Figure 1-35.—Cutaway view of a double receiver

1-44 receiver corresponds to the minute hand. This double receiver has the advantage of requiring less space than two single receivers. However, it also has a disadvantage — when one receiver goes bad, both must be replaced. Figure 1-35.—Cutaway view of a double receiver. In the dual-speed synchro system, data is transmitted by the coarse transmitter, while the system is far out of correspondence and then is shifted to the fine transmitter as the system approaches correspondence. This shifting from coarse to fine control is done automatically when the fine error signal exceeds the coarse error signal. The fine synchro transmitter then drives the system to the point of correspondence. When the dual-speed synchro system includes control transformers, there is always the possibility of a 180º error being present in the system. The reason is the rotor of the CT is not energized and its error- voltage output is zero both at its proper position and also at a point 180º away from that position. To prevent the CT from locking 180º out of phase with the rest of the system, a low voltage is applied across the rotor terminals of the coarse CT as shown in figure 1-36.