TM
11-5835-243-34/EE641-AA-MMl-010/E154
MTT/TO
31S3-4-110-1
motor will increase or decrease as necessary until
b. Tape Motion Command Input Switch. Tape
motion commands FORWARD, FAST FWD,
VELOCITY FEEDBACK is equal to the servo
REVERSE, and FAST RVS are accepted on the
reference voltage. The servo error is amplified by
servo control circuit card by quad bilateral switch
operational amplifier AR2-B for use by output
drivers AR4-B and AR5-B. Voltage swings at the
U2. A high level from the control logic circuit card
output of amplifier AR2-B are limited in both
directions by diodes CR5 through CR8. The output
the corresponding switch section. The enabled
drivers AR4-B, and AR5-B form a push-pull circuit
switch section acts as a transmission gate,
which drives the servo power amplifier assembly
presenting a low impedance which connects the
appropriate reference voltage to servo control
(A9, fig. 1-2, para 2-10). When DRIVE A is
amplifier AR2-A.
positive, tape motion is in the reverse direction.
c. Voltage Reference. Reference voltages for use
Tape motion is in the forward direction when
DRIVE B is positive.
by servo control amplifier AR2-A are developed by
h. Current Sense Amplifier. Capstan motor drive
zener diode VR1 and divider R14, R15. A logic high
current is sampled on the power amplifier assembly
level on either FAST FWD or FST RVS connects
(A9) and fed back on the SENSE A and SENSE B
the full zener voltage (V) through switch U2. A logic
lines to current sense amplifier AR5-A. This am-
high level on either FORWARD or REVERSE
plifier, and the power amplifier assembly, behave as
connects half the zener voltage (V/2) to the servo
control amplifier.
a transconductance amplifier having a gain of 2.2
d. Servo Control Amplifier. The output of switch
amps/volt at the output of error amplifier AR2-B.
As a result of this current feedback, the current
U2 is applied to operational amplifier AR2-A such
which the capstan motor can draw from drivers
that forward motion commands appear at the
AR4-B and AR5-B is limited to prevent
output of the amplifier as a negative servo reference
demagnetization of the motor.
voltage, and reverse motion commands appear as a
i. Schmitt Triggers. Three photo transistors
positive servo reference voltage.
(sensors), hole sense A8A1, low tape sense A8A2,
AR3-A, AR3-B and associated circuit components
and strip sense A8A4, are mounted on the tape deck
form a ramp generator with unity gain. When
assembly. The sensors are activated by light
switch U2 commands REVERSE (a positive
emitting diodes (LEDs), also mounted on the tape
deck assembly. When activated, the sensor output
reference voltage), for example, the output of the
signal level is over 3 vdc. When not activated, the
ramp generator will take 18 milliseconds to reach
sensor output is nearly zero. The sensor signals are
the level of the servo reference voltage. The capstan
HOLE SENSE (holes punched at the beginning and
motor voltage is, thus, a ramp function which
end of the tape are sensed), STRIP SENSE
provides controlled acceleration rather than an
(reflective tape at the beginning or end of the tape is
abrupt on-off switching.
f. Feedback Amplifier and Filter. The velocity of
sensed), and LOW TAPE SENSOR (the amount of
the capstan motor is measured by the voltage
tape on the supply reel has reached a low level).
These signals are input to Schmitt trigger circuits
developed by a dc tachometer mounted on the
AR1-A, AR1-B, and AR1-C, the triggering
capstan motor shaft. The tachometer voltage is fed
threshold of which is set at 2.3 vdc (0.5 vdc
into the servo control circuit card where it is filtered
hysteresis). When triggered, the output is a low
to suppress the ripple voltage caused by the com-
level pulse which is routed to the servo control logic.
mutator segments of the tachometer. Filtering and
buffering is accomplished by feedback amplifier
j. Servo Control Logic. The servo control logic
AR4-A and associated circuit components. The
consists of combinational gating U1 and U3 which
resultant signal (VELOCITY FEEDBACK)
accepts the output of the Schmitt triggers and
generates tape position information. The tape
represents the motor velocity.
g. Error Amplifier. The current from ramp
position signals are used on the control logic circuit
card (para 2-8). When the HOLE SENSE is not
generator AR3 and velocity feedback amplifier
present, the control logic enables BEG HOLE. With
AR4-A is summed at the junction of resistors R47
both the HOLE SENSE and LOW TAPE SENSOR
and R49 to generate the servo error. This error
signals present, BEG HOLE is inhibited and END
signal is the instantaneous difference between the
desired capstan motor velocity (servo reference
HOLE is enabled. With a STRIP SENSE signal
voltage) and the actual capstan motor velocity
causing the AR1 -C to conduct and no LOW TAPE
(VELOCITY FEEDBACK). The polarity of the
SENSOR signal, the control logic inhibits EOT and
enables the BOT signal. This also outputs BOT IND
servo error is such that the velocity of the capstan
2-13
