TRANSPONDER REPAIRS    
            

                              FLIGHT CONTROL SERVICES FAA CRS#OFHR414L 

                        USE THE ORDER FORM LINK ABOVE TO ORDER SERVICE

 Updated DEC 2023

THIS SECTION COVERS THE THREE MAIN AIRFRAME RELATED ISSUES THAT COULD CAUSE A TRANSPONDER FAILURE.  IT IS ASSUMED YOU HAVE PERFORMED THE FUNCTION TESTS AND MAY HAVE AN IDEA WHERE THE PROBLEM IS. 

YOUR DECISION TO PROCEED WITH ANY OF THE TESTS IN THIS SECTION MUST BE WITH THE UNDERSTANDING THAT THIS IS MAINTENANCE AND REQUIRES AT THE VERY MINIMUM THE SUPERVISION OF A LICENSED MECHANIC.  THIS INCLUDED REMOVAL OF THE TRANSPONDER FOR A SIMPLE VOLTAGE TEST, AND IT'S REPLACEMENT.  PERTINENT FAA DATA CAN BE FOUND IN THE RESEARCH LINK OF THIS SECTION.

  POWER ISSUES 14VDC BUSS                     IF YOUR AIRCRAFT HAS A 24VDC BUSS

THE TRANSPONDER MAY NOT BE RECEIVING THE POWER REQUIRED TO FUNCTION. IT MAY ALSO BE AN INTERNAL PROBLEM WITH THE TRANSPONDER POWER SECTION.

STEP 1:  MAKE SURE THE TRANSPONDER IS FIRMLY MOUNTED IN IT'S TRAY AND REAR CONNECTOR IS ATTACHED.  INSPECT LATCH.  IF INTERNAL NIGHT LIGHTING WORKS YOU MAY ASSUME THE CONNECTOR IS ENGAGED.

STEP 2:  WITH MASTER SWITCH ON APPLY POWER TO THE UNIT WITH YOUR EAR CLOSE TO UNIT LISTEN FOR A HIGH FREQUENCY WHINE.  PLACE UNIT IN STANDBY MODE AND PRESS IDENT WHILE WATCHING REPLY LAMP.  IF UNIT HAS A REPLY LAMP DIMMER ASSURE IT IS FULL CLOCKWISE.  IF IT HAS ELECTRONIC DIMMING SHINE A FLASHLIGHT INTO LIGHT SENSOR ON FACE AND PUSH IDENT AGAIN. TURN MASTER OFF.

STEP 3:  CHECK FOR TRIPPED/DEFECTIVE BREAKER OR BLOWN FUSE.  WITH MASTER ON TEST BOTH LOAD AND BUSS VOLTAGE (SHOULD BE IDENTICAL) AND MEASURE AGAIN WITH TRANSPONDER ON.  RECORD VOLTAGE TO COMPARE WITH NEXT STEP.

STEP 4:  REMOVE THE TRANSPONDER FROM TRAY.  INSPECT THE PINS NOTING SOME MAY BE MISSING HOWEVER THEY SHOULD ALL LOOK SIMILAR AND NONE BROKEN OR "SPRUNG".  FIND THE POWER INPUT PIN USING CHART BELOW, TURN ON MASTER AND TEST FOR VOLTAGE.  IF THERE IS VOLTAGE AND IT IS THE SAME AS PREVIOUSLY MEASURED TURN MASTER OFF.  GENTLY PUSH ON POWER PIN TO ASSURE IT IS LOCKED INTO SLOT. (OFTEN AN UNLOCKED PIN WILL SLIDE OUT OF IT'S SOCKET AND "SPRING" BACK INTO POSITION WHEN UNIT IS REMOVED).

MAKE SURE TRANSPONDER TRAY IS MOUNTED/ATTACHED TO AIRFRAME GROUNDED RAILS.

IF STILL NO POWER CONSIDER RUNNING NEW WIRE FROM BREAKER.

IF YOU ARE CERTAIN UNIT HAS POWER BUT SHOWS NO ACTIVITY IT NEEDS AN INTERNAL REPAIR

  IF YOU HAVE AN ARC (CESSNA) TRANSPONDER.

ANTENNA ISSUES

STEP 1: DETERMINE TYPE OF ANTENNA, BLADE OR WHIP AND INSPECT (BOTH ARE ABOUT 3 INCHES LONG).  A BLADE SHOULD HAVE NO CRACKS WHILE THE WHIP MUST SHINE (AT MICROWAVE LEVEL THE RF TRAVELS ON THE SURFACE OF AN ANTENNA CONDUCTOR. IF THE WHIP IS PITTED, BENT OR DIRTY IT WILL IMPACT THE REPLY FREQUENCY.  IF A WHIP ANTENNA TWISTS OR TURNS FREELY AROUND IT'S AXIS IT IS PROBABLY DEFECTIVE.

