5 : T h e o r y o f O p e r a t i o n
CHRISTIE INC. -
CHRISTIE INC. - 35/70 Service/PM Manual
July, 1997
toward zero volts for the fifth time, the pulldown motor has nearly slowed to a stop. At this point both optical pickups are
partially on, and registration begins.
The sensor circuit also has a 90-degree sensor and an index sensor. Their outputs are squared by U2 and sent to the Main
Logic/CPU card, along with the zero-degree and 180-degree outputs.
The Pulldown Power Amplifier Circuit (2275)
The pulldown power amplifier circuit consists of a LM741 operational amplifier followed by an analog amplifier. (See
the pulldown amplifier circuit diagram and component layout in Section 8.11.) During final braking and registration, the
tach signal from the pulldown motor is summed with the pulldown signal to dampen oscillation. The overall voltage gain
of the power amplifier is slightly over 12. The two circuits are identical in operation, differing primarily in their circuit
board layout.
The Pulse-Width-Modulated Power Amplifier (2285)
The 2285 power amplifier (see Section 8.13) contains three LMD18200 integrated power switching amplifiers, U5, U6,
and U7. Their outputs are coupled to the motor through filters formed by L1-L6 and C12-C13. The filters contain the
20KHz switching frequencies to the board and also isolate U5 – U7. Photocouplers U1-U4 isolate the power stages from
the logic level circuits on the 2293 CPU Card. U1 inputs the Pulse Width Modulated signal to U5 – U7, while U2 inputs
the direction signal. U3 and U4 activate bilevel drivers Q2 and Q3. Q1 is used to activate photocouplers on the 2293
card that limit current by decreasing the input pulse width. Q4 reduces and regulates the 24-volt supply to 5 volts as
required by U1, U5, U6, and U7. This supply maintains isolation between signal and power level circuits.
PWA Amplifier
Circuit 1
Circuit 2
Shutter
Douser
N/U
Constant Velocity
Reverse Solenoid
N/U
Upper Torque Motor
Reel (brake)
N/U
Lower Torque Motor
Reel (brake)
READY Relay
Table 5.13: PWM Amplifier Bilevel Outputs
The DC Power Supplies
The projector power supply (see Section 8.4)has regulated outputs. The high power outputs are -34 volts and +24 volts.
The voltages used by the pulldown motor are +24 volts and -34 volts; these are switched by the READY relay, which is
part of the power supply. The constant-velocity, shutter, and torque motors use +24 volts. This voltage is switched by
another READY relay contact.
The regulated low power su5 volts, +15 volts, and -15 volts for the CPU and other logic devices.
The Keypad (2267)
The keypad consists of the keyboard card, card 2267. (See the keyboard component layout and circuit diagram in Section
8.8.) The keyboard card is a 4 x 8 switch matrix. The fifth column and the right-hand four keys of the bottom row are not
used. The four rows of the matrix are scanned by strobes from multiplexer U46 on the CPU/Main Logic card. Each
column is monitored for a switch closure by the PIA, U37, on the CPU/Main Logic card. Latches U1 through U4 control
the LED indicators on each key.
