Figure 2-11. Detector and balancing circuit, simplified schematic diagram.
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TM 11-6660-204-25
Figure 2-11. Detector and balancing circuit, simplified schematic diagram.
phase shifting, creates a phase difference of ap-
(more positive) than the potential on the wiper
proximately 90° between the output of the servo-
arm of the slide-wire. The vibrating reed of G301
power amplifier and the fixed phase of the line
alternately switches this higher potential between
power. This phase difference is either a lag or a
the two ends of the transformer primary winding.
lead, depending on the polarity of unbalance at
Current flow will ocur during each contact and
the detector input. The direction of motor rotation
the resulting alternating magnetic field in the
depends on the lead or lag relationship between
transformer core will induce an alternating sec-
the two field windings; the motor leads must be
ondary voltage and frequency which is still equal
connected properly so that the motor will rotate in
to the line frequency (nominally 60 CPS).
the right direction to cancel an error voltage.
b. The vibrating reed is polarized in its motion
d. The damping filter in series with the dc sig-
and always hits contact No. 2 for a given polarity
nal applied to the vibrator consists of a three
of voltage across its coil, and contact No, 4 for the
section RC filter network, R333, R334, R335, and
other polarity. The polarity of the hot side of the
C315. Resistor R336 is used to damp the primary
secondary windings of transformer T302 depends
winding of transformer T302 and reduce voltage
on two things: which of the two contacts (No, 2
transients. The damping filter performs two func-
or No. 4) the reed has just hit, and whether the
tions: First, it acts as a filter for the higher pulse
signal voltage on the reed is positive or negative
recurrence frequencies so only the dc portion of
with respect to the wiper arm. Therefore, the
the voltage developed in the output gate circuit is
phase of the secondary voltage depends solely on
applied to the detector circuit. Secondly, it acts as
the polarity of unbalance of the two dc voltages.
a damping circuit to prevent overshooting of the
This phase sensitiveness to the direction of unbal-
recorder balance point (hunting. ) This is an im-
ance turns the motor in whatever direction is nec-
portant feature included in the design of the
tssary to establish a dc balance.
equipment and when this equipment is repaired,
or parts are replaced, the proper values of circuit
c. The balancing motor is a low-power two-
components must be used to avoid lose of the
phase squirrel-cage induction type with two field
damping action. Damping action is explained as
windings. One field winding is powered by the ac
follows: Assume that an unbalance exists between
power line as a fixed phase which cannot be
the dc voltage applied to the detector circuit from
changed except by a change in wiring; the other
the output gate and the dc voltage developed by
is the control field winding. Phase-shifting capaci-
the slide-wire, and that the output gate voltage is
tor C317, in conjunction with some amplifier

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