TM 11-5840-281-35/1
time, it will appear as an additional voltage drop across
reference voltage across the demodulator .
resistor R7518. Diode CR7504 becomes back-biased
NOTE
and capacitor C7505 charges negatively through diode
CR7503 towards the value of back bias on diode
The tachometer generator is excited by
CR7504. If the instantaneous error voltage at the
a phase displaced 120 degrees from
junction of diodes CR7501 and CR7502 were positive at
that which precedes the references for
this time (180 degrees out-of-phase with the applied
the demodulator. The generator output
reference voltage), diode CR7503 would be back-biased
is shifted 90 degrees from its excitation,
and capacitor C7505 would charge positive through
and R7528 and C7507 produce another
diode CR7504. Thus, the output of the ring demodulator
30-degree shift.
is a dc voltage approximately equal to the peak value of
the ac error signal whose polarity is determined by the
(6) Transistor Q7505 couples the output of
phase relationship of the ac error signal with respect to
the demodulator to a second ripple filter. Transistor
the 400-Hz reference voltage. When the reference
Q7507 couples the filtered tachometer signal to the
voltage becomes positive at the junction of diodes
differential amplifier at transistor Q7509 where it is
CR7501 and CR7503 with respect to the junction of
combined with the error signal. The amplitude difference
diodes CR7502 and CR7504, all diodes in the ring are
between the error signal and the tachometer signal is
cut off and no voltage change occurs across capacitor
amplified to produce the resultant error current to control
C7505.
the electronic control amplifier.
(4) The voltage appearing across capacitor
(7) COARSE BAL control R7546 is used to
C7505 is coupled through emitter-follower transistor
adjust the quiescent emitter voltage of transistor Q7507
Q7504 to provide a high impedance load for the
to match that of transistor Q7506 and to balance the
demodulator. The quiescent base current for emitter-
output of differential amplifier transistors Q7508 and
follower transistor Q7504 is supplied through resistors
Q7509. FINE BAL control R7558 provides a vernier
R7520 from voltage divider resistors R7521 and R7522.
balance adjustment to differential amplifier transistors
Diode CR7505 provides temperature compensation.
Q7508 and Q7509.
The 400-Hz component of the dc error voltage is filtered
e. Drive Motor Amplifier Circuit (fig. 7-8, 7-44
by a 400-Hz null filter consisting of resistors R7535 and
and 7-35). The drive motor amplifier functions to
R7536 and capacitors C7509 and C7510. The dc error
produce control voltage to the azimuth drive motor from
voltage is also partially integrated by resistors R7532 and
azimuth drive error current received from the electronic
R7533 and capacitor C7508, to reduce the high
control preamplifier. The drive motor amplifier contains
frequency response of the servo loop to prevent hunting.
the electronic control amplifier and primary power control
In certain cases, such as when the system is energized
with Antenna AS-1292/ TPN-8 some distance from its
(1) Pins 6 and 7 of transformer T8801 in the
tracking position, capacitor C7508 would take a large
drive motor amplifier circuit provide 400-Hz reference
charge and cause the servo loop to overcorrect. Limiter
voltage from the primary of transformer T8801 (pins 1
diodes CR7510 and CR7511, with their associated
and 2) to the load winding (pins 3 and 4) of magnetic
voltage dividers, prevent this occurrence by limiting the
amplifier AR8801. During the negative half-cycles of the
maximum charge on capacitor C7508 to +2 volts.
reference voltage, the relatively high resistance of
(5) The dc error voltage is coupled through
resistor R8811, in series with the load winding, limits the
emitter-follower transistor Q7506 to differential amplifier
current to well below the saturation point of the magnetic
transistors Q7508 and Q7509, where it is compared with
amplifier core. During the positive half-cycle, blocking
the tachometer feedback voltage Since the integrator in
diode CR8812 connects the low resistance of resistor
the error signal circuit would reduce the effectiveness of
R8812 in parallel with resistor R8811 and increases the
the tachometer feedback signal, a second identical ring
magnetic amplifier load winding current. With no bias
demodulator consisting of demodulator diodes CR7506,
present at some point during the positive half-cycle, the
CR7507, CR7508, and CR7509 is used. Capacitor
negative field created by the winding current is sufficient
C7507 and. resistor R7528 compensate for the inductive
to saturate the magnetic amplifier core. At the point the
phase shift in the tachometer generator to produce an in-
core becomes saturated, the reactance of the
phase or 180-degree out-of-phase relationship with the
2-26
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