3-216. HEADING SELECTOR - MAINTENANCE INSTRUCTIONS (Cont).
Any voltage that is induced in synchro B1 rotor is applied to the primary of transformer T1.
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 volt- age 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. TM 55-1905-220-14-7