24. EFIS Anomalies and Failures___________________________
 A.  Symbol Generator Anomalies and Failures
 (1)  
If any of the following conditions are present, the EADIs and EHSIs will display SG FAIL message:

 (a)  
Symbol generator not operating

 (b)  
Fault in the symbol generator memory

 (c)  
Instruction execution fault

 (d)  
Any other symbol generator failure that does not result in obvious display indications

 (2)  
During normal operation, the airplane symbol on the EADIs are displayed in normal (horizontal) position. If a failure occurs in the internal display controller circuits of the symbol generator, the airplane symbol will be displayed in an inclined position in the lower right hand side of the EADIs.

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A 737-300/400/500MAINTENANCE MANUAL

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737-300/400/500MAINTENANCE MANUAL
 (3)  
If the power to the EFIS symbol generator is lost, or the power supply within the symbol generator fails, the EADIs and EHSIs will be blanked.

Control Panel Failures

 (1)  
If the mode switch on the EFIS control panel fails, or the mode data becomes invalid, the EHSI will automatically revert to one of the following displays:

 (a)  
If a VOR frequency is selected on the VHF NAV control panel, the EHSI will display the VOR mode.

 (b)  
If an ILS frequency is selected on the VHF NAV control panel, the EHSI will display the ILS mode.

 (2)  
If the range switch on the EFIS control panel fails, or the EFIS range data becomes invalid, the EFIS will display the range being received from the weather radar system.

Beam Test Failure

 (1)  
The beam test discrete causes all three color guns to come on simultaneously to produce a white beam. During a beam test, the three cathode currents and the display alignment of the CRTs are monitored. If a malfunction is detected, a beam test failure discrete is transmitted to the symbol generator, which causes the display to be monochromatic (single color).

Display Unit Anomalies and Failures

 (1)  
If a malfunction is detected in the CRT filament circuits or the CRT power supplies, the display unit will be blanked, and a display unit anomalies discrete will be transmitted to the symbol generator. If the cathode current monitor circuitry detect an open circuit condition for a specific color gun, a display unit anomalies discrete will also be generated. In this case, the remaining color guns will be switched on, to produce a monochromatic (single color) display. In addition, if the cathode current monitor circuitry detect that a specific color gun is continuously on, a display unit anomalies discrete is generated and the color gun is switched off.

Light Sensor Failures

 (1)  
If a light sensor within a display unit fails, the functioning

 sensors are used for automatic brightness control.EFIS Overtemperature

 (1)  
Symbol Generator Overtemperature

 (a)  If cooling air to the symbol generators is lost, it normally takes more than 90 minutes for the symbol generator power supply temperature to increase to 105°C. At 105°C the power supply will shut down, and the CRT displays will be blanked. When the cooling air is restored, within 4 to 6 minutes the temperature should decrease to 93°C threshold point, at which time the CRT displays will be partially restored. If the temperature continues to decrease below 75°C the complete CRT data, including the raster display, will be restored.
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