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GNC 250/300 MAINTENANCE AND REPAIR MANUAL

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1. 11 TROUBLESHOOTING CHART eter eite puce a RR 14 2 5 TROUBLESHOOTING COMM SECTION ve delete qe aane e 16 SECTION 3 DISASSEMBLY INSTRUCTIONS X REMOVAL OFTHE COVERS 18 aus BOARD AND MODULE acea 18 3 2 1 Control Display Unit 18 222 Al t de Decoder Board Remi 18 3 2 3 Interface Board toD 19 3 2 4 CPU Board Subassembly Removal 19 GPS Receiver Assembly Removal 20 3 2 6 Power Volume Switch Removal 20 3 2 7 Rotary Switch Assembly Removal hada 21 3 28 Display Deane Ce HON 21 229 21 ERTS 4 5 ost ea du cotes 21 SECTION 4 SERVICE AND TESTING PUB REPLACEMENT ab 23 42 PINAL TEST osea fon u anu 24 qb Test Equipment Required aa sa
2. Ms on 24 25 Lest Proceduren sued tu do u cub dedu Saa uqta 25 oii Transceiver cena ae ot 41 SECTION 5 ILLUSTRATED PARTS LIST Ad LISTOR REPLACEABLE 45 3 2 ASSEMBLY DRAWINGS uu Sar quat ae eet peau qhaq 46 LIST OF FIGURES FIGURE 1 GNC 250 300 BLOCK DIAGRAM ot itia 3 FIGURE 2 CONTROL DISPLAY UNIT BLOCK ier 4 FIGURE 3 GPU BOARD DIAGRAM uates 5 FIGURE 4 RFF BEOGCGK DIAGRAM FIGURE 5 COMMUNICATION 8 FIGURE 6 CPU BOARD TESTPOINTS ult ala 12 FIGURE 7 CPU BOARD TEST POINTS BOTTOM VIEW 13 FIGURE GOM RECEIVER TEST SETUP iot retten 35 FIGURE 9 TRANSMITTER TEST SETUP acea cot ee dtl pio 39 FIGURE 10 J2 REAR CONNECTOR PINOUT 43 FIGURE 11 Jl REAR CONNECTOR PINOUT DEFINITION 44 FIGURE 12 GN
3. Check connections and verify operation in Test Mode see Section 4 2 3 Replace CPU Board see Section 3 2 4 No external annunciator output s message arrival approach Keycaps do not function properly Replace Control Display Unit see Section 3 2 1 Replace CPU Board see Section 3 2 4 Check Flex Circuit connection from rotary switch to back of Control Display Unit Replace Rotary Switch Assembly see Section 3 2 7 Replace Display Board see Section 3 2 8 Replace CPU Board see Section 3 2 4 Dual concentric rotary knobs do not function properly Check connections and verify operation in Test Mode see Section 4 2 3 Replace external switch Replace CPU Board see Section 3 2 4 Approach arm switch not functioning properly 300 only Check connections and verify operation in Test Mode See Section 4 2 3 Replace external switch Replace Comm Chassis Sub Assy see Section 3 2 2 steps 1 7 ace CPU Board see Section 3 2 4 Sequence Hold low switch not functioning properly 300 Only Rep GPS reception weak or GPS Status Page shows low signal levels Replace Antenna and or antenna cable Replace GPS Receiver Board see Section 3 2 5 14 MESSAGE cont RECOMMENDED REPAIR cont 1 No GPS reception Check Date Time Page for correct UTC time Check Position Page for close approximate starting position within 300 nautical miles 7 Replace Ante
4. GNC 230 300 MAINTENANCE AND REPAIR MANUAL ACTV STBY GARMIN GNC 250 BIET m TA nw eman 221 ml R GARMIN CORPORATION PART NUMBER 190 00067 55 1200 E 151ST STREET REVISION E MARCH 26 1998 OLATHE KS 66062 3426 TABLE OF CONTENTS SECTION 1 INTRODUCTION Paragraph Page DESCRIPTION uu sansa 1 12 DETAILED DESCRIPTIO e estie pulsos 2 1 2 1 Control Display Unit 4 12 2 CPI Board 6 1 22 Central Processing Umt CPU as anasu hasa a yu yasa 6 1 2 2 2 dita 6 Eri AA 6 13 4 Interface Board and Data Cadenas 7 12 3 VHF Communications JU TAHSCOIVET e irae Ge dell eet Sq ont is 7 SECTION 2 TROUBLESHOOTING 2 1 TROUBLESHOOTING EQUIPMENT REQUIRE D ZGG deceat 9 2 9 2 3 INITIAL TROUBLESHOOTING OPERA LOIN S ret 11 2521 Power UICC os soa aues qus pedo co 11 2 212 Mermal
5. GPS Receiver Assembly Module which contains the GPS receiver and high precision crystal oscillator Remote Battery Pack Module containing rechargeable cells fuse and charging circuits 28V to 14VDC Converter Contains circuitry for converting 28V power bus to 14V in 28V aircraft 12 DETAILED DESCRIPTION Internally the GNC 250 300 is divided into seven printed circuit boards the CPU GPS Receiver Comm Receiver Comm Transmitter Interface Display and Keypad Circuit boards The block diagram in Figure 1 shows the relationships between major circuits and modules within the GNC 250 300 4 4 ON OFF VOLUME 4 1 DUAL CONCENTRIC CONTROL SWITCH CDU PHOTOCELL KEY PAD TRANSMITTER pc BOARD J13 J2 POSISTOR R206 EXTERNAL GPS RECEIVER BOARD EXTERNAL WALL CHARGER Figure 1 GNC 250 300 Block Diagram 1 2 1 Control Display Unit CDU Control Display Unit is an assembly consisting of eleven keycaps keypad board optical lens vacuum fluorescent display VFD and board with drivers photocell and screws housed in a die cast bezel with a dual concentric rotary switch and two knobs The is 4 row by 20 column unit with variable intensity This display provides excellent day and night visibility from wide viewing angles The front panel provides function selection and alphanumeric input to the CPU board Assembly
6. 1 Remove the four 4 40 x 250 screws 211 63204 08 holding the cover 115 00154 00 to the base 115 00155 00 Remove the cover To replace the battery pack disconnect the three pin connector J102 from the charger circuit board 012 00126 00 Remove the battery pack 361 00008 00 from the unit To remove the charger circuit board assembly 012 00126 00 remove the four 4 40 x 250 screws 211 63204 00 holding the board to the base Remove the two 4 40 hex standoffs 214 00004 00 holding the J101 connector to the chassis Carefully lift the charger board from the chassis 3 3 REASSEMBLING THE UNIT NOTE During reassembly all threaded fasteners shall be coated with thread locking compound 291 00003 00 1 Attach the power dimmer switch to the chassis by reversing the steps followed in Section 3 2 6 The switch is correctly installed when the contacts and flexible circuit cable are pointed away from the chassis frame Ensure that the power dimmer switch is in the off position Attach the GPS receiver assembly to the main chassis assembly by reversing the steps followed in Section 3 2 5 21 Mount the CPU board Sub Assembly the chassis by reversing the steps followed in Section 3 2 4 Be sure to pull up on the retaining lip of the ZIF connector j804 before inserting the flexible circuit cable Mount the Interface Board to me chassis by reversing the steps followed in Section 3 2 3 Install the Comm Chassis
7. 2 9 sped 8 oo op 0 D n amp o Deedes mag 1101 PINS1 16 4 MHz LAE 0515 COLLECTOR Stoo 17 TO 467 VDC Figure 7 CPU Board Testpoints Bottom View 13 TROUBLESHOOTING CHART MESSAGE RECOMMENDED REPAIR Unit will not power on Check External Power Connection Check Remote External Battery Pack if equipped Check Pwr Volume Switch Connection Replace Pwr Volume Switch see Section 3 2 7 Replace CPU Board see Section 3 2 4 Unit will not power off Replace CPU Board see Section 3 2 4 Replace Pwr Volume Switch see Section 3 2 6 Display has horizontal Replace Display Board see Section 3 2 8 or vertical lines Replace CPU Board see Section 3 2 4 Display has missing or Replace Display Board see Section 3 2 8 dark pixels Display brightness not Replace CPU Board see Section 3 2 4 adjustable manually Replace Control Display Unit see Section 3 2 1 Display adjustab brightness not e via photocell Replace Con Replace CPU trol Display U nit see Section 3 2 1 Board see Section 3 2 4 Keycaps have no backlighting Replace Con Replace CPU trol Display U nit see Section 3 2 1 Board see Section 3 2 4 Keycap backlighting not Replace Con trol Display U nit see Section 3 2 1 uniform Bezel Annunciators do not function Replace Display Control Unit see Section 3 2 1 Replace CPU Board see Section 3 2 4
8. GNC 250 300 OFF b Insert a Jeppesen NavData card c Turn the GNC 250 300 ON normally without entering test mode Verify that the GNC 250 300 passes its self test A failure is indicated by one of the messages listed in Section 2 2 d Verify that the GNC 250 300 correctly reads the NavData card and indicates the type effective date and expiration date e Press the ENT key and verify that the Satellite Status Page is displayed f Press the MSG key and verify that the Message Page is displayed g Pressthe Direct To key and verify that the GO TO Page is displayed h Rotate the inner function knob to select a letter or number on the display Pressthe CLR key and verify that the selected letter number disappears j Pressthe NRST key Verify that a Nearest Waypoint Page is displayed k Pressthe RTE key Verify that a Route Page is displayed Press the WPT key Verify that the Select Waypoint Type is displayed and the annunciator next to the key is on m Press the CRSR key twice and verify that APT flashes n Rotate the outer function knob clockwise and verify that VOR flashes Press the key Verify that a Navigation Page is displayed p Turnthe GNC 250 300 OFF 3l 16 Satellite Tracking Test 17 The Satellite Tracking Test ensures that GNC 250 300 has current almanac information and is able to lock onto a sufficient number of satellites to provide a position This test als
9. 2 00 Figure 16 GNC 250 300 Assembly Drawing View 5 011 00108 51 CONTROL DISPLAY UNIT 011 00107 01 00 1 J JO 011 00183 00 MODULATOR MOD CABLE J2 J13 COMM CHASIS SUB ASSY 011 00108 00 CPU BOARD 012 00066 00 50 PWR DIMMER SW 011 00230 00 J702 INTERFACE BOARD 012 00092 00 DATA CARD Figure 17 GNC 250 300 Connector Diagram 52
