possible KT-76A transponder malfunction

possible KT-76A transponder malfunction

Hello,

I bought an Archer a couple of months ago, which has been a great plane for me so far.

In the avionics stack is a KT-76A transponder. I do not know exactly how old it is. It is at least ten years old, but possibly much more. The plane is a 1976, and the avionics were reworked in the mid-90's with a King Silver Crown package.

When flying IFR, or with flight following, I am often told by ATC that they are not receiving my transponder, and asked to recycle. I flew about six hours of IFR yesterday and it happened twice. After unscientifically fooling around with the transponder and other avionics for a few minutes, the controller usually says the signal is back. This has been happening since almost the day I took position, as the second day I had it I was departing an airport in IMC and a controller asked me to reset (very unnerving with a new plane).

I know older model transponders can be intermittent if they get hot, but I live in Seattle and excessive heat buildup isn't as big of an issue as in other parts of the country. Other components, such as Nav/Coms, the GPS, and the ADF all work well, although the DME (located immediately above the xponder in my installation) only receives intermittently as well.

Has anyone have any similar experience? What has been the problem and what did it take to resolve? I'm obviously planning to get in touch with avionics shops about the issue, but I am looking for some direction ahead of time. I'm hoping not to have to replace several components at an unscrupulous shop's recommendation when a quick repair, or replacement of a single component would do.

Any useful thoughts out there would be very much appreciated! Best regards.

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Re: possible KT-76A transponder malfunction

I has a similar problem with my KT76A transponder. It turned out to be a bad capacitor in the circuitry.

Bill L
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Re: possible KT-76A transponder malfunction

That doesn't sound too bad, thanks. I heard from another source that sometimes the transponder antenna gets dirty as it is usually in the airflow from the exhaust, and that a good scrub could resolve transponder issues. That sounds like a good free thing to try.

Thanks, Bill. Any other thoughts from anyone?

Regards.

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Re: possible KT-76A transponder malfunction

wouldnt hurt to rerack it either.

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Re: possible KT-76A transponder malfunction

Look at avionicsmasters.com, they offer a flat fee $350 repair on any transponder, but that does not include if you need a new cavity/tube, which get weak with age on the old xpdrs. I've never used them, so can't tell you anything about them, just saw their ad. I also hear that Sarasota Avionics repairs the King xpdrs at a reasonable price.

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Re: possible KT-76A transponder malfunction

Mike,

I had the exact same maddening problem with mine. When the shop pulled it out and bench tested it, the said it would report altitude intermittently.  They sold me a reconditioned one for $500 which I thought, at $100/hr to fix it (and not knowing what the problem was), was a pretty good deal.

Since then another avionics shop told me that there are two well-known causes for this in the KT-76A, and both involve dirty electrical contacts within the instrument.

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Re: possible KT-76A transponder malfunction

I thought I'd post an update to this since I have a solution, one that worked for me anyway.

I emailed two avionics shops with a description of the problem. One of the shops was one that I see advertising all the time in TAP and other aviation magazines. The second shop was a smaller shop at a local GA reliever airport (read: lots of bizjets) that I had read catered to the Gulfstream crowd.

The first shop tried to schedule me to have a new transponder installed without really inquiring much about the actual root cause.

The other shop said, "Yes, you can either remove unit from aircraft and send/bring into us. Or you can schedule a time to bring the aircraft to us. I do recommend looking at your transponder antenna on belly of aircraft and make sure its clean. Its usually in the exhaust flow. So it gets covered with oil and exhaust stuff. This will cause it to work poorly.

"If that does not clear the problem. Contact us about troubleshooting box or aircraft."

Well! Wash the transponder antenna - what a concept! We had washed the plane once in the three months since we purchased it, but ignored the belly, and I can't say when the most recent time it was washed prior to our taking ownership of it.

There was a whip-style antenna, about three inches long with a ball on it, and there was a blade style antenna that said King on it. I am not sure which was the transponder and which was the DME antenna, but I scrubbed them both prior to the next flight. The blade antenna had a lot of scum on it that came off. Not as much with the whip antenna, though.

But I have since flown the aircraft more than 20 hours and haven't had a single controller tell me ATC wasn't receiving my transponder.

I love simple, inexpensive solutions! And I was surprised that the local shop sent me that email that was so helpful, even though I'm not a G-II pilot (but I play one on TV).

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Re: possible KT-76A transponder malfunction

I am having a similar issue in my 1976 Archer.  A couple times when mine has "failed", when I turn it to "Test", the interrogation light does not come on.  Did you have the same symptom when yours failed?  I would like to believe cleaning my antenna will resolve, but suspect mine is more of an electrical issue.

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Re: possible KT-76A transponder malfunction

Sorry, but I don't know what the behavior was in the "Test" position. Don't think I ever tried that.

Hope your xponder issue isn't too serious.

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Re: possible KT-76A transponder malfunction

My KT76 was acting up, sent it to Sarasota Avionics, needed alignment, $150.

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Re: possible KT-76A transponder malfunction

mars1259 Wrote:
-------------------------------------------------------
> My KT76 was acting up, sent it to Sarasota
> Avionics, needed alignment, $150.


Common problem for "cavity" transmitters.  The antenna system can pull the transmitter off frequency which also causes changes in the pulse width.  In the end, far enough off, and no one sees you.  It take about 30 minutes to adjust if nothing more serious is wrong.  Usually a ramp test, then a bench test will show what the antenna system is doing to the signal. 

Happy flying,

Bob

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Re: possible KT-76A transponder malfunction

OK, my KT 76 is acting up again. And I was wrong, the front of the unit actually says KT 76 TSO so I don't think it's really a KT 76A.

I am considering just replacing it with a more modern unit, like a GTX-327. I'd like the convenience of just pushing the VFR button when squawking VFR, etc.

The GTX-327 is a 200 watt output unit. What I don't know is how many watts the KT 76 TSO is, and I can't find a reference on line.

Any ideas?

Regards...

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Re: possible KT-76A transponder malfunction

Michael Gibbons [KRNT] Wrote:
-------------------------------------------------------
> OK, my KT 76 is acting up again. And I was wrong,
> the front of the unit actually says KT 76 TSO so I
> don't think it's really a KT 76A.
>
> I am considering just replacing it with a more
> modern unit, like a GTX-327. I'd like the
> convenience of just pushing the VFR button when
> squawking VFR, etc.
>
> The GTX-327 is a 200 watt output unit. What I
> don't know is how many watts the KT 76 TSO is, and
> I can't find a reference on line.
>
> Any ideas?
>
> Regards...

Take a look a the Narco plug and play model, I believe they have one for your unit. It has a couple of nice features and has a VFR button. It's a lot less expensive than the Garmin and just slides in your existing tray.

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Re: possible KT-76A transponder malfunction

Michael, when my transponder started to act up like yours is,  I didn't have the option of a plug in alternative since my old one was a Collins for which there isn't one, so I installed a Garmin 327 which has been flawless for a few years now.  Not cheap but one less headache and sure is nice to use and has a few features the old ones didn't such as instant on, flight timer etc.  I gave my old one to a friend who is flying an experimental on a limited budget since on the bench it tested good on transmit but was weak on receiving so the tube still worked though almost 20 years old.

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Re: possible KT-76A transponder malfunction

I have an older Narco ?? TSO x-ponder that's doing pretty much what Ken describes, but last Sunday ATC gave me a code, but subsequently kept telling me that I kept squawking the old code. Turning off and on the Xpondder didnt help? I also tried to just squawk 7000 for VFR but it kept sending out the old code. ??

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