6 : M e c h a n i c a l a n d E l e c t r o n i c A d j u s t m e n t s
CHRISTIE
CHRISTIE
35/70 Service/PM Manual
March, 1997
Pulldown Sensor Adjustment
The pulldown sensor provides position information to the CPU/Main Logic card. The sensor card has four infrared LED’s
and photo-transistor detectors. An encoder disk is mounted on the pulldown motor shaft next to the sprocket. Zero- and
180-degree signals are used to continuously adjust the drive to the pulldown motor to maintain a consistent jitter-free
picture while running cine forward. Both a 90-degree and a zero-degree signal are used to form a biphase signal that locks
the pulldown sprocket to the constant drive sprocket during slew motion in either direction. A fourth signal, an index
pulse, is used by the CPU to keep the picture in frame.
The sensor card must be correctly positioned relative to the notched disk on the pulldown sprocket shaft. (See Figure 6.3.)
Pulldown
Sensor
Figure 6.3: Pulldown Sensor Position
If the sensor card is adjusted improperly, the pulldown gain can be too low (Figure
6.4
-a), or too high (Figure
6.4
-b).
Either situation can cause excessive jitter. The normal pulldown sensor signal is shown in Figure
6.4
-c.