10. CONTROL oa DATA Figure 5 Communication Transceiver Block Diagram The VHF transmitter is 3 stage RF power amplifier capable of delivering 5 watts minimum of CW power to the antenna connector The input to the transmitter is supplied by the receiver synthesizer and is amplified from a level of 100 mW to the final output of 5 watts The receiver board AM modulates the transmitter using microphone input via the J2 connector to collector modulate the final two stages of the transmitter through power transistor Q34 The microphone circuits contain a compressor which limits the AM modulation depth over a 20 dB microphone input range In addition to the communications functions the receiver board contains the Gillham Greycode altitude decoder circuitry Encoded pressure and altitude data from a parallel altimeter device is received as a 10 bit data word at the 26 pin J2 connector The data is decoded and output to the CPU via connector J13 SECTION 2 TROUBLESHOOTING 2 1 TROUBLESHOOTING EQUIPMENT REQUIRED 0 30 Volt DC 6 Amp Power Supply Digital Multimeter 20 MHz Oscilloscope Adjust Power Supply to 14 Volts 2 2 SELF TEST FAILURES The GNC 250 300 monitors many of its internal functions and displays a message to alert the user should a failure occur The table below shows the recommended repair for each self test failure message MESSAGE RECOMMENDED REPAIR Altitude input fail Verify connections to altitude serializ
11. Repeat this procedure at 127 075 MHz and 136 875 MHz AGC Test Tune the comm receiver to 127 075 MHz Set the RF generator to 5uV 127 075 MHz Using the audio level 5uV as a 0 dB reference increase the RF signal generator level to 100mV and verify that the audio level does not change more than 3 dB Audio Distortion Test Set the RF generator to 10 mV and increase the modulation depth to 8596 Place the COMP DISABLE switch on the test panel in the DOWN position Turn the volume clockwise to obtain at least 7 07 Vrms comm audio output Using the audio analyzer verify that the comm audio distortion is less than 1096 Repeat the procedure with audio tone frequencies of 350 Hz and 2 5 kHz NOTE 1596 distortion is acceptable for the 350 Hz tone Audio Frequency Response Test Set the RF generator to 100 uV with 30 modulation depth Place the COMP DISABLE switch on the test panel in the UP position Re adjust the power volume control for 7 07 Vrms comm audio output Vary the frequency of the modulating tone from 350 Hz to 2500 Hz and find the frequency where the comm audio output level peaks Make this level the 0 dB reference Adjust the modulation frequency to 350 Hz and verify that the comm audio output level drops no more than 6 dB Adjust the modulation frequency to 2 5 KHz and verify that the comm audio output level drops no more than 6 dB 23 Adjust the modulation frequency to 1 KHz and set comm audio
12. a Rotate the outer function knob counter clockwise until the Altitude Decoder Test is displayed b Turn the ALTITUDE DECODER LINE SELECT switch on the test panel to position D4 Verify that D4 on the Altitude Decoder Test Page indicates 1 All other altitude inputs should still indicate 0 c Continue to step through ALTITUDE DECODER LINE SELECT switch positions A1 2 A4 B1 B2 B4 C1 C2 and C4 Verify that only one input A2 A B2 B C1 C2 or C4 indicates 1 for each switch position All other altitude inputs should still indicate 0 29 11 12 13 d Turn the ALTITUDE DECODER LINE SELECT switch to RMT ENT position Verify that all altitude inputs indicate 0 OBS Selected Course Test GNC 300 only a Rotate the outer function knob until the OBS Selected Course Test Page is displayed b Turn the OBS BEARING switch on the test panel to 150 Verify that the input course is 150 9 and CALIB appears in the lower right corner of the test page Note The OBS Normal Cal switch on e test panel must be in the Normal Position c Press the CRSR key twice d Press the ENT key on GNC 300 to calibrate the unit There will be a brief delay while the unit performs the calibration Verify that the input course is now 150 0 2 e Turn the OBS BEARING switch on the test panel to the 60 240 and 330 positions Verify that in each instanc
13. connector the receiver board and the J8 connector on the CPU board Remove the comm chassis by sliding rearward approximately 1 4 until the DCP hinge 125 00020 00 slips out of the hole in the main chassis Remove the modulator transistor 034 630 00017 00 by first unsoldering the brown wire on the P16 connector to the modulator transistor lead Unsolder the P7 cable from the other modulator transistor lead Remove the remaining M3 x 8mm PHP screw 211 50207 11 that connects the red wire on the P16 cable assemble from the modulator Be careful to retain the insulator 250 00025 00 Remove the molded plastic 03 insulator 145 00134 00 modulator transistor 613 00017 00 and thin 03 insulator 250 00026 00 from the unit Remove the three 211 00024 00 screws holding the Modulator casting 125 00019 00 to the main chassis Remove the M3 x 5mm PHP screw 211 50207 05 that holds the posistor to the modulator casting Remove the modulator casting from the unit Interface Board 012 00092 00 Removal Remove the covers as described in Section 3 1 Remove the Control Display Unit as described in Section 3 2 1 Remove the Comm Chassis Sub Assembly amp Modulator Casting as described above Remove the three M1 5 x 5mm flat head screws 211 56201 05 retaining the data card race 145 00056 00 and Interface Board Slip the data card race off the Interface Board connector Carefully pull the Interface Board
14. minutes Before beginning replacement of the memory battery transfer all waypoints and routes to a user data card so that they may be transferred back to the unit following battery replacement This process is covered in the user s manual 1 Remove the covers as described in Section 3 1 2 Open the Comm Chassis Sub Assembly as described in section 3 2 2 steps 1 3 Do not completely remove the assembly but swing it open on its hinge 3 Carefully turn the unit over and remove the CPU board bottom side cover 115 00116 00 3a Remove all electrical grounds from the GNC so if using a grounded tip soldering iron the battery will not be shorted upon removal 4 Unsolder the three battery clip mounting tabs from the bottom side of the CPU board as marked in figure 6 5 Carefully remove the battery by pulling it out from the top side of the CPU board The battery clip leads are welded to the battery and cannot be removed 6 Install a new battery and solder the clip leads from the bottom side of the CPU board 7 Close and secure the Comm Chassis Sub Assembly by reversing the steps in procedure 3 2 2 steps 1 to 3 8 Turn the GNC 250 300 on 9 Press ENT to accept normal mode operation 10 Press ENT again to accept the Database Page 11 Verify that the Memory battery low message does not appear on the message page by pressing the MSG key 12 Turn the GNC250 300 off and carefully turn the unit over 13 Verify that the memory
15. signal generator frequency to 127 100 Mhz Verify that the DMM reading is LESS than the reference AGC voltage 34 8640 Signal Generator 6 dB Comm Antenna Hi Lo 26 Pin Hi Density Power In GPS 150 155 165 GNC 37 Pin D Sub Test Panel Comm Audio p T Adapter 200 MHz HP 331A Distortion Oscilloscope Analyzer 30V 6A Power Supply Figure 8 Comm Receiver Test Setup j Noise Squelch test Set the RF signal generator to 127 075 Mhz 30 modulated with a 1 KHz tone Turn the RF level down below 1uV Turn the power volume control on the radio fully clockwise SLOWLY increase the RF level until the squelch just breaks open and audio can be heard Verify that the RF signal generator level is between 1 and 4uV 35 SLOWLY decrease the level until the squelch closes audio Verify that the RF level is less than 6 dB below that level which opened the squelch Repeat this test at 118 000 Mhz and 136 875 Mhz k Carrier Squelch Test Set the RF signal generator to 127 075 Mhz 85 modulated with an 8 KHz tone Tune the radio to 127 07 Mhz Turn the RF level down below 6uV Slowly increase the RF level until the squelch just breaks open and audio noise can be heard Verify that the RF signal generator level is between 25uV SLOWLY decrease the RF level until the squelch closes no audio Verify that the RF level is less than 6 dB below that level which opened the
16. squelch Repeat this test at 118 000 Mhz and 136 875 Mhz Squelch Key Test Remove the RF input from the GNC 250 300 Verify that the comm audio output is quiet Press the SQ key on the front of the GNC 250 300 Verify that the comm audio output is not quiet m Intercom test Remove the RF input to the GNC 250 300 Apply a 100 mV RMS audio signal at 1 KHz to the MIC INTERCOM input to the test panel NOTE The MIC INTERCOM input has a 500 ohm input impedance Make sure that the audio signal generator is set up to deliver 100 mV RMS into this load impedance Monitor the comm audio output level with the audio analyzer and verify that its input level is greater than 7 07 Vrms and has less than 10 distortion 18 Communication Transmitter Tests a Power Supply Setup Place the unit in test mode according to section 4 3 3 1 Turn the outer function knob to the unit counter clockwise on clock to display the External power input page Key the transmitter by pressing down on the MIC KEY switch on the test panel NOTE Pressing down on the MIC KEY engages a momentary switch which keys the transmitter until it is released Pulling UP on the MIC KEY will operate the transmitter continuously until the switch is returned to the middle position or the transmitter times out about 35 seconds 36 Verify that the External power reading on the display is between 13 5 and 13 9 VDC IF it is outside this range re adjust the power sup
