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Transponder Question

petehowell

petehowell

Well Known Member
Hello,

I have an older transponder and I think there is an issue with it. The reply light flashes intermittently when on the ground and it is on almost solid when in the air. ATC does not report any issues, mode c is dead on, and indent works. It is causing an issue with my traffic scope.

I have heard that a bad antenna cable could cause something like this.

Any thoughts?
 
Only a few components

petehowell said:
Hello,

I have an older transponder and I think there is an issue with it. The reply light flashes intermittently when on the ground and it is on almost solid when in the air. ATC does not report any issues, mode c is dead on, and indent works. It is causing an issue with my traffic scope.

I have heard that a bad antenna cable could cause something like this.

Any thoughts?
Click to expand...
Pete... if it's an old KT-76 there are only about 3 or 4 parts in the "blinking light" circuit.

Power for the "blink" is picked up off a wire loop next to the UHF circuitry. IIRC a diode, transistor and photocell make up the circuitry driving the light.

The photocell apparently has a low reliability, and can make the "blinking light" either come on too much, or act very dim (my case).

If you can find a friendly radio shop that doesn't want to do an entire Yellow Tag certification on the transponder or ship it back to the maker, it should be an easy fix.

Try asking the locals where they take their old radios (such as KX-170s) to get fixed.... :)

The above is the case for some KT-76 models, I imagine other old units are similar...

gil A
 
Pete,
The oscillator in the front end of the receiver is not working and the transponder is replying to electrical noise. This is a simple adjustment that will take a shop about a minute to do. At the worst a mixer diode or tripler diode will be bad. If it takes the shop more than 5 minutes to diagnose I'd be surprised. This is very minor and the last thing you will need is a new transponder.

Best,
 
Thanks Bryan

I'll have them take a look at it. That makes sense -as it seems to be working from the ATC viewpoint - I did think it odd that it would randomly reply in the hangar at times.
 
My KT-76A does the same thing. It has done it since brand new. I had it checked out by two different shops and neither one could find anything wrong with it. It passes the bi-annual check and I've never had ATC complain about it (except when I leave it in standby, oops :eek: ).

I just ignore it now.

Karl
 
the 150 does it

the 150 i use to fly did this the whole time i flew it. over about 3 years. atc never had a problem with it.
 
don't know what area you are flying, but nowadays soooo many people have tcas that you just may be getting interogated that much. As long as atc doesn't complain don't worry about it.
 
Xponder

Wonder if it's the "Fish Finders" on the airliners and other collision avoidance equiped aircraft. I have had this same symptom in two KT-76A's in my -8. Transponder works fine according to ATC.

My hangar is directly underneath the approach from the North East into ATL, so it isn't unusual to look up and see 1 to 4 airliners at any given moment. (Bring on the "user fees"!!!!!!) :mad:
 
I'm OK you're OK

Sounds like this might be common. As long as ATC is ok with it, I'll just leave it alone.
 
Try this... The test feature on the transponder is a circuit that injects noise into the receiver and forces a reply. Hold the switch in the test feature and watch the reply lite. If the rapid flashing is similar to what you are experiencing the problem is definetely what I described above. As for the fishfinder queries... The reply that you should see from the TCAS interegations will look will look like the normal atc interegations as you look at your xpndr.

Best,
 
Test switch

Bryan,

When the I go to test mode on the ground, the reply light comes on steady vs flashing. In the air, same thing, except in the air the flashing (without test switch) is more rapid.
 
petehowell said:
Bryan,

When the I go to test mode on the ground, the reply light comes on steady vs flashing. In the air, same thing, except in the air the flashing (without test switch) is more rapid.
Click to expand...

Very nice! You have your problem.
 
Found the issue

I found the cause for the runaway transponder replies. This summer, right before the Big Sky trip, I switched the top plugs from std BR8ES auto plugs to iridium style auto plugs. I received an off list message telling me to look at the ignition system - sure enough the plug change matched up with the Transponder issue time frame. The iridium plugs are resistor types, but they appear to kick off more electrical noise.

Put in new plugs all around last night and things appear to be back to normal.

Cheers,

Pete
 
Found the Real issue

My transponder problem cropped up again a few weeks ago about the same time I got an email from a Cozy pilot that had similar issues with his transponder. He traced it to his spark plug leads on his LSE EI system. He replaced the wires and solved the problem.

I replaced my plug wires last night at the hangar and was able to get 1.1 in this morning. Giddy up - no more constant replies and the TrafficScope is usable again. :D

I must have wiggled a bad wire when I removed the irridium plugs temporarily solving the issue. Hope this might help someone else.
 
great ideas

Thanks All

It sounds like I need to check that antenna connector before sending the transponder in for repair.
 
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