MOUNTING SCREW (SIX PLACES)
D
Marker Beacon System Component Location Figure 1
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A 737-300/400/500MAINTENANCE MANUAL
FAN MARKERS
WHITE LT WHITE LT AMBER LT BLUE LT
NOTE: OUTER MARKER KEYS - CONTINUOUS DASHES UNLESS OTHERWISE SPECIFIED. MIDDLE MARKER KEYS - ALTERNATE DOTS AND DASHES UNLESS OTHERWISE SPECIFIED. INNER MARKER KEYS - CONTINUOUS DOTS UNLESS OTHERWISE SPECIFIED. BACK COURSE MARKER KEYS - CONTINUOUS PAIRED DOTS UNLESS OTHERWISE SPECIFIED. AIRWAYS MARKER MORSE CODED IDENTIFIER.
Marker Beacon System Figure 2
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A 737-300/400/500MAINTENANCE MANUAL
4. Indicator Lights
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A. The marker beacon lights are color coded blue, amber, and white and are labeled OUTER, MIDDLE, and AIRWAYS, respectively. They come on with detection of the marker audio tones. The blue light comes on with the 400 Hz tone, amber with 1300 Hz, and white with 3000 Hz.
B. The two sets of marker beacon lights are located on the captain's panel, P1 and on the first officer's panel, P3. The captain's and first officer's marker beacon lights are connected in parallel and illuminate on each panel simultaneously.
MARKER BEACON PANELS RECEIVER
Marker Beacon System Block Diagram Figure 3
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A 737-300/400/500MAINTENANCE MANUAL
5. Sensitivity Control
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A. The HIGH/LOW sensitivity switch is installed on the captain's instrument panel. The LOW and HIGH positions of the switch indicate the degree of receiver sensitivity and are normally used when the airplane is flying at low and high altitudes, respectively.
B. The marker beacon receiver is fixed-tuned to receive 75-MHz amplitude modulated signals. The 75-MHz incoming signal from the antenna is applied through a selective LC filter to a crystal controlled converter where it is mixed with a 70.4-MHz local oscillator signal to generate a 4.6-MHz intermediate frequency (IF) signal. This difference or IF frequency is passed through a 4.6-MHz IF filter and three stages of IF amplification.
C. 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.
D. 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.
E. The second audio amplifier output divides and drives the audio power amplifier and the audio light driver. 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 solid state light switches.
F. 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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