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 TM 55-1905-219-14-11
5-117. SHIP'S COURSE INDICATOR - MAINTENANCE INSTRUCTIONS (Continued).
aligned or null position by an amount other than 180 electrical degrees, a voltage is induced in the (control
transformer) rotor such that the phase of the voltage indicates the direction of rotation from the aligned
position. The magnitude of the voltage is, for small angular rotations from the aligned position, proportional
to the extent of angular misalignment.
2
In the two-speed system the 36X control transformer passes through 72 nulls during a revolution of the 1X
dial. The servo circuit is controlled by the 1X speed synchro signal until the control transformer signal is
within 2.5 electrical degrees of position alignment. At this condition, the servo circuit must switch to
operate from the 36X synchro to provide fine position alignment.
(c) Anti-stickoff bias voltage.
1
The 36X control transformer determines the accuracy of the indicator, but because this synchro has 72 null
positions for one revolution of the 1X indicator dial, the 1X synchro is used to determine the proper null.
The two synchro rotors are connected in shunt through a mixing network consisting of pairs of diodes and
two resistors. The mixing network performs three functions. First, it effectively opens the 1X synchro rotor
signal circuit whenever the indicator dial is within 2.5 degrees of null and effectively closes the 36X
synchro rotor signal circuits. Second, it limits or attenuates the 36X synchro signal whenever the 36X
synchro is more than 2.5 degrees from its null. Third, it keeps synchro loading to its minimum allowable
level.
2
The use of the mixing network and anti-stickoff voltage eliminates every false null as an input signal to the
servo amplifier including the one at the 180-degree point out of correspondence of the 1X synchro control
transformer. This null is eliminated by the effect of adding a 2.5-volt 400-cps anti-stickoff voltage in series
with the 1X synchro rotor voltage and shifting the phase of the 1X synchro voltage by 2.5 degrees to bring
the indicator null back to a true reading. The above procedure converts the 180-degree out of
correspondence null point to an unstable (or decentering) null. If the coarse (1X) and fine (36X) control
transformers were adjusted to the same electrical zero as the electrical zero position of the compass
transmitters, there would be a position of the coarse control transformer shaft 180-degrees out of
correspondence with the compass transmitter at which the rotor voltages of
5-1306
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