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ensure that the counter does not overflow and cause a sudden transient to the motor during rollover, pending overflow is
detected and the CPU reduces motor speed so overflow does not occur.
Locking the Constant Velocity motor to the line frequency has advantages where line frequency variations are minor.
Where excessive variation exists, crystal oscillator control may be selected by a jumper on the 2293 board.
P u l l d o w n C o n t r o l C i r c u i t
The primary mechanical elements of the pulldown circuit are the pulldown motor, the pulldown disk sensor, the
tachometer, and the pulldown sprocket, all located on the pulldown assembly. The major electronic assemblies are the
Pulldown Amplifier 2275 and the associated pulldown circuitry on the 2293 Main Electronics Board.
The action of the pulldown circuit is different in slew and in cine mode. During slew operation, the pulldown motor must
be locked to the constant-velocity motor so film moves smoothly through the film gate.
Figure 5.5 shows the pulldown circuit configured for slew operation. To lock the speed of the pulldown motor to the
constant velocity motor, the two tachometer signals are fed into U21 as a differential amplifier. The bi-directional error
output is amplified by U21 and sent to the Pulldown Amplifier Board 2275. On the amplifier board the signal is amplified
by U1 and sent to the motor drive circuitry. To further ensure full synchronization between the two motors, the respective
encoder signals are sent to EPLD U19 operating as an up-down counter. The eight-bit output of this counter is sent to the
DAC U20, amplified by U55, and summed with the pulldown tachometer to provide precise positional control between the
two motors.
PD Disk Sensor
CV Encoder
PULLDWN3
CHRISTIE INC. - 35/70 Service/PM Manual
CHRISTIE INC. -
July, 1997
U1
U20
U55
UP / DN
37
DAC
CNTR
AMP
CV Tach
E
T
Figure 5.5: Slew Operation - Pulldown Circuit
Pulldown Tach
U21
U21
PLL
12
VF
Open
SUMMING
AMP
Main Electronics Card
5 : T h e o r y o f O p e r a t i o n
U1
Pulldown
Drive
2275
D-AMP
Pulldown Amplifier Card
E
T
M
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