17. sub assy and modulator by reversing the steps followed in Section 3 2 2 Carefully observe proper orientation and alignment when reconnecting the ribbon cable to the board Mount the Display Board to the CDU bezel by reversing the steps followed in Section 3 2 8 Mount the Rotary Switch Assembly to the Control Display Unit by reversing the steps followed in Section 3 2 7 The switch is correctly installed when the flex circuit soldered connections point toward the top of the unit Install the Control Display Unit by reversing the steps followed in Section 3 2 1 Gentle slide the CDU onto the two CPU board connectors P9 and P10 observing proper pin alignment during the process Install the top and bottom covers by reversing the steps followed in Section 3 1 22 SECTION 4 SERVICE AND TESTING 4 1 MEMORY BATTERY REPLACEMENT The GNC 250 300 utilizes a built in lithium battery to maintain user waypoints settings stored in RAM This battery has an operational life of up to five years Should the battery require replacement as indicated by the memory battery low message disassembly of the unit will be required to access the battery When replacing the memory battery all user waypoints settings may be lost If this occurs the message stored Data Lost will be displayed The unit must then be attached to an antenna allowed to search the sky and collect new almanac data before it will be usable again This process may take 20 30
18. super heterodyne AM receiver operating from 118 000 to 136 975 MHz with 25 kHz channel spacing for a combined total of 760 channels Received signals are routed from the antenna via the antenna switch to the RF pre selector where they are filtered and amplified The received signal is down converted to a 21 4 MHz IF via the mixer The mixer local oscillator signal LO is generated by the synthesizer which uses a crystal reference oscillator to generate the LO frequency for each channel as well as the transmitter drive signal After down conversion the signal is filtered by a narrowband crystal IF filter amplified by IF AGC amplifiers and the AM is detected by an envelope detector The demodulated audio is filtered amplified and is held at a constant level by the receiver compressor circuits The audio is routed through a digital volume control IC and is transformed coupled out of the unit via connector J2 The serial data which tunes the synthesizer as well as controls the digital volume chip is generated by the CPU board TRANSMITTER ANTENNA MODULATION FROM Q34 ELIPTIC FILTER ANTENNA SWITCH CONNECTOR SYNTHESIZER RECEIVER BOARD RF PRE SELECTOR MIC INPUT 42 CONTROL DATA FROM 14 CRYSTAL FILTER MODULATION ee TO TRANSMITTER ALTITUDE TRANSISTOR DECODER AUDIO FILTER COMPRESSOR IF AGC AMPLIFIERS 100 mw VOLUME POWER eon CONTROL AMPLIFIER 1
19. via 11 push buttons and the dual concentric rotary switch There are two LEDs behind each keycap providing backlighting for night time use The block diagram in Figure 2 shows the interaction between components on the Control Display Unit COLUMN DRIVER VACUUM FLUORESCENT DISPLAY PHOTOCELL KEYPAD BOARD DRIVER DRIVER DISPLAY KEYPAD KEYPAD VED CONTROL BOARD SCAN LIGHTING TO CPU BOARD Figure 2 Control Display Unit Block Diagram The block diagram in Figure 3 below shows the interaction between the various CPU board circuits NavData or User Data Card Flash Memory dieci 5 8 Antenna V S 2 Discrete 1 Transceiver Appr Control To From PG FLAG Left Right GPS Baseband Processor Super Flag GPSSEQ Encoded HOLD Altitude Switch Data 5 Remote Enter 2 Remote Transfer o 5 Blanking 5 5 8 45 8 8 5 5 2 5 5 8 gt E 5 a External Battery Pack Display Board AC Aircraft Adaptor Power Figure 3 CPU Board Block Diagram 1 2 2 Board Assembly The CPU Board Assembly contains the CPU operating system system memory discrete I O serial communication drivers receivers and the power supply These items are discussed in fur
20. 0 gt 125 00025 00 AFTER ROUTING RIBBON CABLE PROPERLY MOUNT PC BOARD TO CASTING USING SCREW 211 50202 15 WITH SPACER 233 00012 00 AND NYLON SCREW 11 00018 00 ATTACH BNC CONNECTOR 330 00115 00 TO CASTING USING SCREWS INDICATED SOLDER BNC CONNECTOR LEAD TO PC BOARD SLIDE GPS RCVR_COVER 015 00104 00 gt OVER PC BOARD SHIELD AND SECURE BY USING REMAINING SPACERS AND SCREWS lt 211 50202 15 PLACE COVER 115 00113 00 OVER CASTING AND SECURE USING SREWS INDICATED 012 00125 00 211 00018 00 SEE NOTE 1 TYPE 3 115 00104 00 330 00115 00 211 50207 05 4 PLCS SEE NOTE 1 PIN 1 BROWN TYPE 2 CREF 211 50202 15 4 PLCS 211 56201 03 1 PLCS 115 00113 00 Figure 15 GNC 250 300 Assembly Drawing View 4 011 00183 00 50 00 29000 51 3NUN 37725 8 719130 20 40205 8 719130 335 20 0000 016 8 119134 335 10 0000 01 Z has OOH HONOSHL 31n0N 5 00 79000 6 832914 2 1 9100 511 3NON 31 25 8214 2 9 19134 80 02 9 112 341540 00 01100 51 00 18000 00 20100 511 0 20206 112 NAUHS SY 00 1 000 0S2 319201 00 69200 110 00 ZOI00 SIT 0310N 3SIAY3HLO 5531 3441 1v NO 00 8 000 100 ON 2345 ONIX201 0 3 1 Addy 1 5310 v 1 335 700 1 000 0
21. 00 by unscrewing the pivot pin screw 211 00030 00 Remove the hex nut securing the power dimmer switch to the chassis This nut is 7 16 and may be removed with a deep thin walled 12 point socket Slide the power dimmer switch out of the mounting hole on the chassis 20 3 2 7 Rotary Switch Assembly 011 00231 00 Removal 6 Remove the covers as described in Section 3 1 Remove the Control Display Unit as described in Section 3 2 1 Remove the two rotary switch knobs by loosening the two M3 x 4mm set screws on the inner knob 430 00002 00 After removing the inner knob gently pull the outer knob off of the rotary switch shaft Disconnect the rotary switch flex circuit from the connector on the Display Board Remove the hex nut securing the rotary switch to the Control Display Unit This nut is 1 2 and may be removed with a deep 12 point socket Push the rotary switch assembly out of the back of the Control Display Unit 3 2 8 Display Board 012 00065 00 Removal 1 2 Remove the covers as described in Section 3 1 Remove the Control Display Unit as described in Section 3 2 1 Disconnect the rotary switch wiring harness from the connector on the Display Board Remove the four screws 211 50201 05 that secure the Display Board to the bezel Carefully pull the Display Board away from the CDU bezel disengaging it from the Keypad Board connector 3 2 9 Remote Battery Pack and Charger 011 00182 00 Disassembly
22. 0025 00 2 PLCS 50 00037 00 145 00056 00 012 00092 00 125 00019 00 lt gt 250 00026 00 630 00017 00 pute 145 00134 00 2 Se gt ex 211 50207 05 5 3 211 63204 08 PLCS 211 00024 00 3 PLCS 9 211 56201 05 PLCS SEE NOTE 2 011 00183 00 1 115 00062 00 211 50207 05 b LI SOLDER 1 211 00030 00 1 SEE NOTE 2 312 30125 00 42 cm LONG 011 00230 00 125 00027 00 325 00025 02 210 00021 00 325 00037 00 E E e P16 50 00002 00 CONNECTS 16 POSISTOR SEE NOT ON 012 00066 NOT USED CONNECTS TO J7 ON 012 00066 DETAIL A 012 00066 XX 250 00035 00 CINSIDE VIEW NOTE 4 SCALE N A 21 50207 330 00154 00 NOTES 211 50207 05 1 APPLY THREAD LOCKING COMPOUND PER GARMIN SPEC 001 00048 02 ON ALL THREADED FASTENERS UNLESS OTHERWISE NOTED 2 APPLY THREAD LOCKING COMPOUND PER GARMIN SPEC NO 001 00048 01 ON THREADED FASTENERS WHERE THIS NOTE IS REFERENCED 3 INSURE THAT POSISTOR IS MOUNTED TO 125 00019 00 USING 211 50207 05 SCREW AS SHOWN 4 SEE LIST OF REPLACEABLE PARTS Figure 14 GNC 250 300 Assembly Drawing View 3 49 NOTES 1 APPLY THREAD LOCKING COMPOUND PER GARMIN SPEC NO 001 00048 00 ON ALL THREADED FASTENERS TYPE 1 UNLESS OTHERWISE NOTED 2 ASSEMBLY ORDER PLACE PC BOARD 012 00125 00 gt INTD CASTING 125 00025 0
23. 02 212 00 61000 06 00 00 09 1000 652 043389 00 96000 501 5214 B 0296 12 1 3104 00 20000 01 30 1294 19 0C100 S v1 y 1 130335 20100 5 11130 335 80100 691 0 10 0 00 49000 210 gt gt 00 11000 621 Dy 5274 lt 21 310N 335 5245 00 lt 9000 210 lt NO DINI 1 5 QA 00 16200 110 g GNC 250 300 Assembly Drawing View 1 011 00107 01 00 Figure 12 47 211 00024 00 2 PLCS 115 00061 00 211 50207 05 2 PLCS 125 00020 00 211 50207 05 011 00108 00 211 50207 05 SEE DETAIL A 335 00026 01 211 50207 11 DETAIL CINSIDE VIEW SCALE N A 211 00024 00 4 PLCS 211 50207 05 011 00107 5 NOTES 1 APPLY THREAD LOCKING COMPOUND PER GARMIN SPEC NO 001 00048 00 ON ALL THREADED FASTENERS 2 UNLESS OTHERWISE NOTED DISASSEMBLE UPPER SCREW ON MODULATOR AND RE ASSEMBLE WITH MODULATOR CABLE 3 APPLY THREAD LOCKING COMPOUND PER GARMIN SPEC NO 001 00048 01 WHERE THIS NOTE IS REFERENCED 4 ALIGN HOLE IN TAG WITH COUNTERSINK IN CHASSIS AS SHOWN 5 SEE LIST OF REPLACEABLE PARTS 010 10038 XX CREF SEE DETAIL A 011 00105 00 161 00105 00 4 fu 211 00037 00 3 250 00033 00 227 430 00014 00 211 50207 05 2 PLCS 115 00060 00 Figure 13 GNC 250 300 Assembly Drawing View 2 48 211 50207 11 2 PLCS 250 0
24. 250 300 011 00230 00 Pwr Vol swch w flx ckt cbl GNC 011 00231 00 Rtry Swch Assy w flex circuit GNC 011 00265 00 Transmitter Sub Assy GNC 012 00049 00 Receiver Bd Assy GNC 012 00066 00 CPU Board GNC 250 012 00066 50 CPU Board GNC 300 012 00067 00 Key Bd Assy GNC 012 00092 00 Intrface Bd GNC 145 00134 00 PMP Insulator TO 3GNC 250 00026 00 Insulator TO 3 253 00011 00 Gasket Neoprene 320 00088 00 RG58 10dB cable 1 5 GHz 360 00009 00 Lithium Cell 361 00008 00 Btry Pk 9 6V Nicad 362 00014 00 Wall Charger GNC 410 00004 01 Cont Rotary sw pot 410 00007 00 Cont Rotary Sw Dual Cnctrc 430 00002 00 Knob Inner w o Trim ring w set screws 430 00010 00 Knob Power dimmer with shaft 430 00011 00 Knob Outer 430 00013 00 Knob Assy skirted 430 00014 00 Knob Power Volume 470 00008 00 Lens 300 GNC 470 00008 01 Lens 250 GNC 45 5 2 ASSEMBLY DRAWINGS The following assembly drawings are provided for part identification and to aid in assembly disassembly of the GNC 250 300 46 V 119134 0 20100 lt 1 20 Z0100 Srt 10 20100 591 40 40100 6 1 MEER EHE On 2 90 20100 591 50 10100 6 1 90 20100 591 0100 5 1 20 80100 571 20 80100 5 1 10 08100 65 1 80000 0 90 60000 621 00 20000 06 30 1394 7 gt 07 00 000
