HF Communication System Schematic  5C8 
23-11-01  Figure 2 (Sheet 1)  Jun 20/85 
Page 12 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

5C8  HF Communication System Schematic 
Jun 20/85  Figure 2 (Sheet 20  23-11-01 
Page 13 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

4)  During segment C, either the shunt capacitor of the shunt coil (as selected in segments A and B) is servo-tuned to produce zero loading error. The series capacitor is fine-tuned to retain a zero phasing error. At this point, the sequence counter advances to the operate position.
(d) 
 Operate

1)  Upon reaching the operate sequence, the tune power and key lines have unlatched and the coupler is ready to transmit and receive. If a new frequency is selected, the sequence counter advances to the home step, and the complete turning sequence is repeated.

(e)  
Fault

1)  The coupler has circuits that detect faults and provide such indications on the HF fault detection panel. COUPLER FAULT circuit - indicates a failure of the coupler to home or tune within 15 seconds. RF FAULT circuit - indicates a lack of RF power due to a defective transceiver, an open coaxial, or insufficient power to allow the coupler to tune. PRESSURE FAULT circuit - indicates low pressure in the coupler case. TUNE IN PROCESS circuit - indicates when the coupler is in a tune mode. OPERATE circuit - indicates when the coupler has tuned.
5C8 23-11-01 Jun 20/85 Page 14
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

HF COMMUNICATION SYSTEM - TROUBLE SHOOTING
EFFECTIVITY
Airplanes with Collins 628T-1 Transceivers
1. General
A.  Trouble spots in the HF communication system can be most easily corrected by replacing a suspected malfunctioning component. If the system is not operative after assumed malfunctioning components have been replaced by known operative components, check airplane wiring.
B.  To decrease the number of unscheduled removals, confirm that the condition stated in the flight report exists prior to replacing system components. If HF system is reported to be inoperative on one or two specific frequencies, a precautionary check of the equipment should be made to confirm the report and that the problem is repetitive. The trouble may be due to the area and position of the airplane in respect to the ground station, or to other conditions existing at the time. Also ensure the flight interphone system is operating properly so that poor communication attributed to the HF system is not actually due to a faulty isolation amplifier in an audio selector panel.
C.  Before attempting to operate HF system, ensure all HF circuit breakers on load control center P18 and P6 are closed (pushed in) and power is available to them. Allow the system to warm up for 15 minutes before trouble shooting.
5C8
Feb 20/88 23-11-01 Page 101
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
2. Trouble Shooting Chart

T52027
557 
23-11-01 Page 102  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Nov 15/78 

HF COMMUNICATIONS SYSTEM -ADJUSTMENT/TEST
EFFECTIVITY
Airplanes with Collins 628T-1 Transceivers
1.  General
A.  The following test should be used to check the operation of an HF system after a system component has been replaced or repaired. Conduct test using only authorized test frequencies and check to ensure frequency is not in use before transmitting.
B.  The airplane should not be in or near a large metallic structure, such as a hangar or tower, which may attenuate RF energy.
2.  HF Communication System Test
A.  Prepare to Test
(1)  
Provide ground power to the airplane and energize buses on load control centers P6 and P18.

(2)  
Ensure all HF circuit breakers and interphone audio selector panel circuit breakers are closed.

(3)  
Turn on HF systems and allow 15 minutes for warmup.

(4)  
Check flight interphone system and ensure audio selector panels are operable (Ref 23-52-0).

B.  Test HF Communication System
(1)  
At captain's station, press HF-1 mike selector switch and push up HF-1 toggle switch on audio selector panel. At first officer's station, press HF-2 mike selector switch and push up HF-2 toggle switch on audio selector panel.

(2)  
Turn each HF control panel mode selector switch to AM and tune system to several WWV frequencies.
　
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