VHF NAV CONTROL PANEL - REMOVAL/INSTALLATION
1. General
A.  The VOR/ILS control panels are located on aft electronics panel P8. Each panel is held in place by four fasteners.
2.  Remove Control Panel
A.  Open circuit breakers on electronic load control center circuit breaker panels P6 and P18 as follows:
Control Panel  Circuit Breaker/Panel
 P6 Panel  P18 Panel
NO. 1 ELECTRONICS PANEL LIGHTS  G/S-1 VOR/LOC-1
NO. 2  ELECTRONICS PANEL LIGHTS
G/S-2
VOR/LOC-2

B.  Loosen fasteners securing panel to mounting.
C.  Withdraw panel from mounting and disconnect electrical connector(s).
3.  Install Control Panel
A.  Connect electrical connector(s) to control panel.
B.  Secure control panel to mounting.
C.  Provide electrical power.
D.  Close circuit breaker opened in par. 2.A. above and check that panel lights are on.
NOTE: The flight deck panel lights control must be on bright.
E.  Remove electrical power if no longer required.
501 
Oct 20/84  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  34-31-51 Page 401 

MARKER BEACON SYSTEM - DESCRIPTION AND OPERATION
1. General
A.  The marker beacon system provides the pilots with both visual and aural indication of airplane passage over airways fen markers, station location Z markers and instrument landing system (ILS) middle and outer markers.
B.  The marker beacon system consists of a receiver, an antenna, two sets of three indicator lights and a sensitivity switch. (See figure 1.)
C.  The ground marker beacon transmitters all operate on a fixed frequency of 75 MHz and are modulated by one of four audio signals, depending on the type of marker beacon. The signals, received by the airborne receiver as the airplane passes over the transmitter, are made to light indicator lights. The audio signal is also made available to the crew by actuation of the MKR control on the audio selector panels. Following are the four types of marker beacons with their tone frequency and indicator light color.
(1)  
ILS Outer Marker - An intermittent 400-Hz tone with the blue light flashing.

(2)  
ILS Middle Marker - An intermittent 1300-Hz tone with the amber light flashing.

(3)  
Airways Fan Marker - An intermittent 3000-Hz tone with the clear light flashing.

(4)  
Station Location Z Marker - A steady 3000-Hz tone with the clear light steadily lighted.

D.  When an ILS middle marker signal is received, this information is fed to the flight director system for the low approach mode. (Refer to 34-26-0, Flight Director System.)
2.  Receiver
A.  The receiver is a single conversion crystal-controlled superheterodyne operating at a fixed frequency of 75 MHz Sensitivity of the receiver is variable and controlled by a switch located an the captain's instrument panel. Audio filters tuned to 400, 1300, and 3000 Hz are used for separation of the three types of tone modulation used by the beacons. The receiver, which contains an integral power supply, is installed on the electronic equipment rack (E-2).
3.  Antenna
A.  The antenna is a blade-type unit located on the bottom centerline of the airplane at body station 620. The antenna is connected to the receiver by a coaxial cable.
543 
Feb 15/73  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  34-35-0 Page 1 

L36404
Marker Beacon System Component Location  5C8 
34-35-0  Figure 1  Jun 20/88 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

4.  Indicator Lights
A.  There are two sets of three indicator lights, one set is on the captain's instrument panel, and the other is on the first officer's instrument panel. The lights are covered by white, blue and amber lens and are the push-to-test type.
5.  Sensitivity Control
A.  On GJ ALL EXCEPT B-2504 and B-2505, the sensitivity switch is located on the captain's instrument panel adjacent to the marker beacon indicator lights. On GJ B-2504 and B-2505, the sensitivity switch is located on the selcal control panel, installed in the forward electronic control panel. The LOW and HI positions of the control indicate the degree of receiver sensitivity and are normally used when the airplane is flying at low or high altitudes respectively.
6.  Operation (Fig. 2)
A.  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 selected by a 4.6-MHz IF filter and applied through three stages of IF amplification.
　
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