25. C 250 300 ASSEMBLY DRAWING VIEW e 47 FIGURE 13 GNC 250 300 ASSEMBLY DRAWING VIEW 2925 48 FIGURE 14 GNC 250 300 ASSEMBLY DRAWING VIEW 49 FIGURE 15 GNC 250 300 ASSEMBLY DRAWING VIEW 50 FIGURE 16 GNC 250 300 ASSEMBLY DRAWING VIEW 51 FIGURE 15 GNC 250 300 CONNECTOR DIAGRAM en aa ea 52 1 INTRODUCTION This manual is intended to provide instructions and guidelines for the maintenance and repair of the GARMIN GNC 250 and GNC 300 WARNING This unit contains static sensitive devices Service personnel must insure that proper precautions are taken to prevent damage to this equipment from Electro Static Discharge ESD Disassembly and repair should only be accomplished at an approved ESD workstations by properly trained and grounded personnel 11 GENERAL DESCRIPTION The GNC 250 300 is a full featured aviation GPS navigation and communication instrument designed for a fixed panel mounted installation The GNC 250 300 utilizes the patented GARMIN MultiTrac system for tracking up to eight satellites simultaneously The unit features a GPS receiver a 4 line 20 character high brightness vacuum fluorescent display and a front loading NavData card The unit contains a fully TSO d 760 channel VHF communications transceiver operating from 118 000 to 136 975 Mhz wi
26. SR key twice Rotate the outer function knob clockwise until VALUE is flashing Rotate the inner function knob clockwise to select CLOCK Turn the FUNCTION SELECT switch on the test panel to position 14 Rotate the outer function knob clockwise until LOW flashed Verify that the DMM voltage is less than 1 0 VDC Rotate the inner function knob to select HIGH Verify that the DMM voltage is greater than 4 5 VDC Rotate the outer function knob counterclockwise until CLOCK flashes Rotate the inner function knob clockwise to select DATA Turn the FUNCTION SELECT switch on the test panel to position 15 Rotate the outer function knob clockwise until LOW flashes Verify that the DMM voltage is less than 1 0 VDC Rotate the inner function knob to select HIGH Verify that the DMM voltage is greater than 4 5 VDC Rotate the outer function knob counterclockwise until DATA flashes Rotate the inner function knob clockwise to select SYNC Turn the FUNCTION SELECT switch on the test panel to position 16 Rotate the outer function knob clockwise until LOW flashes Verify that the DMM voltage is less than 1 0 VDC Rotate the inner function knob to select High Verify that the DMM voltage is greater than 4 5 VDC 7 5 232 RS 422 ARINC 429 Loopback Tests NOTE RS 422 Not available on GNC 250 a b Press the CRSR key Rotate the outer function knob clockwise until the 1 0 Test Page is displayed 27 C d e Veri
27. TUDE C4 ALTITUDE COMMON GROUND REMOTE ENTER FIGURE 11 JI REAR CONNECTOR PINOUT DEFINITION 20 21 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 VIEW OF 91 CONNECTOR FROM BACK OF UNIT PIN DESCRIPTION GPS D BAR LEFT GPS TO GPS FLAG COMMON RESERVED RESERVED GPS OBI CLOCK GPS OBI DATA APPROACH ARM LOW GNC 300 ONLY GPS SUPER FLAG HIGH RS 422 IN A GNC 300 ONLY ARRIVAL ANNUNCIATOR APPROACH ACTV ANNUNCIATOR GNC 300 ONLY GPS ROTOR C GNC 300 ONLY ARINC 429 OUT B ARINC 429 OUT A RS 232 IN 1 RS 232 IN 2 USED FOR TESTING ONLY RE 232 OUT 2 MESSAGE ANNUNCIATOR POWER 13 8 VDC GROUND GPS OBI SYNC RS 232 OUT 1 POWER 13 8VDC GROUND RS 422 IN B GNC 300 ONLY APPROACH ARM ANNUNCIATOR GNC 300 ONLY CHARGE ENABLE REMOTE BATTERY SPARE ARINC 429 IN A GNC 250 USE IS LIMITED TO TESTING ARINC 429 IN B GNC 250 USE IS LIMITED TO TESTING REMOTE BATTERY GPS STATOR D GNC 300 ONLY TEST SELECT GPS STATOR F GNC 300 ONLY 44 5 ILLUSTRATED PARTS LIST 5 1 LIST OF REPLACEABLE PARTS The table below lists the parts which are available to authorized maintenance shops 011 00107 00 CDU GNC300 011 00107 01 CDU GNC250 000 00108 00 Comm Chassis Assy GNC 250 300 14V 011 00147 00 Order 010 10040 02 Antenna Flange mt 011 00183 00 GPS revr assy GNC
28. anel Connect the negative output of the power supply to the black DC IN jack at the back of the text panel Connect the J2 and J1 wiring harness assemblies to the front of the test panel Connect the positive and negative digital multimeter leads to the and LO FUNCTION SELECT outputs on the front of the test panel Set the DMM to measure DC voltage Set the CAL NORMAL switch to NORMAL and set the GPS 150 155 165 GNC switch to GNC set the A B switch in the A position Plug one end of the AC power cord into the back of the test panel and the other end into a 120 VAC outlet Turn the FUNCTION SELECT switch to POWER IN Ensure that the PANEL POWER and UNIT POWER push button switches are OFF not lighted Turn on the power supply push the PANEL POWER switch and verify that it lights up Push the UNIT POWER switch and verify that it lights up Monitor the POWER IN voltage with the DMM and set the power supply for 14 8 VDC 0 1 VDC Push the UNIT POWER switch to turn unit power OFF Connect the J2 and J1 wiring harness assemblies to the back of the GNC 250 300 Push the UNIT POWER switch to turn unit power ON 4 2 3 Final Test Procedure 1 2 Test Mode Activation Display Test Hold the ENT key down while rotating the power volume knob clockwise from the OFF position b Verify that the GNC 250 300 enters test mode There are three test patterns displayed when first activating test mode The first
29. ay Unit 011 00107 00 01 Removal 1 2 Remove the covers as described above Remove the power volume control knob and shaft 430 00014 00 by loosening the M3 x 2 6mm set screw 211 00037 00 on the coupling sleeve 125 00027 00 Remove the four M3 x 5mm flat head screws 211 00024 00 which attach the CDU to the chassis Carefully remove the CDU from the chassis pulling the assembly away from the two CPU board connectors P9 and P10 and the power volume switch Communication Chassis Sub Assembly 011 00108 00 amp Modulator Removal Remove the covers as described in section 3 1 Remove the M3 x 5 PHP 211 50207 05 and the M3 x 8mm PHP 211 50207 11 screws that connect the transmitter casting to the GPS receiver casting At this point the comm chassis sub assembly can be hinged open by lifting up on the transmitter casting while holding the main chassis down thus giving access to the top side of the CPU board This is beneficial to troubleshoot the CPU board without completely removing the comm sub assembly The comm chassis can be completely disassembled by first removing the M3 x 5mm PHP screw 211 50207 05 on the back of the unit that holds the receiver connector plate 115 00081 00 to the unit Remove the M3 x 8mm PHP screw 211 50207 11 that connected the modulator cable to the modulator Be careful to retain the insulator 250 00025 00 18 3 2 3 3 2 4 Remove the ribbon cable 335 00026 00 from the J13
30. ay unit Replace comm chassis sub assembly see Section 3 2 2 steps 1 7 Replace CPU Board see Section 3 2 4 Transmitter sidetone audio is noisy or distorted Check comm antenna in installation for intermittent connection and or high VSWR Verify connections from CPU Board to modulator transistor Q34 and from Q34 to transmitter Verify connection between CPU Board and Comm Receiver Board Replace comm chassis sub assembly see Section 3 2 2 steps 1 7 Replace modulator transistor Q34 see Section 3 2 2 steps 8 14 Replace CPU Board see Section 3 2 4 Transmitter has low or no power output Check comm antenna in installation for intermittent connection and or high VSWR Verify connection from CPU Board to modulator transistor Q34 and from Q34 to transmitter Verify connection between CPU Board and Comm Receiver Board Replace comm chassis sub assembly see Section 3 2 2 steps 1 7 Replace modulator transistor Q34 see Section 3 2 2 steps 8 14 Replace CPU Board see Section 3 2 4 16 MESSAGE cont RECOMMENDED REPAIR cont Transmitter demodulated audio is distorted or noisy Check output from 28v to 14v converter if so equipped Replace converter if output voltage is higher than 15VDC Check comm antenna in installation for intermittent connection and or high VSWR Verify connections from CPU Board to modulator transistor Q34 and from Q34 to transmitter Verify connection bet
31. battery voltage from test point to ground see Figure 6 is at least 2 9 VDC 14 Re install the CPU board bottom side cover 15 Re install the top and bottom covers by reversing the steps followed in Section 3 1 23 4 2 FINAL TEST After completing all necessary maintenance or service the following test procedure is provided to ensure that all GNC 250 300 functions are operating properly 4 2 1 Test Equipment Required 0 30 Volt DC 6 Amp Power Supply GPS 150 155 165 GNC Manual Test Panel T10 00003 02 Red and Black Cables with Banana Plugs 3 length recommended Digital Multimeter Signal Generator HP8640B or equivalent external frequency counter capability desirable Frequency Counter capable of measuring up to 137 MHz not necessary if RF generator has external counter capability RF Watt meter Bird 6811 or equivalent 25 Watt 50 ohm RF load Bird 8080 or equivalent Audio Signal Generator HP200CD or equivalent Audio distortion analyzer HP 331A or equivalent Linear Detector see figure below Pad minicircuits CAT 6 or equivalent 200 MHz oscilloscope FR GPS 101 Global Positioning Simulator or equivalent LINEAR DETECTOR TO BE USED IN TRANSMITTER TEST SETUP FIGURE 9 24 4 2 2 Setup Preset the power supply to 14 8 VDC and set current limit to 6A Connect the positive output of the power supply to the red DC IN jack at the back of the test p
