Transponder going bad?

rbridges

rbridges

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rbridges
I have a kt-76 transponder. Had ifr certification done about 6 months ago. Some adjustments needed but has worked fine for the last 3 years that I've owned the plane.

Leaving hilton head last week, savannah approach asked if my tx was turned on. I checked and it was, but no flashing orange light. Recycled, no luck. Ident, no light. After 10 minutes it started flashing. I figured it was a fluke since it worked fine the whole trip.

Yesterday I was leaving towards destin and had the same issue. Recycle doesn't help, ident doesn't work but test will make it glow. Same thing, it worked fine after 10-15 minutes. I'm going to have a shop look at it, but I'd like some guesses before I go. I'm leaning towards a kt74 if the tx needs work.
 
I have an '81 Mooney, but no idea what transponder. Looks original. Anyway, a few months ago mine started pretty much the exact same thing. I could get the light to light with the test mode, but ATC still couldn't see anything. Only thing I did, that you didn't say you did, was pull the breaker (didn't know if there was phantom power to it with the power switch off). Nothing.

There's an avionics shop on the field, so I had them look at it. I'm not sure what they adjusted but it was $180 to fix it. The guy said "I was able to get it into spec... barely. You're probably going to need a new one soon."

edit:

Got lots of advice to pull the transponder and clean the contacts. Make sure the tray is firmly seated, etc. I could have, and probably should have tried before taking it to the shop (electronics is a hobby, so this would have been easy), but with no real way to test it except go fly, I didn't bother. That $180 may have just been the above. Dunno.
 
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I took off from a class D and got the same thing from the tower. After going through the whole recycle stuff, I unturned and returned to the airport. Later, maintenance itdentified a loose wire. Three months later, it is still in the aircraft and still works.
Sometimes it is a cheap fix. Then again, this is aviation. The occurances of thatis so few and so far between!
 
Other than cleaning the antenna (oil from the breather getting on the antenna can interfere with transmission) or pulling and reseating the unit in the tray, there's not much you can do. Further, if you weren't getting the reply light, it's probably not a dirty antenna since lack of a reply light means the transponder isn't sensing a pulse and responding. Unless you have the skill to trace and check the wires behind the panel and leading to the antenna, the next stop after that is your avionics shop.
 
Thanks. I did check circuit breaker, no problems. It is getting power since test position works. Oh well, guess it's a trip to the avionics shop.
 
I have a KT76A with an auto dimming eye doodad and I cannot see the orange reply indictator flash at night without shining a light on the eye.








Was the knob between functions? Instead in the detent.
 
The KT-76 uses a Cavity Tube that has both a heater (to bring it up to proper operation temps quickly) and a heat sink (to keep it at the proper temp during ops).
If the heating element is out, mis-adjusted or has come loose from the CT hull the transponder will not work until the power going thru the CT itself warms everything up to working temp. The heater is there to cut down the warm up time.

It's possible this is your problem...

Chris
 
First thing I do with stuff like that is trace the ground circuit looking for corrosion. Ground circuits are the simplest and most common point of intermittent electric/electronic problems and the cheapest and easiest to fix (unless you discover that the entire airframe is so corroded that it's scrap metal, had that happen on a C-170 I looked at with a radio static problem. :eek:) so it's the best place to start and something you can legally do yourself.
 
Hiperbiper said:
The KT-76 uses a Cavity Tube that has both a heater (to bring it up to proper operation temps quickly) and a heat sink (to keep it at the proper temp during ops).
If the heating element is out, mis-adjusted or has come loose from the CT hull the transponder will not work until the power going thru the CT itself warms everything up to working temp. The heater is there to cut down the warm up time.

It's possible this is your problem...

Chris
Click to expand...

I read something about it having a heater. I'm at the point of deciding whether to put any more money into it or getting a mode s.
 
Borrow that magic allen wrench from someone, unscrew the transponder's captive screw and then pull the unit part way out and shove it all the way back in a half dozen times. Tighten the screw. Go find something else to spend money on.
 
Dr. O said:
Borrow that magic allen wrench from someone, unscrew the transponder's captive screw and then pull the unit part way out and shove it all the way back in a half dozen times. Tighten the screw. Go find something else to spend money on.
Click to expand...

A couple of my landings would do the trick also. :eek:

;)
 
I have a KT 76 A transponder with a 8130, I bought it as a spare, but have not used it. It also has the mounting tray, I would sell it if you are interested. PM me.
 
