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737-300/400/500MAINTENANCE MANUAL
 INSTRUMENT COMPARATOR SYSTEM - DESCRIPTION AND OPERATION
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 1.  General_______
 A.  The instrument comparator system provides annunciation of system discrepency through actuation of comparator lights, after a comparison has been made between output signals as fed from each of the following No. 1 and 2 systems: Inertial Reference, Radio Altimeter, and VOR/ILS Navigation. The signals compared are heading (compass), roll and pitch (attitude), altitude (radio altimeter), localizer deviation and glide slope deviation (VOR/ILS navigation). The system also monitors its own power supply.
 B.  The system consists of the comparator monitor unit, two annunciator panels, and a test switch. The location of these components is shown on Fig. 1. The system schematic is shown on Fig. 4.
 2.  Comparator Monitor Unit_______________________
 A.  The comparator monitor unit is a 3/8-ATR short case rack mounted unit installed on shelf E3-4 in the main equipment center. Connection with airplane wiring is made at the rear of the component. Two 115 volt ac power inputs are required. One input provides power for the instrument comparators, and the other is used for the power monitor.
 B.  The comparator unit provides a signal output for visual display if a significant difference exists between the captain's and first officer's navigation instruments.
 C.  The comparator unit is a solid state device consisting of two power supplies and six comparator monitors. The monitor power (MON PWR) light illuminates if the comparator power circuit detects loss of power. The comparator monitors compare heading, bank attitude, pitch attitude, LOC deviation, GS deviation and radio altitude.
 D.  Front Panel Features
 (1)  
Seven test lights are located on the front panel. The MON PWR light is red and all others are amber when illuminated.

 (2)  
A PUSH TO TEST switch provides a means of testing the comparator unit. All six amber lights should come on when you push the test switch. The amber lights are also turned on by a comparator warning output.

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737-300/400/500MAINTENANCE MANUAL
 (3)  AIRPLANES WITH A COLLINS -002 COMPARATOR MONITOR UNIT; The red MON PWR light will also come on when you push the test switch.
Annunciator Panels
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 A.  Instrument comparator annunciator panels are located on each pilot's panel. The annunciator panels contain the comparator lights, which illuminate to give an indication when the output of the two systems disagree.
 B.  The MON PWR light illuminates when the instrument comparator power supply fails.
 C.  The HDG light illuminates when the captain's and first officer's HSI compass cards are displaying different headings.
 D.  The PITCH or ROLL lights illuminate when the captain's and first officer's ADI attitude displays are different. The HDG, PITCH and ROLL annunciators may all come on simultaneously if excitation voltage from the comparator unit is lost. In such a case, the Heading, Roll and Pitch comparison is unreliable.
 E.  The GS and LOC lights illuminate when the glide slope and localizer deviation signals from the captain's and first officer's VHF NAV receivers are different.
 F.  The ALT light illuminates when the altitude signals from the No. 1 and 2 Low Range Radio Altimeter receiver/transmitters are different.
 G.  If a light illuminates, it can be dimmed by pressing the clear plastic
 cover over the lights (PUSH-TO-DIM).Test Switch
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 A.  The test switch, on the captain's panel, permits testing of all monitor channels. When you push the button, all six amber annunciator lights will come on, indicating that the instrument comparator unit is functioning.
 B.  AIRPLANES WITH A COLLINS -002 COMPARATOR MONITOR UNIT;
 The red MON PWR light will also come on.Flight Instrument Accessory Unit
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 A.  A flight instrument accessory unit is located in electronic equipment rack E1-2. The unit contains relays and 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.
　
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