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7970B/7970C
2,35.
The one-second delay (generated on the power
regulator printed-circuit assembly) following a high-speed
reverse command prevents additional high speed commands
from being processed by the control and status circuits. The
delay also prevents the load point search from occurring for
one-second following a rewind operation.
2-36.
CAPSTAN SERVO CIRCUITS.
2-37.
The capstan servo circuits control the speed and
direction of tape motion across the magnetic head assembly.
The capstan servo consists of a capstan motor/tachometer
and a capstan servo printed-circuit assembly. The servo cir-
cuit employs current and velocity feedback. The velocity
feedback is provided by the magnetic moving coil tach-
ometer attached to the capstan motor shaft. Current feed-
back is provided by a pair of sensing resistors in the capstan
motor return circuit. Figure 2-4 is a block diagram of the
capstan servo circuit.
HSFWD------~----------------~_:~::~
HSFWD
LPSEARCH-----------------+~----~
Theory of Operation
2-38.
Motion commands from the control and status
printed-circuit assembly control switching circuits and a bi-
polar ramp generator. The output of the ramp generator
and high-speed control signals control the capstan closed-
loop servo amplifier. The capstan servo amplifier consists of
an integrated circuit servo preamplifier (U4) and a capstan
power amplifier (Ql through Q6).
2-39.
The forward and reverse switching circuits contain
temperature compensated zener diodes that provide a basic
reference voltage for forward and reverse speeds. When a
FWD command from the control and status circuits of FWD
service switch S2 is placed in the on position (up), Q7 is re-
verse biased. Bias current for CR6 is then provided through
R33 to establish a -6.2 volt reference for the ramp generator
(U2 and U3). A REV command or placing the REV service
switch in the on position (up) will provide bias current for
CR2 through R27. CR2 will provide a +6.2 volt reference
for the ramp generator. The forward and reverse reference
voltages at the output of U3 are independently adjustable
to control the forward or reverse speeds.
.12V
L---o-~ OFFSET
12V
TENSION
TENSION
----------------4~
' -. . . . - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - _ TO REE L SE RVO
cCA
7970511A
Figure 2-4. Capstan Servo, Block Diagram
Part 2
2-7
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