I'm seeing some arcing inside my KT76a transponder. It took weeks to track this down, originally hearing what I thought was a mechanical 'click' about once per hour when flying. That's a grand story in itself, but I finally caught the culprit on video, my KT76a arcing inside the shell. The transponder is fully functional, as per ATC telling me I'm transmitting to them. It's very occasional, not repeatable, etc. The GoPro was recording for 1.5 hours before it happened on this flight.

Has anyone seen this? the engineer in me want's to squirt some 50cents of RTV up there so insulate whatever is shorting out, but the rule-following pilot says that's probably a no-no.

Avionics shops want $110 to even set it on their bench. Used replacements $200-500.

Thoughts?

Thanks for any/all help. Not ready to spend $4-5k on ADS-B yet.

-asa

Video: (watch the holes in the bottom of the transponder. This view is look upwards from under the copilot side of the panel.

Can you take the outer case off and see if there is any carbon arc residue or BURN marks between components. Can't think of any positive long term outcome if left as is. Sometimes a layer of DUST can cause a path for arcing.

Shot in the daytime

Chris C

PS the $$s in the shop could end up being cheap ins.

Maybe just try a blast of shop air up through the ventilation holes?
You might take out the transponder & remove the bottom cover & see what is actually sparking.
Pretty easy to do.
I used to have a KT76 that blew an internal fuse about once a year.
Cost me $68 to have the avionics guy fix it the first time, pretty cheap really,
but every time after that it cost me less than a buck for the fuse and just a few minutes of my time.

Are there any symptoms other than the occasional "click"?

An inflight electrical fire would REALLY suuuuuck. I would probably pull the cover off to see if it was something obvious then shell out the money for the shop to work their magic and yellow tag it. Good find by the way.

CW

No other symptoms. And yes, clippwagon, the only reason I'm concerned is the safety of my plane and my self. If I wasn't based inside Denver's class B, I'd pull the breaker and let it ride.

Gosh, how I hate throwing money at something! When I bought my airplane, it had a KT78 in it, but that failed within a year or so, so I replaced it with another KT78 that a local avionics shop (now out of business) had. I just looked at it (it's in the garage), and it has a hand written marker note on it "repaired, (date)," with the shop's owner's signature--not exactly a legal return to service sort of thing. It started glitching in another couple of years, so I replaced it with a Narco slip-in. Although that passed its last 2 year check 3 years ago, I replaced it with a KT74 tied to my 430W a little over a year ago to accomplish ADS-B Out. A bench check of the Narco indicated that it was failing anyway and probably wouldn't have passed the next 2 year check.

So with all that, I spent about $2500 between the used KT78 and the Narco, and had I not decided to meet the ADS-B Out requirements early, I'd still have had to replace the Narco anyway. So my recommendation is just bite the bullet and get a new one. If you decide to get a used one like what you have, Bennett Avionics has the KT76A for $895, or the newest version of the KT76A for $995. Past experience with Bennett is that they're a good outfit to deal with. Their normal warranty is 90 days, or you can buy an extended warranty for 10% more, based on the purchase price of the transponder, and that is good for a year.

If you want my old KT78, if Bennett would give you anything for it, you can have it. Check with them, and if so, PM me your address and I'll ship it to you, or if you're coming up here, I'll give it to you in person.

FWIW, I sure wouldn't fly with any arcing electronics.

Cary

Ready or not, it might be a good time to get set for ADS-B. Might be cheaper in the long run...

Find a shop with used ones in stock, lots of kt76a's out there with changing to ADSB, then let them take a look at yours on the basis that they will wave the bench fee is you buy a used transponder from them. Last fall when I needed the transponder in my C170A checked it didn't work, pulled it out and you could hear something rattling around in the case. The shop owner who was doing the check said he had several on the shelf that he had removed and would sell me one bench checked and certified for $400. Seemed like a good deal for both of us, since I needed one and hek new that in 3 years the ones he had would not be worth the value of the scrap metal in them.

Tim

Thanks for the responses guys. Opened her up and there's nothing obvious in the area that it appeared to be arcing. No residue, broken connections, carbon, or bad insulation. There's a lot of high voltage building up in those capacitors so it's probably something less obvious.

A couple developments:
- I realized it was actually a KT-78. No real difference expect a lower power transmitter. Same manuals and all, I guess I was just so accustomed to seeing KT-76's that I assumed it was one.
- My IA (not avionics shop) says he has a functioning KT-76A on his shelf he'd let me have for $150. The plane and I are going to see him next week so he can help install firewall battery and BAS handles, so I will most likely just drop that thing in. It's a direct slide in replacement for the 78, same rack, connectors, etc, and actually a slight upgrade.

-asa

Glad you found a quick, easy and cost effective solution. I'm still impressed at your investigative work!

I'm also currious what might be happening inside a transponder on one hour intervals that may be at the root of the issue? I tried to do some internet investigating but didn't come up with any answers.

CW

clippwagon wrote:Glad you found a quick, easy and cost effective solution. I'm still impressed at your investigative work!

I'm also currious what might be happening inside a transponder on one hour intervals that may be at the root of the issue? I tried to do some internet investigating but didn't come up with any answers.

CW

Investigative work is because I'm young, dumb, and not willing to just throw money at the situation more than I have to. Didn't realize it was an electrical problem until i saw blue flashing at my passengers feet coinciding with the clicking on a night flight, where the arc was illuminating the floor. That time it happened 5 times in a row pretty quickly, like it was making sure I got the memo that it was electrical. That was when I figured I needed to fix it...

I wouldn't say it's happening on hour intervals, just that around once per hour is it's current average. The first time it happened I was at a high altitude (>13k) so I thought it was something to do with pressure differentials, air pockets, etc, but it happened probably 10 times in 15 minutes. The one I caught on this video was in the middle of a 2 hours flight and it only did it the one time I got on video.

My IA says that the KT-76/78 have a common issue with blowing the capacitor which produces smoke in the cockpit and blows the unit, but never starts cascading failures or fires, and he thought that maybe this was an early symptom of that capacitor going bad. Hard to say.

-asa

asa wrote:Thanks for the responses guys. Opened her up and there's nothing obvious in the area that it appeared to be arcing. No residue, broken connections, carbon, or bad insulation. There's a lot of high voltage building up in those capacitors so it's probably something less obvious.

A couple developments:
- I realized it was actually a KT-78. No real difference expect a lower power transmitter. Same manuals and all, I guess I was just so accustomed to seeing KT-76's that I assumed it was one.
- My IA (not avionics shop) says he has a functioning KT-76A on his shelf he'd let me have for $150. The plane and I are going to see him next week so he can help install firewall battery and BAS handles, so I will most likely just drop that thing in. It's a direct slide in replacement for the 78, same rack, connectors, etc, and actually a slight upgrade.

-asa

Great find!!