reel is placed on the supply arm and the
empty supply reel is moved to the takeup arm.
Check the instructional manual for the projector
with which you are working to determine the
steps for rewinding film on that specific pro-
SOUND AND SILENT SPEEDS.Projec-
tors designed to project both silent and sound
films are equipped with a silent/sound selector.
Sound film is projected at the rate of 24 frames
per second (fps), Silent film is projected at the
speed of 16 fps in some projectors and 18 fps in
others. When the sound/silent control lever is
placed in the SILENT position, it rotates an
eccentric* that is attached to it, approximately
180°, This slows down the speed of the film.
When the sound/silent control lever is in the
normal conditions are
restored and the film moves at the speed of 24
frames per second.
PROJECTION OPTICAL SYSTEM.The
projection optical system, shown in figure
7-16, consists of the projection lamp, a re-
flector, condensing lens assembly, projection
lens, a blower motor
and cooling ducts, and
* Eccentric: A mechanical device consisting of a disk
through which a shaft is keyed eccentrically and a circular
strap which works freely round the rim of the disk for
communicating its motion to one end of a rod. The other
end is constrained to move in a straight line so as to
produce a reciprocating motion.
Figure 7-16.Projection and sound optical system.