Bell & Howell - Projector - Filmosound 641 - 642 ... - Cine Information

50. TIMING BELT IDLER ADJUSTMENT
Belt tension is not critical, although
excessive tension will reduce belt life. Set
the rewind belt idler so that the midpoint of
the slack side of the belt can be pushed
inward approximately i/8 inch by light
pressure. Set the take-up belt idler so that
the belt can be pushed down against the
motor relay mounting bracket by light
pressure.
51. PROJECTOR SPEED CHECKS
The speed of the projector is not
adjustable. Therefore, speed checks are
primarily for the purpose of determining
that the equipment is operating properly
and as a means of detecting excessive
mechanism loads, damaged drive belt or
similar conditions.
A. Methods of Measurement
Various devices and procedures can
be used to check projector speed. The
most common ones are as follows.
(1) Photocell and Frequency Meter.
Used to measure the number of
interruptions of the projection
beam per second, pulses per
second are then converted to the
projector speed. This method is
quite practical if a quantity of
equipment is handled.
(2) "Strobotac" or similar strobe light.
Usually synchronized with the
interrupter shutter or shuttle.
The shutter makes one revolution
per frame. The shuttle makes
one stroke per frame.
(3) Tachometer (preferably having a
speed range with a maximum
speed of 150-200 RPM). Used to
measure RPM of sprocket.
(4) Timed Loop* Make a loop of
exactly 120 frames. At sound
speed the splice will pass the
38
aperture 12 times per minute +
the permissible variation in speed
and the timing error.
B. Speeds at 240 Volts 50 CPS
(1) Sound Speed
Shutter - 1440 RPM + 2%
Sprocket - 102.86 RPM + 5%
(2) Silent Speed
Shutter - 960 RPM + 5%
Sprocket - 68.6 RPM + 5%
52. LOOP RESTORER ADJUSTMENTS
A. Test Film
Since the film is transported by means
of the sprocket holes and the shuttle and
sprockets are synchronized by the gear
train, there will always be the same
number of frames in the lower loop, when
the shuttle is at top of stroke position. The
size of the loop, however, is dependent
upon the actual length of the film in the
loop. If the film is either stretched or
shrunk, the size of the loop will change.
This possible variation in the size of the
loop, due to film condition must be recognized
and allowance made for it in adjusting
the loop restorer. Therefore, it is necessary
to measure the stretch or shrinkage
of the piece of film which is to be used in
adjusting the loop restorer in order that
the adjustment can be made with the
highest possible degree of accuracy.
Proceed as follows:
(1) Count 40 frames of film and mark
the first and last sprocket holes.
(2) Place the trailing edge of the first
sprocket hole at one end of a
12 inch steel rule.
(3) Smooth the film along the rule.
Do not apply more than 2 to 3 oz.
tension to the film as this might
produce an erroneous measurement*
(4) If there is zero shrinkage in the
film, the trailing edge of the last