32. board see Section 3 2 5 Replace CPU Board see Section 3 2 4 Battery rqrs service optional battery If battery charger pack not installed change configuration to remote battery none Verify connections to remote battery charger pack Replace remote battery pack Where multiple repair actions are listed they are listed with most likely failure cause first 10 2 3 INITIAL TROUBLESHOOTING OPERATIONS The first step in troubleshooting the GNC 250 300 is to remove the top and bottom covers see Section 3 1 and the CPU bottom shield and swing the comm assembly open Verify that the unit is being properly powered and the internal clock is functional NOTE The NavData card should be removed before disassembling the unit 2 3 1 Power Supply Check 1 Check the DC voltage between 0511 and ground as shown in Figure 6 The voltage should be between 9 60 and 14 40 volts If the voltage is out of the limits specified replace the CPU board 2 Check the DC voltage between L502 and ground as shown in Figure 6 The voltage should be between 4 8 and 5 2 volts If the voltage is out of the limits specified replace the CPU board 3 Check the DC voltage between 0515 and ground as shown in Figure 7 The voltage should be between 17 00 and 67 00 volts If the voltage is out of the limits specified replace the CPU board 4 Check the DC voltage between L504 and ground as shown in Figure 6 The voltage should be between 9 6 a
33. ch on the test panel to position 2 If necessary rotate the inner function knob to select FLAG Verify that the DMM voltage is 0 15 millivolts Units with Mod Status 1 DMM voltage is 0 10 millivolts Rotate the inner function knob clockwise to select FROM Verify that the DMM voltage is 83 20 millivolts Units with Mod Status 1 DMM voltage is 190 40 millivolts Rotate the inner function knob clockwise to select TO Verify that the DMM voltage is 83 20 millivolts Units with Mod Status 1 DMM voltage is 190 40 millivolts 4 CDI Nav Flag Test a b Rotate the outer function knob clockwise Turn the FUNCTION SELECT switch on the test panel to position 1 If necessary rotate the inner function knob to select IN VIEW Verify that the DMM voltage is 0 10 millivolts Rotate the inner function knob clockwise to select OUT OF VIEW Verify that the DMM voltage is 375 80 millivolts 5 Super Flag Output Test Rotate the outer function knob clockwise Turn the FUNCTION SELECT switch on the test panel to position 4 If necessary rotate the inner function knob to select IN VIEW Verify that the DMM voltage is less than 0 25 VDC Rotate the inner function knob clockwise to select OUT OF VIEW Verify that the DMM voltage is greater than 12 8 VDC 26 6 OBI Output Test a b r Press the CRSR key Rotate the outer function knob counter clockwise until the OBI Test Page is displayed Press the CR
34. e comm audio output on the audio analyzer Key the transmitter and measure the sidetone level Verify that it is equal to 1 4 Vrms or 20 38 30V 6A Power Supply HP 200 CD Audio Osc 26 Pin Hi Dens UUT Test Panel Comm Antenna Mic Audio Hi To Distortion Analyzer for Sidetone Measurements Keep Connection as short as Possible Linear Detector 200 MHz O Scope T Junction Distortion Analyzer 500 25W Load ___ 40 dB Coupler Output To O Scope For Modulation Measurements To HP8640B or Equivelant Counter Input for Frequency Tolerance Measurements Figure 9 Transmitter Test Setup 39 External COM Switches Test With the unit still in test mode turn the outer function knob counter clockwise until the External COM Switches test page is displayed fr off Place the ALT DECODER LINE SELECT switch on the test panel in the RMT ENT position Verify that RMT ENT ON is displayed on the unit Place the ALT DECODER LINE SELECT switch on the test panel in the C4 position Verify the RMT ENT OFF is displayed on the unit Place the REMOTE TRANSFER switch on the test panel in the UP position Verify that THE ACTV AND STBY channels on the display change places and that RMT XFR ON is displayed on the unit Place the REMOTE TRANSFER switch on the test panel in the DOWN position Verify that RMT XFR OFF is displayed on the
35. e potentiometer fully clockwise Key the transmitter and monitor the demodulated audio output on the oscilloscope SLOWLY adjust the potentiometer counter clockwise increasing the demodulated audio output level until it just reached the point where no further increase is obtainable Perform test procedure 4 3 3 18 d to verify that this adjustment has been done properly Transmitter Sidetone Monitor the comm audio output level with audio analyzer Use the 510 ohm load on the analyzer input Apply a 275 mV RMS audio signal at 1000 Hz to the MIC AUDIO HI input to the test panel nsert a tuning tool in the SIDETONE hole in the top cover until it fits in the slot of the sidetone potentiometer on the comm receiver board Key the transmitter and adjust the potentiometer until the comm audio output level equals 1 4 Vrms or 5 42 FIGURE 10 J2 REAR CONNECTOR PINOUT DEFINITION 9 1 0000000 20000000 VIEW 2 CONNECTOR FROM BACK OF UNIT 5 O N IS DESCRIPTION COMPRESSOR TEST MIC AUDIO HI MIC AUDIO LO MIC KEY COMM AUDIO HI COMM AUDIO LO GROUND MIC INTERCOM IF AGC TEST REMOTE TRANSFER UNSQUELCHED AUDIO TRANSMIT INTERLOCK SEQUENCE HOLD LOW GNC 300 ONLY ALTITUDE D4 ALTITUDE Al ALTITUDE A2 ALTITUDE A4 ALTITUDE ALTITUDE B2 ALTITUDE B4 ALTITUDE Cl ALTITUDE C2 ALTI
36. e the input course is within 2 0 External Power Battery Tests a Rotate the outer function knob counter clockwise until the External Power Battery test page is displayed 1 36 47 118 00 External Bat None Bat ok b Verify that line two of the display indicates External Power and the voltage reading is within 0 3 VDC of the reading taken in section 4 2 2 step 9 approximately 14 8 VDC c Connect the wall charger to the unit and verify that line two of the display now indicates Wall adapter d Unplug the wall adapter Line two of the display should go back to indicating External Power e Verify that line 3 of the display indicates Rmt Pak none f Verify that line 4 of the display indicates Mem Bat ok Display Intensity Test a Rotate the outer control knob until the Display Intensity test page is displayed 30 b Cover the photo diode detector on the display with a finger and verify that the Display Intensity percentage number decreases and that the display grows dimmer 14 GPS Receiver Noise Test a Depress the ENT key while powering up the unit This puts the unit into TEST MODE b Turn the outer knob 8 pages until you see the following display c Terminate the GPS receiver into a 50 ohm load d The bottom line of the display represents the receiver noise level Verify that this number is within the range of 1800 and 2200 15 Operational Test a Turn
37. er encoder Verify connection between CPU Board and Comm Receiver Board Replace comm chassis sub assembly Replace CPU Board Course input fail Check ARINC 429 RS422 course input connections 300 only Replace CPU Board Data card failed Replace NavData card Replace Interface Board see Section 3 2 3 Data card write fail Replace user data card Replace Interface Board see Section 3 2 3 Fuel Air input fail Check connections to fuel air data system Replace CPU Board see Section 3 2 4 Memory battery low Replace memory battery see Section 4 1 No altitude input Verify connections to altitude serializer encoder 300 only Verify connections between CPU Board and Comm Receiver Board Replace Com Chassis sub assembly see Section 3 2 2 Replace CPU Board see Section 3 2 4 Osc needs adjustment Replace GPS Receiver Board see Section 3 2 5 Replace CPU Board see Section 3 2 4 RAM failed Replace CPU Board see Section 3 2 4 Receiver failed Replace CPU Board see Section 3 2 4 Replace GPS Receiver Board see Section 3 2 5 Stored data lost Replace memory battery see Section 4 1 Configuration reset Replace CPU Board see Section 3 2 4 EEPROM write fail Replace CPU Board see Section 3 2 4 MESSAGE cont RECOMMENDED REPAIR cont Battery low optional battery IF battery Charger pack not installed change Configuration to Remote Battery None If battery is
38. ess the NAV key on the unit and then the MSG key and verify that the remote XFR key stuck message has gone away k Stuck Remote Enter Key test Place the ALT DECODER LINE SELECT switch in the RMT ENT position Verify that after 35 seconds the MSG annunciator flashes Press the MSG key on the unit and verify that Remote ENT key stuck is displayed Press the CRSR key on the unit and tune the STBY channel to a different frequency Press the transfer key lt gt and verify that the ACTV and STBY channels change places Place the ALT DECODER LINE SELECT switch in the C4 position Press the NAV key and then the MSK key on the unit Verify that the Remote ENT key stuck message has gone away 4 2 4 Comm Transceiver Alignment few receiver and transmitter alignments are accessible throughout the top of the unit without removing the unit cover The following procedure describes how to perform these adjustments 1 Comm Receiver Carrier Squelch Configure the test equipment as shown in section 4 3 3 17 Make sure the COMP DISABLE switch on the test panel is in the DOWN position Turn the unit on in normal operation mode Turn the power volume control knob fully clockwise Tune the GNC 250 300 active channel to 127 07 Mhz Set the RF signal generator for 127 075 Mhz 8596 modulated with an 8 KHz tone and an output level of 12 5uV Insert a tuning tool the CARRIER SQ hole in the top cover until it fits in the
