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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 - Unpublished Work - See title page for details.  34-21-0 Page 7 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 - Unpublished Work - See title page for details.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.
557 
May 01/76  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  34-21-0 Page 9 

K79843 K80009
Compass System Schematic  5C8 
34-21-0  Figure 5 (Sheet 1)  Jun 20/83 
Page 10 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

5C8  Compass System Schematic 
Jun 20/83  Figure 5 (Sheet 2)  34-21-0 
Page 11 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

COMPASS SYSTEM -TROUBLE SHOOTING
EFFECTIVITY
WA N4502W thru N4530W GJ ALL EXCEPT B-25l09 and B-25110
1.  General
A.  The location of troubles in the compass system can best be determined by a systematic functional test. Use of unit electrical schematics, block diagrams, and electronic wiring diagrams of the compass system will aid in trouble shooting analysis. Trouble shooting the compass system is best accomplished by substituting components where possible.
B.  The flux valve (remote compass transmitter) is extremely sensitive to magnetic fields and will malfunction if it is near magnetic materials which distort the earth's magnetic field.
CAUTION:  DO NOT CHECK THE FLUX VALVE WITH AN OHMMETER. DIRECT CURRENT IN THE WINDINGS MAY MAGNETIZE THE CORES.
C.  Before proceeding with the trouble shooting procedures, recheck that the compass transfer switch is in normal position and all compass system circuit breakers are closed. Check that cable connectors are properly mated and securely connected. Check that 400 cycle ac synchro excitation is present to all units. When substituting known good units in place of suspected malfunctioning units, leave replacement units in the circuit until trouble is cleared and then replace original good units into the operating system.
D.  The following trouble shooting procedures are for compass system No. 1. If trouble is encountered in compass system No. 2, use the same procedure substituting, in the procedure, with an equivalent compass system No. 2 component.
　
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