D.  Random drift in the gyro is overcome by means of the flux valve and servo slaving circuits. Assume there are no error signals in the system and that the airplane is headed towards magnetic north once more. The gyro now drifts slightly off course. This results in a signal being generated in the gyro synchro which is passed to the servo-amplifier via the heading synchro in the RMI. As in the case of the heading change, the servo-amplifier drives the servomotor to null out the error in the heading synchro and automatically turns the rotor of the slaving synchro in the RMI along with it. However, in this case the stator of the slaving synchro is still referenced to north by the flux valve; hence, an error signal is now generated in the slaving synchro which is amplified by the slaving amplifier and fed to the torquemotor to precess the gyro back to a northerly heading. The error in the gyro synchro is thus nulled out and the system returns to its original zero error condition with the gyro again "pointing" north.
E.  In practice, when the compass system is first activated, some disagreement will exist between the airplane heading and that of the directional gyro. The system will immediately start to precess the gyro to take out this error, but the precession rate for the slaving circuits is a relatively slow one, and the time lag involved could not always be tolerated. Therefore, the heading synchronization knob on the RMI (ADF) is used to introduce an artificial error signal into the heading servo circuit, which rapidly produces the desired synchronization. An annunciator indication of a dot or cross, will show that the slaving circuits are operating, and will also serve to indicate which way the HDG/SYNC knob should be turned to synchronize the system. When the knob is turned, it turns the stator of the heading synchro, thus generating an error voltage in the rotor which is amplified by the servo-amplifier and drives the servomotor and compass cards to the correct airplane heading (and nulls out the error in the heading synchro as before). Since the servo-amplifier circuit is able to respond far more rapidly to an error signal than the slaving circuit, the "manual" method of synchronizing the system is preferred.
F.  The operation of the compass transfer switch and relays is illustrated in Fig. 4 and 5. Figure 4 shows how the relay coils are connected with the dc supply through the relay contacts. The switch is shown in NORMAL position. Assume that the transfer switch is now set in BOTH ON NO. 2 position. This switch puts a ground on the lower end of the BOTH ON NO. 2 latching relay coil via BOTH ON NO. 2 and BOTH ON NO. 1 relay contacts. Since the other end of the coil is connected to 28 volts dc, it will now latch over to the alternate set of contacts and remain until energized once more. Note that the ground is removed from the coil by the transfer of BOTH ON NO. 2 relay contacts. In order to ground the coil once more and switch the relay back again, the switch must be returned to NORMAL position. The switching to BOTH ON NO. 1 position is carried out in a similar manner. 
557 
May 01/76  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  34-21-0 Page 7 

G.  The switching contacts shown in Fig. 5 are a continuation of those in Fig. 4., but they take care of the transfer of signals within the system. In the normal configuration, the captain's RMI receives its signals direct from directional gyro No. 1 and the first officer's RMI receives its signals from directional gyro No. 2. AC power for each system servo-amplifier is taken from COMPASS-1 and COMPASS-2 circuit breakers respectively. When the transfer relay is switched over to BOTH ON NO. 2, the first officer's system receives its signals and power supplies as before but the captain's system now receives its signals from a synchro in the first officer's RMI and the power supply for No. 1 servo-amplifier is taken from the captain's RMI (ALT) circuit breaker. A similar transfer takes place on switching to BOTH ON NO. 1. The autopilot normally gets its heading reference from No. 1 directional gyro, but in the BOTH ON NO. 2 position this signal is taken from directional gyro No. 2.

K42066
Compass Switching Circuit  557 
34-21-0  Figure 4  May 01/76 
Page 8 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

H.  The compass system contains flag warning circuits which are used to detect and display malfunctions. Malfunctions are displayed by warning flags on the captain's and first officer's Radio Magnetic Indicators (RMI's) and horizontal situation indicators (HSI's). A heading valid signal is also sent to the captain's and first officer's Flight Director Systems. When all components are operating normally, the warning flags are not in view. Flag logic circuits in the flight instrument accessory box monitor the No. 1 and 2 compass racks, flux valves, instrument transformers, and Directional Gyro (DG) monitor outputs. Power for the circuits is provided by full wave rectifiers in the compass racks.
　
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