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Theory of Operation
2-40.
The bi-polar ramp generator consists of two inte-
grated circuit operational amplifiers (U2 and U3), a symetri-
cal 8-volt clipping network (CR7 through CR11), and a
ramp control network
(R42/C28).
2-41.
Operational amplifier U2 initially operates as a
saturating comparator when a forward or reverse command
is given or removed. Near the end of a ramp-up or ramp-
down U2 changes from a saturating comparator to a linear
amplifier with the non-inverting input at O-volts.
2-42.
The output of U2 is held to 8 volts by a symetrical
clipping network (CR7 through CR11) which establishes
current for the ramp control network. The slope of the
ramp is determined by the current through R42 (RAMP
control) into integrating capacitor C28. Feedback through
R66 nulls the reference input voltage to U2 and the output
voltage of the ramp generator (U3-6) is held steady by the
ratio determined by R66 (R35 + R34) or R66 (R29 + R28).
2-43.
When the Forward or Reverse command is removed,
the current through R66 drives operational amplifier U2 in-
to saturation and the ramp integrates the 0 volts. As the
output of the ramp generator (U3-6) approaches 0 volts, U2
reverts to a linear amplifier and the output of the ramp
generator is maintained at 0 volts.
2-44.
A High-Speed Forward command (HSFWD) from
the control and status circuits or placing the + 160 service
switch in the on position (up) will allow CR12 and CR13 to
conduct and reverse bias Q8. With Q8 reverse biased, C25
is charged through R49. The exponential voltage at the base
of Q9 rises to + 12 volts but is clipped at +6 volts when Q9
saturates. When the HSFWD command is removed or the
+160 service switch is placed in the off position (down),
U1B conducts placing U1B-4, CR12, and CR13 at 0 volts.
This allows Q8 to conduct and C25 discharges through R48.
The threshold caused by the base emitter turn-on voltage of
Q9 and the diode drop across CR14 results in a delay of
approximately 100 ms before motion starts or stops.
2-45.
A High-Speed Reverse command (HSREV) or plac-
ing the -160 service switch in the on position, (up) will cause
the high-speed reverse ramp circuit to function the same as
the high-speed forward ramp circuit, except that voltage
polarities are reversed. Capacitor C26 is charged through
R56 and discharged through R54 and R55.
2-46.
The LOAD command from the control and status
circuits does not control a ramp circuit. The load switch of
the capstan servo is a single step input to the capstan servo
amplifier resulting in a nominal 20 ips tape motion.
2-47.
The outputs of the bi-polar ramp generator, high-
speed forward ramp generator, high-speed reverse ramp gen-
erator and load switch form a summing junction at the input
of the capstan servo preamplifier (U4). Diodes CR17 and
CR18 provide clipping to protect the amplifier from over-
load. The preamplifier drives the capstan motor power am-
plifier (Q1 through Q6). The dc gain of the power amplifier
2-8
7970B/7970C
is 10 volts per volt determined by RIO and R11. The power
amplifier is operated in class B with Q6 providing negative
current for forward motion and Q5 providing positive cur-
rent for reverse motion.
2-48.
A notch filter in the velocity feedback circuit from
the tachometer is selected to attenuate the mechanical re-
sponse of the motor-tachometer combination. A compensat-
ing network in the current feedback circuit is also selected
depending upon synchronous speed of the tape unit.
2-49.
Transistor switch Q22 senses the presence of
tape tension. While the tape is tensioned, Q22 is on,
keeping switch Q20 off. However, when tension is lost
Q22 turns off, allowing Q20 to turn on and switch the
input of motor drive amplifiers Q2, Q4, and Q6 to
ground. This disables the capstan servo. The capstan
motor circuit is completed through the B contacts of
relay K1 on the reel servo amplifier PCA when K1 is
energized. Relay K1 is energized when the LOAD push-
button is pressed. Once energized, it remains energized
until tension is removed.
2-50.
REEL SERVO CIRCUITS.
2-51.
The reel servo circuits consist of a tension circuit,
a voltage switching circuit, a delay circuit, voltage/current
feedback switches, tension arm photosense circuits, pream-
plifiers, motor power amplifiers, and reel motors. Figure 2-5
is a block diagram of the reel servo circuit.
2-52.
At initial power-on, the tension circuit is disabled.
The normally closed contacts of LOAD pushbutton switch
prevent Q13 of the tension circuit from conducting. Pressing
the LOAD control allows Q13 to conduct, energizing relay
K1. With K1 energized, the capstan and reel servo motor
circuits are completed. As tape is tensioned and the tension
arms swing away from the limit switches, power through the
limit switches maintain a forward bias of Q13. When power
is removed, or tape tension is lost, the relay contacts short
across the reel motor windings to provide dynamic breaking.
2-53.
The voltage switching circuit is located on the
power regulator printed-circuit assembly. During a high-
speed operation forward or reverse, power to the motor
power amplifiers is switched from 22.5 volts to 57.5 volts
(nominal).
2-54.
During a high-speed reverse operation (rewind), the
HSREV command from the control and status circuits is
gated with TENSION. When both reel motors approach full
r/min, the motor voltage exceeds the break-down voltage of
CR4. The condition established by the gating of HSREV
and Tension allows current through CR4 to forward bias
Q5. Voltage switch Q6/Q7 conducts placing +57.5 volts on
the +20/40 line. Diode CR2 on the power distribution
printed-circuit assembly is back-biased preventing +57.5
volts from entering the +20 volts line.
Part 2
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