39. fy that the 1 0 Test Page indicates CH 1 OK and CH 2 OK Verify that the 1 0 Test Page indicates RS422 OK Verify that the 1 0 Test Page indicates A429 gt 8 Annunciator Output Test a b Press CRSR key Rotate the outer function knob until the Annunciator Test Page is displayed Press the CRSR key twice Rotate the inner function knob to turn the front panel annunciators ON Verify that all five annunciators on the GNC 250 300 are lit Rotate the inner knob to turn the front panel annunciators OFF Verify that all five annunciators on the GNC 250 300 are off Turn the FUNCTION SELECT switch on the test panel to position 6 Rotate the outer function knob clockwise to select the MSG annunciator With the MSG annunciator OFF verify that the DMM voltage is greater than 4 5 VDC Rotate the inner function knob to turn the MSG annunciator ON Verify that the DMM voltage is less than 0 3 VDC Turn the FUNCTION SELECT switch on the test panel to position 7 Rotate the outer function knob clockwise to select the ARRVL annunciator test With the ARRVL annunciator OFF verify that the DMM voltage is greater than 4 5 VDC Rotate the inner function knob to turn the ARRVL annunciator ON Verify that the DMM voltage is less than 0 3 VDC Turn the FUNCTION SELECT switch on the test panel to position 5 Rotate the outer function knob clockwise to select the APPR annunciator test With the Appr annunciator off verif
40. he normal lock on time Display of low velocity numbers when not moving can occur when the units is in 2D mode and should not be considered a problem However this should seldom occur in 3D mode Turning message appears below approximately 9 VDC external power 15 2 5 TROUBLESHOOTING CHART COMM SECTION MESSAGE RECOMMENDED REPAIR Receiver has poor signal reception low sensitivity Check comm antenna in installation for intermittent connection and or high VSWR Verify connection between power volume control knob and CPU Board Verify connection between CPU Board and Comm Receiver Board Replace comm chassis sub assembly see Section 3 2 2 steps 1 7 Replace CPU Board see Section 3 2 4 Receiver audio 15 distorted Verify connection between CPU Board and Comm Receiver Board Replace comm chassis sub assembly see Section 3 2 2 steps 1 7 Replace CPU Board see Section 3 2 4 Receiver audio volume is low or intermittent Verify connection between power volume control know and CPU Board Verify connection between CPU Board and Comm receiver board Replace comm chassis sub assembly see Section 3 2 2 steps 1 7 Replace CPU Board see Section 3 2 4 Receiver squelch is stuck open noise through audio at all times Verify connection between Control Display unit and CPU Board Verify connection between CPU Board and Comm Receiver Board Replace Control Displ
41. installed verify that the charge is set up for auto and allow to charge for 1 hr Verify Connections to remote battery Charger pack Replace Remote battery pack PWR down and re init Turn unit off and then back on again Verify connection between CPU Board and BPS receiver board Replace GPS receiver see Section 3 2 5 Replace CPU board see Section 3 2 4 No course input 300 only Verify connection to OBS Replace CPU board see Section 3 2 4 COM failed Verify connection between CPU Board and Comm Receiver Board Replaced comm chassis sub assembly see Section 3 2 2 Replace CPU Board see Section 3 2 4 Stuck mic TX disabled Verify that mic key input to unit is not grounded Verify connection between CPU Board and Comm Receiver Board Replace comm chassis sub assembly see Section 3 2 2 Replace CPU board see Section 3 2 4 Remote ENT key stuck Verify that remote enter input to unit is not grounded Verify connection between CPU Board and Comm Receiver Board Replace comm chassis sub assembly see Section 3 2 2 Replace CPU Board see Section 3 2 4 Remote XFR key stuck Verify that remote transfer input to unit is not grounded Verify connection between CPU Board and Comm Receiver Board Replace comm chassis sub assembly see Section 3 2 2 Replace CPU Board see Section 3 2 4 Rcvr needs service Verify connection between CPU Board and GPS receiver board Replace GPS receiver
42. made via a BNC connector The RF IF supplies 5 volts to the antenna s preamplifier through this BNC connector The block diagram in Figure 4 shows the interaction between components the Assembly Figure 4 GPS Receiver Block Diagram 1 2 4 Interface Board and Data Cards The Interface Board provides two way data transfer capability between the CPU board and a data card NavData cards allow for data transfer to the CPU board only User data cards with 128 kilobytes of flash memory data storage may be used to save user data settings from the GNC 250 300 or restore that information to the GNC 250 300 Address and data lines from the CPU board route through the Interface Board s output buffers to the data card A plastic race secures the Interface Board at the front of the unit and serves to guide the data card onto the 40 pin connector NOTE When servicing the GNC 250 300 user defined waypoints routes settings etc may be saved on a user data card 010 10032 03 first In the event these data are lost they may then be restored from the user data card before the unit is returned to serve Refer to Appendix A of the GNC 250 300 Pilot s Guide for additional information on user data card operation 1 2 5 VHF Communications Transceiver The VHF communications transceiver consists of the comm receiver board assembly and the transmitter board and casting assembly The VHF receiver is a single conversion
43. nd 14 4 volts If the voltage is out of the limits specified replace the CPU board 5 Checkthe DC voltage between 0511 and ground as shown in Figure 7 The voltage should be between 25 and 45 volts If the voltage is out of the limits specified replace the CPU board 6 Checkthe DC voltage between D508 and ground as shown in Figure 6 The voltage should read between 12 and 18 volts If the voltage is out of the limits specified replace the CPU board 2 3 2 Internal Clock Check Check the clock speed by placing an oscilloscope probe at 1101 pin 31 as shown in Figure 7 The frequency should be approximately 16 4 MHz If the clock is not operational replace the GPS receiver If the clock still fails to function replace the CPU board 11 0508 15VDC L502 5VDC L504 12VDC Figure 6 CPU Board Testpoints Top View 12 0511 COLLECTOR MOUNTING TABS TEST POINT 25 45 VDC 3 VDC I 0097 anos 00 oo on oo 5 1 ef E c sec 20 g Ll V p MUERE o od AIAX 5 2726 1471 1 o Ne emo s SES ge exea fen 9 5 c s s 0 2 0 2 CL D 221 2 2 000 0 0 EX 2000 05 oo 3800 2 0000 00 0 0
44. nector External battery pack power is also applied to the rear connector 1 All of these sources are diode isolated to prevent parallel sourcing The unit is turned on and off by a digital latch circuit operated by a knob located on the front panel adjacent to the NavData card opening The switching circuit is a fly back design The switching regulator operates at a frequency of approximately 129 kHz and uses a high efficiency switching regulator for the switching device The high efficiency switching regulator drives a transformer that is configured in a fly back mode The 15 volt output is post regulated to provide 11 75 to 12 25 volts The 5 volt output is regulated at 4 8 to 5 2 volts All other voltages are 20 1 2 3 GPS Receiver The RF IF assembly consists of a dual conversion receiver a frequency synthesizer and a high precision crystal oscillator The receiver consists of a ceramic RF filter RF and IF amplifiers mixers and an IF filter The frequency synthesizer uses the high precision oscillator as a reference frequency for the phase detector in the synthesizer The resultant frequency is used in the mixer section to produce the first IF After further amplification and mixing with a product of the crystal oscillator the baseband IF is passed to the CPU Board for processing The crystal output is also used for the system clock pulse The RF IF assembly is contained within a shielding fence cover and connection to the antenna is
45. nna and or antenna cable Replace GPS Receiver Board see Section 3 2 5 Replace CPU Board see Section 3 2 4 Unit displays velocity Replace GPS Receiver Board see Section 3 2 5 when not moving Ni Cad Battery life is Replace External Battery Pack short or Ni Cad battery Replace CPU Board see Section 3 2 4 mop Operational Remote Battery Unit loses memory in Replace CPU Board see Section 3 2 4 spite of new memory battery Unit does not detect low Replace CPU Board see Section 3 2 4 memory battery Unit does not detect Check external power connection external power Replace CPU Board see Section 3 2 4 No serial data output Replace CPU Board see Section 3 2 4 No external CDI output Check connections and verify operation in Test Mode or external CDI functions see Section 4 2 3 improperly Replace CPU Board see Section 3 2 4 Unit detects low NICAD Adjust battery alarm setpoint from config page battery too soon or too late If UTC Time is incorrect unit may be reset by holding the CLR key down while turning the unit on The unit must then be attached to an antenna allowed to search the sky and collect new almanac data This process may take 20 30 minutes CAUTION user waypoints settings will be lost if the unit is reset 250 300 will eventually perform AutoLocate M and lock onto satellites however this can add an additional 10 minutes to t