STEP 2: USE AN OHM METER TO TEST CONTINUITY DIRECTLY FROM THE WHIP ANTENNA TO THE CENTER CONDUCTOR OF THE TRANSPONDER'S ANTENNA CONNECTOR.  USE THE SAME METER TO TEST FOR A SHORT ACROSS THE CENTER ANTENNA CONDUCTOR AND GROUND.

STEP 3: REMOVE BNC CONNECTOR AND INSPECT CENTER PIN.  FIRMLY PRESS A FINGERTIP ON/INTO THE CENTER OF THE BNC, AND WITHDRAW IT. NOW LOOK AT YOUR FINGER AND OBSERVE THE CIRCULAR IMPRINT LEFT BEHIND.  MORE IMPORTANT IS THE TINY DOT IN THE CENTER LEFT BY THE BNC CENTER PIN.  IF THERE IS NO DOT THE BNC CENTER PIN HAS PUSHED BACK.

DO NOT ATTEMPT TO PULL IT BACK AS YOU MAY ALSO PULL THE COAX SHIELD INSIDE THE BNC AND SHORT OUT THE TRANSPONDER.  HAVE THE BNC REPLACED.

IF ANTENNA IS A BLADE YOU MAY TEST BY REMOVING BNC CONNECTOR FROM ANTENNA AND CHECK FOR CONTINUITY OF CENTER CONDUCTORS ON BOTH ENDS.  DON'T FORGET TO TEST FOR SHORT ACROSS GROUND.

MODE C ISSUES

MODE C ISSUES ARE THE HARDEST TO TROUBLESHOOT IN AIRCRAFT WITHOUT SPECIALIZED EQUIPMENT AND EXPERIENCE.  THE PROBLEM CAN ORIGINATE IN THE ENCODER, TRANSPONDER, THE WIRING THAT CONNECTS THEM, AND EVEN THROUGH AN INTERFACE SUCH AS A GPS.

IF A CONTROLLER ADVISES HE HAS LOST YOUR MODE C, ASK IF HE CAN STILL SEE YOUR MODE A SQUAWK. THIS COULD BE THE DIFFERENCE BETWEEN THE LOSS OF YOUR ALTITUDE REPORTING OR THE ENTIRE TRANSPONDER.

ENCODER CAUSING ERROR   (MODE C WORKS JUST NOT ACCURATELY)

LINEAR ERROR:  REPORTS THE SAME ERROR NO MATTER WHAT ALTITUDE.  FOR EXAMPLE 300 FEET ERROR AT 1000 AGL, 300 FEET ERROR AT 10,000 AGL, AND SO ON.

PROGRESSIVE ERROR:  THE ERROR BECOMES GREATER THE HIGHER (OR LOWER) YOU FLY. 

BOTH LINEAR AND PROGRESSIVE ERROR ARE CORRECTED THROUGH ALIGNMENT OF THE ENCODER.  IT IS UNLIKELY THE TRANSPONDER IS AT FAULT.

SOLUTION:  HAVE THE ENCODER REMOVED AND ALIGNED.

TRANSPONDER OR WIRING CAUSING ERROR  (MODE C MAY WORK BUT IS OFTEN EXTREMELY INACCURATE)

UNFORTUNATELY THERE IS NO EASY TROUBLESHOOTING SOLUTION FOR THIS CONDITION WITHOUT THE PROPER TEST EQUIPMENT, OTHER THAN PROCESS OF ELIMINATION.  THE COMMUNICATION BETWEEN YOUR TRANSPONDER AND IT'S MODE C ENCODER IS THROUGH A SERIES OF 9 OR 10 WIRES AND THE CIRCUITRY ASSOCIATED.  IF ONE OF THESE WIRES (OR CIRCUITS) IS NOT COMMUNICATING THE TRANSPONDER MAY REPORT EITHER ACCURATELY, THE WRONG ALTITUDE, OR NOT AT ALL.

SOLUTION:  WE SUGGEST YOU HAVE BOTH THE ENCODER AND TRANSPONDER REMOVED FOR A BENCH TEST.  IF BOTH CHECK OUT THEN ADDRESS A WIRING ISSUE.

ALTERNATELY, CHECK THE ENCODER CONNECTOR, LOOSE WIRES IN THE BUNDLE TRANSPONDER PROPERLY SEATED AND TRANSPONDER PINS NOT BROKEN OR SPRUNG.

FLIGHT TESTING TO TROUBLESHOOT MODE C ISSUES IS NEVER ADVISABLE.

  CLICK NEXT FOR FLIGHT TESTING ADVICE