TM 5-6675-319-14
(c) When SLOW speed is selected the SLOW speed potentiometer is
connected to P1, P2 and P3 of the motor control TB. A voltage divider network
consisting of R10, TH (thermistor), Zener diode, maximum speed potentiometer P4, R12,
Slow speed potentiometer, minimum speed potentiometer P5, torque potentiometer P3,
and the dc motor armature is established.
Maximum speed potentiometer P4 is adjusted
for an armature voltage of 125 V dc.
Minimum speed potentiometer P5 is normally set
fully left and torque potentiometer P3 limits motor current and is normally set fully
r i g h t .
The SLOW speed potentiometer establishes the bias on Q2 during slow speed
o p e r a t i o n .
The conduction of Q2 controls the charging rate ofC5. As the armature
rotates the ac wave is felt on the base of Q2.
If Q2’s conduction is increased, the
charging rate of C5 increases.
When the charging voltage of C5 is approximately 2/3
Q2’s collector voltage, Q1 conducts, discharging C5.
The discharge of C5 is coupled
to the secondary of pulse transformer T1.
The SCR conducts and motor speed in-
creases due tc increased armature current.
If Q2’s conduction is decreased, the
charging rate of C5 decreases.
Q1 and SCR remain off and motor speed remains
constant.
The motor speed is regulated by conduction or nonconduction time of the
SCR. The slower the motor turns, the lower the ac signal amplitude; thus the
conduction of the SCR is less.
(d) When FAST (SLEW) speed is selected the SLEW speed potentiometer
is connected to P1, P2 and P3 of motor control TB.
The voltage divider network is
e s t a b l i s h e d.
The SLEW speed potentiometer dial is set to 100 (right). Q2’s
conduction increases, C5 charging rate increases,
f r e q u e n t l y .
causing the SCR to conduct more
This increases armature current and motor speed. The ac signal
amplitude increases, which causes the SCR to conduct for long periods of time.
Torque potentiometer P2 is used to prevent hunting when the motor is stopped.
b.
R e c t i f i c a t i o n.
To rectify photographs,
i t i s n e c e s s a r y t o r o t a t e t he
negative in its plane, aline the negative horizon with the rectifier, and move the
n e g a t i v e r i g h t o r l e f t.
These movements are accomplished by:
Swing Drive
Y-Drive
X-Drive
7-20
