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 8.  RF Attenuator
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 A.  Two RF attenuators are installed on shelf 4 in electronic equipment rack E3. The attenuators are connected between the localizer antennas and the receivers to reduce electromagnetic interference. The attenuators are mounted directly to the ILS relay input rf coaxial connector.
 9.  ILS Relays
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 A.  Two coaxial relays are installed on shelf No. 4 of electronic equipment rack E3 in the main equipment center. The relays are energized by mode logic in the DFCS when the associated navigation unit is tuned to an ILS frequency. The logic requires that an ILS frequency be tuned; DFCS approach or VOR/LOC mode must be selected; and either VOR/LOC arm or VOR/LOC engage logic available in the associated DFCS flight computer. The relays, when energized, switch the navigation unit VOR/LOC rf input from the fin tip VOR/LOC antenna to the nose radome localizer antenna. The forward antenna selected ground signals to flight computers A and B of the DFCS are also routed through energized contacts of the relays.
 10. VHF NAV Transfer Switch
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 A.  The VHF NAV transfer switch is part of the VHF NAV and IRS transfer switching module installed on the pilots' overhead panel. The switch controls the associated VHF NAV transfer relays which enable either VOR/ILS system to feed both the captain's and first officer's flight instruments and the GPWS at the same time.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. AIRPLANES WITH EFIS ú

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 11. VHF NAV Transfer Relays
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 A.  Two stepping-type transfer relays are installed in rack E3-4. When the VHF NAV transfer switch is in the NORMAL position, the captain is on system No. 1 and the first officer is on system No. 2. Placing the VHF NAV transfer switch to BOTH ON 2 position momentarily energizes transfer relay No. 1 and the relay steps to the BOTH ON 2 position. Positioning the VHF NAV transfer switch back to the NORMAL position, again momentarily energizes transfer relay No. 1 and the relay steps back to the normal position. The first officer's transfer relay is controlled in the same manner as the captain's by positioning the VHF NAV transfer switch to BOTH ON 1.
 12. Flight Instrument Accessory Unit (AIRPLANES WITH A FLIGHT INSTRUMENT
________________________________
 ACCESSORY UNIT)

 A.  A flight instrument accessory unit is located in electronic equipment rack E1-2. The unit contains relays and a printed circuit for signal conditioning and interface between the navigation systems and the flight instruments. In addition, the unit contains numerous relays, time delay gates, a circuit interrupter and other components used for interconnecting the DFCS system.
 B.  The flight instrument accessory unit circuits associated with the VOR/ILS navigation system consist of the diodes across the coils of the transfer relays; four relays for inhibiting the VOR/ILS self-tests when the autopilot is operating in an approach mode and the ground return for energizing ILS relays No. 1 and No. 2; and two DME agility mode relays for inhibiting the RDDMI VOR bearing displays when the DME and FMC systems are operating in the agility mode.
 13. Integrated Flight System Accessory Unit (AIRPLANES WITH AN INTEGRATED
_______________________________________ FLIGHT SYSTEM ACCESSORY UNIT)
 A.  An integrated flight system accessory unit is located in electronic equipment rack E1-3. The unit contains solid state relays, solenoid operated relays, and printed circuits for signal conditioning and interface between the navigation systems and the flight instruments. In addition, the unit contains numerous relays, time delay gates, a circuit interrupter and other components used for interconnecting the DFCS system.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. AIRPLANES WITH EFIS ú

 34-31-02

 ú ú 06 Page 10 ú Mar 15/92
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 B.  The integrated flight system accessory unit circuits associated with the VOR/ILS navigation system consist of the diodes across the coils of the transfer relays; six solid state relays, four relays for inhibiting the VOR/ILS self-tests when the autopilot is operating in an approach mode and two to control the ground return for energizing ILS relays No. 1 and No. 2; and two solenoid operated mode relays for inhibiting the RDDMI VOR bearing displays when the DME and FMC systems are operating in the agility mode.
　
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