46. normal operating mode Press ENT key to acknowledge the Database page Volume Control Test Apply a 20uV RF signal to the comm antenna input at 118 000 MHz AM modulate the signal 3096 with a 1000 Hz tone Make sure the COMP DISABLE switch on the test panel is in the down position Tune the comm receiver to 118 00 MHz Turn the power volume control knob fully clockwise Verify that the comm audio output level is greater than or equal to 7 07 Vrms on the audio level meter Turn the power volume control knob fully counterclockwise but not past the detent off position Verify that the comm audio output level is less than or equal to 22 mVrms on the audio level meter 32 Sensitivity and Quieting Test Set the COMP DISABLE switch on the test panel in the UP position Turn the power volume control knob clockwise until the comm audio output level is equal to 7 07 Vrms and the audio is not visibly clipping on the oscilloscope NOTE The receiver has a digital volume controller and it may not be possible to set the volume to exactly 7 07 Vrms It is acceptable to use the first level greater than or equal to 7 07 Vrms Set the RF signal generator output level to 2 Using the audio analyzer verify that the comm audio output signal noise noise ratio is at least 698 the RF signal generator output to 100uV Using the audio analyzer verify that the comm audio output Signal noise noise ratio is at least 354
47. o verifies that the signal strength of the satellites received is at an acceptable level Turn the unit off b Connect the unit to a GA56 antenna mounted outside with a clear view of the sky through a coax with a 10 dB 1 dB cable loss 1 5 GHZ Note attenuation pads must not be used since the unit supplies the GA56 with DC power c on the unit Go to the Satellite Data Page see Pilot s Guide for instructions d Allow the unit to lock on to the satellites If the unit has current almanac data it will list the visible satellites on the row labeled SAT and it will begin acquiring satellites in a period of a few minutes If the unit indicates SEARCH SKY instead of ACQUIRING it must search the sky for available satellites and collect almanac data A search the sky operation may take 20 30 minutes to complete e Then unit must enter 3D navigation mode f The unit must display the following 1 4 satellites with signal strength or greater 2 The average signal strength on satellites above 20 elevation shall be greater than or equal to 3 5 see Pilot s Guide for instructions Satellites that are acquiring should not be included in the average signal strength calculation Communications Receiver Tests NOTE receiver input levels are specified in hard uV which implies a 6 db pad on the output of the RF signal generator See figure 8 for typical comm receiver test set up a b Turn GNC 250 300 on in
48. out of the unit upward disengaging it from the CPU board Connector J12 CPU Board Removal 012 00066 00 50 Remove the covers as described in Section 3 1 Remove the Control Display Unit as described in Section 3 2 1 Remove the Comm Chassis Sub Assembly as described in Section 3 2 2 Remove the 211 50207 05 screw holding the 1501 Regulator to the main chassis Carefully turn the unit over The power volume switch uses a flexible circuit cable for connection to the CPU board This flexible circuit cable is inserted into a ZIF connector J11 Pull up on the retaining lip of the ZIF connector Gently pull the flexible circuit cable out of the ZIF connector Remove the bottom shield cover 155 00116 00 from the CPU and disconnect the GPS receiver cable from the J6 CPU connector Remove the two 4 40x 250 flat head screws 211 63204 08 which hold the 37 pin DSUB connector to the chassis 19 3 2 5 3 2 6 10 11 Remove the two M3 x 5mm screws 211 50207 05 and the M3 x 8mm screw 211 50207 11 holding the CPU board to the chassis Remove the bottom shield cover 115 00116 00 from the CPU and disconnect the GPS receiver cable from the J6 CPU connector Carefully slide the CPU board upward pulling it away from the power volume switch GPS Receiver Assembly 011 00183 00 Removal amp Disassembly 1 2 Remove the covers as described in Section 3 1 Disconnect the Comm Chassis sub assembly from the GPS recei
49. output reference level to 0 Adjust the modulation frequency to 4 KHz and verify that comm audio output level drops at least 18 dB Compressor Test Set RF generator modulation frequency to 1 KHz and the modulation depth to 20 Place the COMP DISABLE switch on the test panel in the DOWN position Re adjust the power volume control for 7 07 Vrms comm audio output Set the comm audio output reference level to 0 dB Adjust the modulation depth to 85 and verify that the comm audio output level does not vary more than 3 dB Selectivity Test Turn the modulation OFF on the RF signal generator Turn the power volume control knob fully counterclockwise but not past the detent OFF position Set the RF output level of the signal generator to 10uV Set the FUNCTION SELECT switch on the test panel to position 18 and monitor the AGC voltage with the DMM DC The DC voltage reading on the DMM is the AGC reference voltage write it down as you will need it for the rest of the test Adjust the signal generator frequency to 127 067 MHz and adjust the RF level to 20uV Verify that the DMM reading is greater than the reference AGC voltage Adjust the signal generator frequency to 127 083 MHz Verify that the DMM reading is GREATER than the reference AGC voltage Adjust the signal generator frequency to 127 050 MHz and adjust the RF level to 10mV Verify that the DMM reading is LESS than the reference AGC voltage Adjust the
50. ply output Only make this adjustment with the transmitter keyed b Carrier Power Test Change the comm actv channel to 118 10 Mhz Remove the audio signal generator input to the MIC AUDIO HI input on the panel This test must be done with no modulation Key the transmitter and verify that the carrier power is greater than 5 Watts Repeat this test at 126 97 and 136 97 Mhz Frequency Stability Test Change the channel to 136 97 Mhz Monitor the output frequency with a counter Verify that the output frequency is between 136 974 and 136 976 Mhz 1 KHz error Modulation Capability Note Either of the following two methods may be used to perform this test Method 1 Modulation analyzer HP 8920B or equivalent Change the actv channel to 118 10 MHz Apply a 275 mV RMS audio signal at 1000 HZ to the MIC AUDIO HI input to the test panel NOTE The Mic AUDIO HI input has a 500 ohm input impedance Make sure that the audio signal generator is set up to deliver 275 mV RMS into this load impedance Key the transmitter and read the modulation depth on the modulation analyzer Verify that the modulation depth is between 70 and 90 Increase the MIC AUDIO HI input level to 2 75 Vrms and verify that the modulation depth is between 70 and 90 Repeat this test at 126 97 and 136 97 MHz Method 2 Oscilloscope Change the actv channel to 118 10 MHz Apply a 275 mV RMS audio signal at 1000 HZ to the MIC AUDIO HI in
51. put to the test panel NOTE The Mic AUDIO HI input has a 500 ohm input impedance Make sure that the audio signal generator is set up to deliver 275 mV RMS into this load impedance Key the transmitter and monitor output from the RF coupler on a 200 Mhz oscilloscope Trigger the scope off the demodulated audio from the linear detector 37 Verify that the modulation depth is between 70 90 using the diagram and formula below 9 Modulation x 100 A B Increase the MIC AUDIO HI input level to 2 75 Vrms and verify that the modulation depth is between 70 amp 90 Repeat this test at 126 97 and 136 97 Mhz Carrier Noise Level Test Change the actv channel to 118 100 Mhz Monitor the demodulated audio output from the linear detector with the audio analyzer Decrease the mic audio HI input level to 275mV RMS Key the transmitter and measure the demodulated output signal level Remove the modulation and verify that the demodulated noise level is at least 45 dB below the modulated level Repeat the test at 126 97 and 136 97 Mhz Demodulated Audio Distortion Change the actv channel to 118 10 Mhz Set the audio generator frequency to 350 Hz and with amplitude at 275mV RMS Measure the distortion of the demodulated audio output and verify that it is less than 10 Repeat at 1 and 2 5 KHz Repeat entire test at 126 97 and 136 97 Mhz Sidetone Level Change the active channel to 126 97 Mhz Monitor th
52. slot of the carrier squelch potentiometer on the comm receiver board the potentiometer counter clockwise until the audio is squelched if it is not already so SLOWLY turn the potentiometer clockwise until the squelch is just broken audio can be heard Run test procedure 4 3 3 17 k to verify that the carrier squelch has been adjusted properly 41 Comm Receiver Noised Squelch Tune the RF signal generator for 127 075 Mhz 30 modulated with a 1 KHz tone and an output level of 2uV Insert a tuning tool in the NOISE SQ hole in the top cover until it fits in the slot of the noise squelch potentiometer on the comm receiver board Turn the potentiometer clockwise until the audio is squelched if it is not already so SLOWLY turn the potentiometer counter clockwise until the squelch is just broken audio can be heard Run test procedure 4 3 3 17 to verify that the squelch has been adjust properly Transmitter Mic Gain Configure the equipment as shown in section 4 3 3 18 Tune the GNC 250 300 ACTV channel to 126 97 Mhz Apply a 275mV RMS audio signal at 1000 Hz to the MIC AUDIO HI input to the test panel NOTE The MIC AUDIO HI input as a 500 ohm input impedance make sure that the audio signal generator is set up to deliver 275 mV RMS into this load impedance nsert a tuning tool in the MIC GAIN hole in the tope cover until it fits in the slot of the mic gain potentiometer on the comm receiver board Turn th
