BACK         SIMPLIFIED PRINTABLE TROUBLESHOOTING GUIDELINES

 

 

 

 

 

 

Text Box: START HERE These following 5 conditions indicate the transponder needs repair.  If any of the following occur there is no need for you to troubleshoot possible wiring or antenna problems.  Follow instructions for removal of transponder (below) and send it to us for repair.  1. Transponder is intermittent even though it's getting DC power. 2. Wrong squawk codes.  Example:  ATC sees 1600 instead of 1200. 3. Missing squawk codes, yet altitude is reporting accurately. 4. One ATC sees you fine, while the next cannot.  Fading in & out. 5. Wrong or missing altitude (could also be encoder.  See ALTITUDE REPORTING ISSUES below). Text Box: BEFORE YOU LEAVE FOR AIRPORT Gather your tools.  You'll need  3/32 & 5/64 allen wrenches, small slotted screwdriver and a flashlight at the minimum.  You may also need a voltmeter with test leads.   Text Box: The most common reasons for transponder failures are listed below. 1. Internal problems inside the box itself. 2. DC power from the circuit breaker or wiring has failed. 3. Antenna is broken, or contact between antenna & unit is severed. The following 6 conditions are signs of a healthy transponder: 1.   Reply lamp comes on during warm-up. 2.   A high pitch whine might be heard if you listen very closely. 3.   Reply lamp goes off in about 20-30 seconds. 4.   Reply lamp will come on if unit is placed in test mode. 5.   Reply lamp will come on briefly when IDENT button is pressed. 6.   Reply lamp flashes during flight. If all 6 occur, and your aircraft cannot be seen by ATC, your transponder needs to be repaired. If none of the 6 occur it could be the transponder.  It could also be a wiring problem with the aircraft. Check DC power input to be sure. If all but item #6 occur it could be the transponder.  It could also be a bad antenna or poor connection between transponder & antenna. Text Box: REMOVE TRANSPONDER FROM RACK (NOTE: Some units use a small screwdriver rather than an allen wrench.) Locate the release hole located on the front panel, usually below the five knobs and towards the middle. Gently insert the allen wrench into the release hole and engage.  Rotate the allen counterclockwise about 5 turns until it free-wheels.  The unit will slide towards you about 3/8". Grab onto the sides of the unit and pull it toward you. Do not pull by the knobs or they may break. Re-installation is the reverse of the above process.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Text Box: ALTITUDE REPORTING ISSUES If your transponder reports an incorrect altitude that is always off by about the same linear amount, no matter what your actual altitude, the encoder must be removed and adjusted.  For example an error of +300 feet would show you at 1300 ft when you are at 1000.  It would also report you at 5300 ft when you are at 5000. If your transponder reports a highly erratic altitude, often thousands of feet off, but only about 20% of the time (meaning it's accurate about 80% of the time), it is probably not the encoder. This type of problem is usually with the transponder, but could be from a broken wire, splice or pin from the harness that runs between the transponder and encoder.  Look under the panel and find the encoder and it's harness.  If you do not see an obvious problem, send both transponder and encoder for a bench test.

 

 

 

 

 

 

 

 

Text Box: CHECK DC INPUT 1. Using your flashlight look into the transponder's slot towards the back of the tray and locate 2 connectors.  The rectangular multi pin connector has the DC power input pin. 2. Using the chart identify the exact pin for your type of transponder. 3. Set your voltmeter to measure either 12VDC or 24VDC. Find a solid ground for the black probe (the tray is a good grounding point). 4. Turn on aircraft master and avionics master.  Place red probe on power input pin. If power is present, your transponder needs repair. If there is no DC power you may have a wiring issue, However do this first:  Recheck the chart and your connections.  Test your volt meter by measuring power at a live circuit breaker. If DC power input is still dead you will need to follow the line to the circuit breaker and find where it is severed. Your transponder should operate after power is restored. MODEL NUMBER PIN # LOCATION AS VIEWED FROM INSIDE TRAY KT-76, KT-78 5 5TH PIN KT-76A, KT-78A, KT-76C 11 TOP ROW 2ND PIN KT-79 11 & 12 TOP ROW 1ST & 2ND PINS ALL NARCO TRANSPONDERS 16 3RD PIN TDR-950, TDR-950L 14 2ND PIN RT-358A, RT-459A, RT-859A 22 BOTTOM ROW 1ST PIN ALL BENDIX TRANSPONDERS 1 1ST PIN GTX-320, GTX320A, GTX-327 15 TOP ROW 2ND PIN SL-70 1 TOP ROW LAST PIN CHECK ANTENNA Your transponder's antenna will be belly mounted, generally near the front half of the aircraft.  It is either a metal whip with a ball on the end or a plastic blade, each a little over 3 inches in length. The most common antenna problems are from broken, shorted or severed coax lines (coax is the wire leading to the antenna), disconnected or damaged coax connectors, or damaged or missing antennas. 1. Testing will require the use of your voltmeter to perform a continuity test.  Place your meter in the Ohms Ώ mode.  Touch the probes together and note how your meter reacts. Remove the transponder from it's tray and note the 2 connectors, one round and one rectangular. 2. If your antenna is a metal whip, attach black probe to antenna.  Momentarily touch the antenna with red probe.  If meter reacts it means black probe has good contact. 3. Next touch the center pin of the round (antenna) connector at rear of tray with the red probe (you may need to temporarily splice in some extra wire if probe is not long enough).  If you have continuity and antenna appears undamaged, the transponder may need repair.  Let's try one more test first. Keep black probe on antenna. 4. Now touch the red probe to ground.  If you have continuity it means your coax is shorted and transponder may not need repair. If you have the blade type (plastic) antenna you must first remove the coax connector at the antenna.  Connect the black probe to the inner pin of the connector and test as above.  This is more difficult because you will need to gain access to the connector under the floor.  DME blade antennas are identical and it is possible to confuse them.  Some blade type antennas will appear "shorted" if you attempt to test them as in step 4 above.  Always disconnect a blade antenna before testing for shorted coax.