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Old 09-05-2011, 04:12   #1
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Transponder Issues

Started having intermittent transponder problems a little while back. ATC would lose my mode C, but still have my mode A, and periodically they would lose both, and sometimes they get it back, and sometimes not.

Took it to an avionics shop, and they did a number of things, none of which corrected the problem. They changed the antenna wire from the tray to the pigtail, installed a new antenna, swapped out the actual transponder, but none of these fixed the problem.

Generally seems to work for about 30-45 minutes before it starts acting up...normally. Even when its acting up, and they are not getting anything from me, I still get a reply light on the box, so the unit is still getting power.

Seems to me that there is likely a wire that is not making a solid connection somewhere, and the heating up of the firewall area is causing some kind of expansion that is loosening things up. Just a wild guess.

Anyone have any ideas that could point me in the right direction on fixing this nightmare?? Could this be a grounding issue of some sort? Loose connections on the harness?

I have 2 encoders in the airplane, and when the problem is occurring, switching encoders doesnt seem to affect anything.

BTW: Its a PA28-180, and the transponder(s) is a narco AT50A.

Thanks for the help!
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Old 09-05-2011, 14:28   #2
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Without a wiring print its hard to say. My first guess would've been a bad coax to the antenna, antenna, or antenna connections.

What about suppression? Not sure on your set up but perhaps there transponder suppression inputs are the issue.
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Old 09-06-2011, 11:56   #3
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The most common issue I see with transponders is the coax, make sure the ends are in good condition (give the BNC a good pull, 50/50 it'll come off in your hand if it's old and oily from 40 years of grime). Check the antenna (I'm assuming it's a blade type) to make sure it is secure, they get smacked a lot when scrubbing the belly with a brush, could be halfway broken while still looking good from a distance.
Like you said, check for good grounds and power, while giving the wiring haress a good wiggle. If you have a Ryan TCAD installed there may be a suppression issue, check for correct installation of the supressor box, the coax routing is very specific and the grounding requirements must be adhered to.
after checking all the possibilities of a physical fault (broken coax,wire,antenna) have the avionics shop check the power output and sensitivity.
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Old 09-06-2011, 19:43   #4
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Have you swapped transponders? if the known good transponder does the same thing the old one does, it's a wiring/antenna issue in the aircraft.
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Old 09-08-2011, 04:39   #5
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PA-28 probably only has 1 transponder.
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