Transponder issue KT76A

Dayron Nunez

Dayron Nunez

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Dayron N.
Hi guys. I have installed a KT76A classic transponder on a 1957 C172, did a bench test on it at a shop about 2 months ago and it was just fine. Last week I flew and was told by atc upon arriving to the terminal area the they were not getting my transponder, just raw primary radar, I cycled my box to stay and back to alt, hit the ident button and light was coming on on the box and all that, but they still could not get me.
Any ideas or suggestion for thing to try? Before I head back to the shop!

Thanks in advance

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Possibly the encoder. Often times just a failed connection between transponder and encoder.
 
The good thing about transponders is, that most of the time they are pretty cheap and easy to diagnose and fix. 2nd hand encoders are dirt cheap.
 
I'd check the antenna and connector. The cavity oscillator is sensitive to antenna missmatch and a bad enough missmatch will pull the Transponder of frequency.
 
If it checks out on the bench and fails in the airplane there is something going on with the airplane, wiring, coax or antenna issues. Even if the altitude encoder is bad (or its wiring) they should still see the transponder.
 
You had it checked in the airplane after the bench check before you flew it, right?

91.413 b Following any installation or maintenance on an ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), appendix E, of part 43 of this chapter.

If it's not plugged into the tray properly, they you won't get the right result.

Is the yellow light flashing when you fly?
 
I had the same problem. Turned out the antenna pigtail was bad. The shop explained that time and heat the wire resistance changes enough to cause problems. Cheap and easy fix.
 
Thanks for all the replies, I will go check on the antenna connection today and also look at the encoder wires, I did checked it on the first flight after installation and it was ok, actually it was fine on the flight that it failed, because I left out of a class D airspace and flew under controlled airspace at all times to my destination airport, and was only told about the failure when 12 miles out of destination.
 
Chip Sylverne said:
The shop explained that time and heat the wire resistance changes enough to cause problems. Cheap and easy fix.
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That's NOT the mechanism that causes coax to go bad, but it's always entertaining to hear how radio engineering shop folks simplify things down to the point they're not accurate anymore. :)

Take a piece of copper wire and heat it up and tell me how hot you have to get it to change its resistance after it has cooled back off. :)
 
denverpilot said:
That's NOT the mechanism that causes coax to go bad, but it's always entertaining to hear how radio engineering shop folks simplify things down to the point they're not accurate anymore. :)

Take a piece of copper wire and heat it up and tell me how hot you have to get it to change its resistance after it has cooled back off. :)
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Not very helpful if you don't close the circle. If not heat, what?
Bottom line, cleaning the connections did not, but replacing the pigtail did solve the problem, for whatever reason.
 
Chip Sylverne said:
Not very helpful if you don't close the circle. If not heat, what?
Bottom line, cleaning the connections did not, but replacing the pigtail did solve the problem, for whatever reason.
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Coax goes bad by the dielectric melting/softening and the center conductor "migrating" toward the outside of curves in bends... until it gets close enough to the outer braid that it doesn't have the original impedance characteristics, or it shorts out to the braid, changing the resistive characteristics. (e.g. a short)

The copper wire itself doesn't change characteristics over time... it's just copper wire. Vibration in aircraft makes the "migration" happen faster... happens in other mobile installations too, but airplanes, especially piston powered ones, are particularly wiggly. :)

But no avionics shop is going to bother explaining that. If they're smart, they'll just say the coax went bad.

When they start adding silly stuff like "the resistance changed" they're muddying the electronic waters.

The coax approved and used in aircraft is fairly crappy, really... there's better stuff on the market. But it's not FAA approved, and often doesn't have a jacket that won't emit toxic smoke if it's on fire... so avionics shops use the old crap. Even the stuff we were quoted to replace our "really old crap" in our airplane to get coax that can handle GPS frequencies, in my opinion was still crappy performing coax at those frequencies...

But it's sure better than the stuff they installed in the airplane in 1975, or maybe 1992... if they replaced the 1975 stuff when they put the King radios in... the paperwork is vague on whether the shop did or not... it shouldn't be... but it is.
 
denverpilot said:
The copper wire itself doesn't change characteristics over time... it's just copper wire. Vibration in aircraft makes the "migration" happen faster... happens in other mobile installations too, but airplanes, especially piston powered ones, are particularly wiggly. :)
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Do you need An explanation why coax has such a large minimum bend radius, with the single wire center conductor? It's not just the dielectric.
 
I went to the airport today as I was nearby anyways and checked that if I press ident the light will illuminate on the box, but when I set it on ALT or TEST position I don't see any lights!
 
GlennAB1 said:
Do you need An explanation why coax has such a large minimum bend radius, with the single wire center conductor? It's not just the dielectric.
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No. It's pretty obvious.

Usually I use hardline when I can for my stuff. It has limits too, but I'll never have to worry about center conductor migration.

Do have to worry about bullet holes in it, though. Why some people shoot at radio towers, I'll never fathom.

The loss numbers on most of the Aviation stuff when up in the GHz are terrible.

Even the venerable LMR series of coax that's been a staple of "flexible" connections on the ground is getting old by modern standards and getting something of that performance level aboard an aircraft is nigh on impossible it would appear.

And there's better stuff than LMR now.
 
Dayron Nunez said:
I went to the airport today as I was nearby anyways and checked that if I press ident the light will illuminate on the box, but when I set it on ALT or TEST position I don't see any lights!
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The light illuminates when a radar signal strong enough to be received by the transponder triggers it to transmit back. There's all sorts of places on airports that might not be reached by an FAA radar. Or if you're inside a typical steel hangar, welcome to being in a weak Faraday cage.