53. test pattern illuminates all thirty five pixels in the first character column of each row and cycles through the remaining nineteen character columns The second test pattern illuminates the first column of pixels in each of the 20 character columns and cycles through the remaining four pixel columns The third test pattern illuminates the first row of pixels in each of the four character rows and cycles through the remaining six pixel rows c Observe the test patterns and verify that the display is working properly CDI Deflection Test a Rotate the outer function knob counter clockwise until the CDI Test Page is displayed flag 3 out of view in View b Turn the FUNCTION SELECT switch on the test panel to position 3 25 Press the CRSR key twice If necessary rotate the inner function knob to select CENTERED Verify that the DMM voltage is 0 15 millivolts Rotate the inner function knob clockwise to select FULL RIGHT Verify that the DMM voltage is 150 15 millivolts Rotate the inner function knob clockwise to select MAX RIGHT Verify that the DMM voltage is 300 30 millivolts Rotate the inner function knob clockwise to select MAX LEFT Verify that the DMM voltage is 300 30 millivolts Rotate the inner function knob clockwise to select FULL LEFT Verify that the DMM voltage is 150 15 millivolts 3 CDI To From Flag Test a b Rotate the outer function knob clockwise Turn the FUNCTION SELECT swit
54. th 5 watt minimum transmitter output power The unit is constructed from high quality materials and uses the latest techniques in manufacturing technology In order to achieve the desired reliability size and power requirements surface mount components are used extensively Specialized equipment and procedures are required to repair circuit boards having surface mounted components GARMIN does not authorize the repair of GNC 250 300 circuit boards All circuit boards and assemblies for the GNC 250 300 can be economically replaced through the GARMIN board exchange program if necessary A remote mounted battery pack with built in charger is available for the GNC 250 300 as well as a 28v to 14v converter assembly The following modules and printed circuit board assemblies are available for field replacement Display Board Assembly Contains the vacuum fluorescent display and display circuit board Rotary Switch Assembly Contains dual concentric switch and an attached flexible circuit Control Display Unit Contains bezel buttons lens keypad board light pipes plus the Display Board Assembly and Rotary Switch Assembly CPU Board Printed circuit board which contains the microprocessor LSI and power supply circuitry Interface Board Printed circuit board which connects the CPU board to the NavData card Communications Transceiver Module Contains VHF Receiver printed circuit board transmitter casing and altitude decoder circuits
55. ther detail below 1 2 2 1 Central Processing Unit CPU The CPU board is a microprocessor based computer board This board contains an Intel 80L186EB microprocessor running at 16 MHz a reprogrammable memory chip and random access memory chips RAM Data stored in RAM is maintained by a 3 volt lithium battery when the unit is switched off and by the regulated 5 volt supply when the unit is powered on A custom LSI is used to decode the signals from the GPS satellites A real time clock IC is used to keep track of the date and time Other circuits on the board are used for input output functions such as controlling the display reading the keypad and controlling the receiver Discrete input output lines are provided for CDI course deviation CDI to from flag CDI VAN flag Super Flag external annunciators OBS course GNC 300 only and to activate the Approach mode GNC 300 only Serial communication lines for OBI clock data sync ARINC 429 RS 422 GNC 300 only and RS 232 two channels are also included 1 2 2 2 Power Supply Circuits The power supply section of the CPU board outputs 60 and 40 volts for the display drivers 12 volts 15 volts and 5 volts VCC In addition the CPU board contains comparator circuits for detecting low Ni Cad battery level low memory battery voltage and external power The external power input is through the rear connector 1 and from the AC adapter through a DC power jack located near the rear con
56. unit Key the transmitter with the MIC KEY switch and verify that PTT ON is displayed Un key the transmitter and verify that PTT OFF is displayed Stuck MIC Key Test Turn the unit off and back on again in normal operating mode Key the transmitter by switching the MIC KEY switch in the UP position Maintain the MIC KEY in this position for approximately 35 seconds Verify that the unit automatically disables the transmitter and the MSG annunciator flashes NOTE If this test fails do not allow the transmitter to remain keyed for more than 60 seconds Press the MSG key on the unit and verify that the message Stuck mic Tx disabled is displayed Put the MIC KEY switch back in the un keyed middle position Press the key on the unit and then the MSG key and verify that the stuck mic message has gone away Stuck Remote Transfer Test Place the REMOTE TRANSFER switch in the UP position Verify that the ACTV and STBY channels change places Verify that after 2 seconds the ACTV channel is automatically tuned to 121 50 Mhz 40 Verify that after 35 seconds the MSG annunciator flashes Press the MSG key and verify that the message Remote XFR Key Stuck is displayed Press the transfer key lt gt on the GNC 250 300 and verify that the ACTV and STBY channels change places and that the ACTV channel DOES NOT tune back to 121 50 Mhz automatically Place the REMOTE TRANSFER key in the DOWN position Pr
57. ver as described in 3 2 2 1 3 Do not completely remove comm chassis sub assembly but swing it open on its hinge Remove the bottom shield cover 115 00016 00 from the CPU board and disconnecting the GPS receiver cable from the J6 CPU connector Remove the two screws 211 50207 05 which secure the GPS Receiver Assembly to the Main chassis Remove the GPS receiver assembly from the main chassis Remove the 11 screws 211 56201 05 which secure the GPS receiver outer cover 115 00113 00 from the casting 125 00025 00 Remove the four screws 211 50202 15 holding the GPS receiver inner cover to the assembly Lift the screws out along with the 233 00012 00 spacers Remove the inner GPS receiver cover from the assembly Pry out with a flat blade screw driver Remove the plastic screw 211 00018 00 holding the PC board to the casting To remove the BNC connector 330 001115 00 from the casting remove the four screws 211 50207 03 from the casting and pull the connector out Unsolder the BNC connector 330 00015 00 from the PC board Remove the PC board 012 00125 00 from the casting Carefully guide the cable assembly through the slot in the casting Power Volume Switch Assembly 011 00230 00 Removal Remove the covers as described in Section 3 1 Remove the Control Display Unit as described in Section 3 2 1 Remove the CPU Board Sub Assembly as described in Section 3 2 4 Remove the coupling sleeve 125 00027
58. ween CPU Board and Comm Receiver Board Replace comm chassis sub assembly see Section 3 2 2 steps 1 7 Replace modulator transistor 034 see Section 3 2 2 steps 8 14 Replace CPU Board see Section 3 2 4 Display does not show tx indicator when mic is keyed Verify connection to external mic key Verify connection between CPU Board and Comm Receiver Board Replace comm chassis sub assembly see Section 3 2 2 steps 1 7 Replace CPU Board see Section 3 2 4 17 3 DISASSEMBLY INSTRUCTIONS To avoid damaging the GNC 250 300 s circuit boards and assemblies the following procedures should be carefully followed Assembly drawings are included in Section 5 for reference Figures 11 to 14 3 1 3 2 3 2 1 3 2 2 REMOVAL THE COVERS 1 2 Remove the NavData card Remove the two M3 x 5mm screws 211 50207 05 retaining the top cover 115 00061 00 and the two 211 00024 00 screws Remove the top cover by sliding it rearward approximately 1 4 until the retaining lip is fully exposed The cover can then be lifted off the unit Carefully turn the unit over CAUTION the GNC 250 300 contains numerous static sensitive components Always observe proper anti static precautions Remove the bottom cover by sliding it rearward about 1 4 until the retaining lip is fully exposed The cover can then be lifted off the unit BOARD AND MODULE DISASSEMBLY Control Displ
59. y that the DMM voltage is greater than 4 5 VDC Rotate the inner function knob to turn the Appr annunciator to ACTV Verify that the DMM voltage is less than 0 3 VDC 28 10 Rotate the inner function knob to turn the Appr annunciator arm Verify that DMM voltage is greater than 4 5 VDC s Turn the FUNCTION SELECT switch on the test panel to position 19 t Verify that the DMM voltage is less than 0 3 VDC u Rotate the inner function knob to turn the Appr annunciator to off Verify that the DMM voltage is greater than 4 5 VAC v Rotate inner knob to select appr actv Verify that the DMM voltage is greater than 4 5 VDC External Switches Test GNC 300 a Press the CRSR key b Rotate the outer function knob two clicks until the External Switches test page is displayed bypassing the External COM Switches page c Toggle APPR SELECT switch on test panel and verify that the External Switches Test Page indicates APPR OFF in the down position d Toggle the OBS HOLD switch on test panel and verify that the External Switches Test Page indicates Hold OFF in the down position and HOLD ON in the up position Altitude Decoder Test This test exercises each altitude date line individually Most altitude codes presented will be invalid however ALTITUDE DECODER LINE SELECT switch positions 9 input line C1 valid and 410 input line C2 valid indicate altitude readings of 800 and 1000 feet respectively

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