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B.  The audio component (400, 1300, or 3000 Hz) of the 4.6-MHz IF signal is detected in the audio and AGC detector and amplified by the first audio amplifier.
C.  Two audio outputs are provided by the first audio amplifier. The AGC audio output is rectified through the AGC gate and applied to all three IF amplifier stages where it provides automatic gain control of the incoming signal. The second audio output drives the second audio amplifier.
D.  The second audio amplifier output divides and drives the audio power amplifier and the audio light drives. The output signal from the audio power amplifier is fed through an output transformer to the flight interphone system for aural monitoring of the marker beacon system. Audio output from the audio light driver is fed through a three-stage frequency selective filter which separates the audio into its 400, 1300 or 3000 Hz component and drives the respective 400, 1300, or 3000 Hz light switches which turn on the corresponding marker beacon lights.
E.  The self-test circuit supplies a 75-MHz amplitude modulated test signal to the input of the crystal controlled converter stage when the push-to-test (PTT) button on the front of the receiver is pressed. The test signal is generated by a switched three-frequency (400, 1300, and 3000 Hz) multivibrator which sequentially modulates a 75-MHz test signal oscillator.
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Marker Beacon System Block Diagram  538 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

MARKER BEACON SYSTEM -TROUBLE SHOOTING
1. General
A.  The following trouble shooting procedures are based on performance of the system operational test and are presented in a tree-type format to aid in rapid fault isolation.
B.  When a test step does not check out, find the box containing the trouble symptom and perform the stated action. Continue to follow a single line by analyzing the results of each test step until the required corrective action is determined. Perform the specified corrective action, then repeat the step at which the failure was encountered and complete the test to check out the system.
C.  All trouble shooting procedures are based on the assumption that wiring is OK and that electrical power is available. If the corrective action in the procedure does not correct the problem, check wiring using the wiring diagram.
2.  Prepare for Trouble Shooting
A.  Verify that marker beacon and flight interphone circuit breaker on P18 circuit breaker panel is closed.
B.  Set sensitivity control to HIGH (or HI) position.
C.  Activate the marker beacon control on the audio selecting panel.
D.  Apply electrical power.
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MARKER BEACON SYSTEM - ADJUSTNENT/TEST
1.  Marker Beacon System Test
A.  Equipment and Materials
(1)  Test Oscillator - BC-376 or equivalent
B.  Test Marker Beacon System
(1)  
Supply external power to the airplane and energize load control centers P18 and P6.

(2)  
With the marker beacon and interphone circuit breakers on load control centers P18 and P6 closed, allow 10 minutes for receiver to warm up. Energize interphone system.

(3)  
Select MKR position on captain's audio selector panel and connect headphone to interphone system.

(4)  
Push-to-test marker beacon indicator lights.

(5)  
Set marker beacon control to TEST position or press and hold test switch on front panel of marker beacon receiver. After approximately 5 seconds, check that the blue, amber, and clear marker indicator lights come on in sequence and that 400-, 1300-, and 3000-Hz tones are heard over interphone, respectively.

(6)  
Place test oscillator on ground, approximately 15 feet away from marker beacon antenna., Extend antenna on test oscillator to its fullest extent, and place parallel to fore and aft axis of the airplane. Turn test oscillator on and allow to warm up.

(7)  
Set marker sensitivity control switch to HIGH, With carrier frequency set at 75.00 MHz, turn oscillator ON and set modulation to 400 Hz.
　
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