A lack of being "interrogated" by a radar on the ground is somewhat meaningless unless you know you've normally seen the transponder responding from that exact location before.

The light comes on when you hit IDENT because you're forcing the transponder to transmit without being interrogated. Not to mention if you ARE within range of a radar site, you just flashed your target on all the controller's screens fed by that radar. IDENT means "transmit right NOW".

TEST however for most transponders will illuminate the light. That one might be a bad sign you have something wrong.

Someone else said it already but I'll repeat it. You said you had the transponder out of the aircraft recently. The most common problem after bench work is connections. If you didn't get it back into the tray properly or a pin got bent or broken or ... all sorts of things... well, as the other person said, it's not supposed to be put back in the panel without a proper test on the aircraft. That test includes putting a test unit and its antenna at particular locations around the aircraft and interrogating the transponder like a regular radar site would and measuring its response through the complete system, connectors, cable, and antenna. The test gear gives a report on how much power it took to interrogate the transponder and how much power it responded with amongst other things it tests.

Was the transponder tested properly when it was reinstalled? Start there.
 
denverpilot said:
No. It's pretty obvious.

Usually I use hardline when I can for my stuff. It has limits too, but I'll never have to worry about center conductor migration.

Do have to worry about bullet holes in it, though. Why some people shoot at radio towers, I'll never fathom.

The loss numbers on most of the Aviation stuff when up in the GHz are terrible.

Even the venerable LMR series of coax that's been a staple of "flexible" connections on the ground is getting old by modern standards and getting something of that performance level aboard an aircraft is nigh on impossible it would appear.

And there's better stuff than LMR now.
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I have an old friend that was installing STC'ed satellite entertainment systems on airliners, I may ask him what coax they are using.
 
GlennAB1 said:
I have an old friend that was installing STC'ed satellite entertainment systems on airliners, I may ask him what coax they are using.
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That'd be interesting. Of course they only need that run to go from the sat antenna to the sat receiver and they probably push D.C. power down it to an active amplifier inside the antenna which helps alleviate any problems with line loss coming back to the receiver.
 
denverpilot said:
That'd be interesting. Of course they only need that run to go from the sat antenna to the sat receiver and they probably push D.C. power down it to an active amplifier inside the antenna which helps alleviate any problems with line loss coming back to the receiver.
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Yes, and the antenna, I've seen in pictures, is on top of the fuselage (naturally), back by the tail. The receiver may be in the ceiling right by the antenna. I texted him, will see if he responds.
 
Coax is a waveguide and has "characteristic impedance" (and the unit is ohms so it can be referred to as resistance) that is dependent on the geometry of the cable, not the resistance of the wire. Put a DC ohm meter on a 50 ohm coax and you will never see 50 ohms. It doesnt work that way. It is 50 ohms for the frequency range the cable is built for. So the cable changing shape can and does affect the characteristic impedance aka resistance of a coax.

Its hard to understand, but just remember, you cant measure coax impedance with a DC ohm meter. It takes a $10k instrument to measure it although there are complicated ways to measure it with less expensive equipment.
 
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Clean the antenna first. Maybe won't fix the problem, but it will make you feel good.
Never trust a single ATC report of no transponder to be definitive.

I know you're just making an example Colorado, but there isn't likely 75 ohm cable in the aircraft. While you can't measure impedence or do fancy things with a DC meter, you can measure simple continuity. The cable conductors should have near zero resistance end to end and infinite between them (when disconnected) at DC.
 
Good point. Aircraft coax is typically 50 ohm.
 
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I've had mine (same model) not work twice. Once we forgot to reattach the coax after doing some work under the belly pan. Maintenance error.
The second time ATC complained (a couple years later) we pulled the unit, cleaned the terminals, reinserted, and it fixed it. Possible maintenance error. It might have been pulled and reinserted by the A&P who installed my new ELT. (Not my regular A&P). He might not have seated it very well.
YMMV.
 
flyingron said:
Never trust a single ATC report of no transponder to be definitive.
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This. No kidding, just yesterday on my flight to AKO, the ZDV controller on the way out told me they "lost me on radar and I won't pick you back up"... they obviously knew of a problem with the radar covering the KAKO area.

On the way back, I called for Flight Following a little bit before that point where now I knew the previous controller had lost me, and the new controller on shift was quite worried sounding that he couldn't see me out there.

Someone missed a memo somewhere.

And I *assume* but don't know for sure, that probably SOME aircraft do show up out there if they're already doing the consolidated image thing at ZDV with ADS-B. You'd be seen via UAT out there via a tower, I'm pretty sure. If you were UAT and not doing it via the transponder.

Don't know but the controller had me try various stuff, even had me wondering if the old transponder had finally bit the dust, because I wasn't seeing return hits on the test light... but... I knew it was their end when I offered to IDENT for him and he never saw it. 250W of forced transmit RF to where I *know* that radar lives, is going to make it there.

So anyway... with that and multiple other stories over the years of controller's stuff being the culprit in multiple airplanes and not the aircraft having a real problem when tested... always take a "I can't see your transponder" with at least a small grain of salt.

Nobody had any trouble seeing it once I was into the coverage of the terminal area radar for DEN. And no complaints since then.
 
I've had a KT-76A for about 23 years. In that time my endoder has required attention a couple of times and my transponder itself required major attention once, done under warranty authorization many years after the warranty had expired. I figure that means there was a known problem. It's worked well since.
 
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Hey....what ever it takes for you to discharge....knock your socks off. ;)
 
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