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 TM 55-1905-219-14-7
3-216. HEADING SELECTOR - MAINTENANCE INSTRUCTIONS (Continued).
Any voltage that is induced in synchro B1 rotor is applied to the primary of transformer T1. A demodulator ring is
connected in series with a leg of each secondary winding of transformer T1.
The same 115-volt, 400-hertz supply that excites the gyrocompass synchro must also be used for reference
purposes in the Heading Selector. This reference voltage is applied to the primary of transformer T2. The output from
transformer T2 secondaries is applied across the demodulator rings. This voltage serves as a trigger to turn the
demodulator rings on or off. Ring 1 (resistors R1, R2; diodes CR1-CR4) conducts during the negative portions of each
reference voltage cycle, and ring 2 (resistors R3, R4; diodes CR5-CR8) conducts only when the reference (trigger) signal
is positive.
When there is any induced voltage across synchro B1 rotor, it will appear across both secondaries of transformer
T1. This voltage can be either positive or negative with respect to the reference voltage at any given instant. The
reference voltage is alternating continuously, and the output from the Heading Selector must be a direct current.
The reference voltage, in effect, switches the demodulator rings on and off in step with its cycles. This forces the
proper transformer T2 secondary to be connected to the output at the right time. The net result is a pulsating d-c voltage
that is proportional to the difference between the actual heading and the ordered heading. Resistor R5 and capacitor C1
provide filtering for the output.
3-3316
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