HP 7970B Operating And Service Manual page 12

Theory of Operation
2-55. During a high-speed forward operation, the voltage
switching circuits function the same as in the fast reverse
operation. HSFWD is gated with tension to allow motor
voltage to switch the -40 volt switch (QI2/QI3).
2-56. The delay circuit is located on the power regulator
printed-circuit assembly. The delay circuit provides an addi-
tional one-second delay following the end of a high-speed
command. The one-second delay prevents further high-speed
commands from the interface from being processed during
the one-second period. The one-second delay is also used to
switch the reel servo operating mode from voltage feedback
to current feedback. The additional one-second following a
high speed operation allows the reel servo to operate in a
current feedback mode until reels are slowed.
2-57. During normal operation, U5A-3 and U5B-4 rest at
o
volts. A HSFWD or HSREV command from the control
and status circuits will cut-off U5A or U5B and base current
for Q15 is supplied through R28 or R29. When Q15 con-
ducts, capacitor C8 immediately discharges through Q15
causing Q16 and Q18 to be cut-off. Feedback control to the
reel servo changes from approximately +8 volts to approxi-
mately -8 volts. The negative potential also reverse biases
Q17 and the DELAY line switches to +5 volts.
2-58. When the high-speed command (HSFWD or
HSREV) is removed and Q15 loses base current, capacitor
2-10
7970B/7970C
C8 is then charged through R33 (one-second time constant).
When C8 is charged to +5V, Q16 and Q18 conduct and the
feedback control changes from -8V to +8V. The DELAY
switches to 0 volts.
2-59. The reel servo tension arm assemblies contain dual
element photo-conductors that are illuminated by a lamp
shining through a slotted disc. The slot is in the form of a
spiral attached to the tension arm. As the arm moves, the
slot exposes different areas of the photo-conductor. As a
result, the output of the photo-conductor is proportional
to the position of the tension arm.
2-60. The reel servo preamplifier is an integrated opera-
tional amplifier that amplifies the position error of the ten-
sion arm. The tension arm photo-conductor output is single-
ended, therefore, an off-set is provided by R39 or R41.
The preamplifier drives a class B motor drive amplifier.
The motor power amplifier has a gain of 10 volts per volt.
2-61. When operating in a normal mode (synchronous
or load speed), the servo operates with voltage feedback.
The +8 volts from the feedback switching network (feed-
back control) back-biases the feedback FET switch (Q14
and QI5). Feedback is then provided through R47 and
RI05. During a high-speed operation, the feedback control
changes to -8 volts. The feedback FET switch is forward
biased and feedback is current through the switch.
Part 2