Hiperbiper said:
The KT-76 uses a Cavity Tube that has both a heater (to bring it up to proper operation temps quickly) and a heat sink (to keep it at the proper temp during ops).
If the heating element is out, mis-adjusted or has come loose from the CT hull the transponder will not work until the power going thru the CT itself warms everything up to working temp. The heater is there to cut down the warm up time.

It's possible this is your problem...

Chris
Click to expand...


I spoke with someone over the phone and he said the heater is integrated with the cavity tube, and it's the most expensive part of the transponder. I'll probably just save my money and go with a solid state such as the KT74. He siad it will work for now but just needs to warm up for 10-15 minutes before taking off.
 
The cavity tube is over $600.

It is a very common problem on these old transponders.

Look at the antenna. If it is a blade style, look for cracks in the coating. Water gets into the cracks and corrodes the antenna blade inside. This causes weak transmission and is easily detected with a watt meter, if your shop has one.

But if you see cracks and rust stains in them, just have it replaced. You cannot do it yourself because it voids the correspondence check portion of the 24 month transponder inspection.
 
Ron Levy said:
Further, if you weren't getting the reply light, it's probably not a dirty antenna since lack of a reply light means the transponder isn't sensing a pulse and responding.
Click to expand...

That's about the most convoluted answer in the thread thus far. Care to move around more on the sticky flypaper and try to extricate yourself?

Jim
 
Hiperbiper said:
The KT-76 uses a Cavity Tube that has both a heater (to bring it up to proper operation temps quickly) and a heat sink (to keep it at the proper temp during ops).
If the heating element is out, mis-adjusted or has come loose from the CT hull the transponder will not work until the power going thru the CT itself warms everything up to working temp. The heater is there to cut down the warm up time.

It's possible this is your problem...

Chris
Click to expand...

DING DING, we have a WINNER folks. Best answer yet.

,
 
Henning said:
First thing I do with stuff like that is trace the ground circuit looking for corrosion. Ground circuits are the simplest and most common point of intermittent electric/electronic problems and the cheapest and easiest to fix (unless you discover that the entire airframe is so corroded that it's scrap metal, had that happen on a C-170 I looked at with a radio static problem. :eek:) so it's the best place to start and something you can legally do yourself.
Click to expand...


Go away. PLEASE, just go away. You have not a clue.

Jim
.
 
philiplane said:
The cavity tube is over $600.

Horsefeathers. How many would you like for $200 or thereabouts?
​
It is a very common problem on these old transponders.

Look at the antenna. If it is a blade style, look for cracks in the coating. Water gets into the cracks and corrodes the antenna blade inside. This causes weak transmission and is easily detected with a watt meter, if your shop has one.

I love answers from people who have only done theory and never had to really fix something. Detected with a watt meter? Sonny, what don't you understand about a pulsed power system?
​
But if you see cracks and rust stains in them, just have it replaced. You cannot do it yourself because it voids the correspondence check portion of the 24 month transponder inspection.

NEVER let this person near my airplane. (S)he has not a CLUE about maintenance.
​
Click to expand...
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The problem is more likely a leaky capacitor (220uf) in the turn-on delay/SPI timing circuit. I've had to replace several over the years. PM sent.
 
weirdjim said:
.....
Click to expand...

I am quoting list price installed from an Authorized Bendix King dealer, from a current BK parts price list. Please list your sources and the prices they offered, with contact information for verification.

You detect transponder radiated power with an ATC IFR 400 or equivalent test set. It is a required test with established minimum performance. A KT 76A is expected to produce 200 watts at six feet.

What do you not understand about a plastic coated blade antenna with a rusty center blade? Rusty internals do not radiate well because the rust is an insulator.

My experience level: Former Avionics repair station chief inspector, avionics technician, A&P/IA, several decades of training and experience, and a customer base throughout the US, Caribbean, and Europe bringing highly complex airplanes to me with highly complex problems that other shops cannot solve.

Please list your credentials.
 
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philiplane said:
I am quoting list price installed from an Authorized Bendix King dealer, from a current BK parts price list. Please list your sources and the prices they offered, with contact information for verification.

Ah, no sir. I asked how many you wanted to buy and I don't generally reveal my sources since it has taken me the better part of a lifetime to develop them. You evidently buy list. I buy wholesale and sell for the same plus a small profit.

You detect transponder radiated power with an ATC IFR 400 or equivalent test set. It is a required test with established minimum performance. A KT 76A is expected to produce 200 watts at six feet.

That right there tells me that you don't have the foggiest frikkin grasp of RF fundamentals. You can't produce POWER at a distance, you can only produce FIELD STRENGTH. If you can generate 200 watts at more than 2*D^2/lambda then you have beaten Maxwell, Hertz, and a whole bunch of really smart guys and are in line for the Nobel next year. (BTW, you DO understand the implication of 2*D^2/lambda, do you not?)

What do you not understand about a plastic coated blade antenna with a rusty center blade? Rusty internals do not radiate well because the rust is an insulator.

What do YOU not understand about ferric oxide ("rust") being a pretty good insulator, so that then electromagnetic waves would pass right through it, wouldn't they? Or do you not understand about insulators being transparent to radio waves? It goes through the plastic coating, doesn't it? Then if rust is an insulator then it goes right through it as well.

My experience level: Former Avionics repair station chief inspector, avionics technician, A&P/IA, several decades of training and experience, and a customer base throughout the US, Caribbean, and Europe bringing highly complex airplanes to me with highly complex problems that other shops cannot solve.

Please list your credentials.

Sure. Fixed Narco Superhomers when I was 15-18 (1958-61) in return for flight time. Worked my way through college in the radio shop of a major airline. BS Electronic Physics. MSEE specializing in microwave antennas with a subspecialty in electronically steerable phased arrays. Surveyor (soft moon landing) landing radar team. Apollo (soft manned moon landing) landing radar team. Two NASA patents for microwave electronically steerable phased arrays.

Founded an avionics manufacturing company in 1973 (RST Engineering) that is still in business. Teach electronic and mechanical engineering technology at the local community college (part time) for the last 40 years. Author for Kitplanes Magazine monthly column on avionics.

A&P/IA for thirty years. Commercial airplane/glider, instrument. Flight instructor airplane/glider. Ground instructor basic. Retired pilot examiner.

FCC First 'Phone with radar. Amateur Extra.

Howzat?
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rbridges said:
I have a kt-76 transponder. Had ifr certification done about 6 months ago. Some adjustments needed but has worked fine for the last 3 years that I've owned the plane.

Leaving hilton head last week, savannah approach asked if my tx was turned on. I checked and it was, but no flashing orange light. Recycled, no luck. Ident, no light. After 10 minutes it started flashing. I figured it was a fluke since it worked fine the whole trip.

Yesterday I was leaving towards destin and had the same issue. Recycle doesn't help, ident doesn't work but test will make it glow. Same thing, it worked fine after 10-15 minutes. I'm going to have a shop look at it, but I'd like some guesses before I go. I'm leaning towards a kt74 if the tx needs work.
Click to expand...

Those older transponders can usually be fixed pretty cheaply, even at avionics shops that are notoriously expensive.:D
 
Mistake Not... said:
Rusty Bolt Effect: http://en.wikipedia.org/wiki/Rusty_bolt_effect

Passive Intermodulation (PIM): http://www.radio-electronics.com/in...ssive-intermodulation-pim/basics-tutorial.php
Click to expand...

Yessir, I understand rusty bolt effect fairly well, as well as all the other x-mod (crossmod, intermod, half-if mod, etc.) as I have fought them my whole professional life. As a matter of fact, I take great pleasure in going into the theory behind the WWII "Razor Blade Radio" with my classes and explaining that diodes are where you find them. And why Gillette was the razor blade of choice for these "foxhole radios".

What I did NOT say, and didn't want to start another holy war, is that antennas are not made of iron or steel -- especially UHF/uW antennas. Copper? Yes. Aluminum? Yes. Silver plated base metal? Yes. But as yet Grumman has not started to make iron antennas and so far as I know the Grumman Iron Works on Bethpage, Long Island is the only purveyor of large chunks of ferrous metals in aircraft.

Anybody ever seen a steel/iron general aviation aircraft antenna?

Jim
 
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Cut the plastic coating off a King, Narco, or Comant blade type transponder antenna and you will find thin, flat metal inside that rusts.
 
Dr. O said:
Borrow that magic allen wrench from someone, unscrew the transponder's captive screw and then pull the unit part way out and shove it all the way back in a half dozen times. Tighten the screw. Go find something else to spend money on.
Click to expand...

That is NOT a legal owner maintenance operation. Autopilots, transponders, and DME are excluded from the tray mounted avionics monkeying provisions in part 43.
 
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The radiator itself isn't likely to be the problem (though oxidation other than ferric rust can still be an issue). More often than not it's corrosion of the connectors which I can bet are probably just sort of (perhaps no longer intact) plated steel.
 
flyingron said:
The radiator itself isn't likely to be the problem (though oxidation other than ferric rust can still be an issue). More often than not it's corrosion of the connectors which I can bet are probably just sort of (perhaps no longer intact) plated steel.
Click to expand...

Ron ... most BNC/TNC connectors are silver or tin plated BRASS for just that reason. Both silver and tin (sulfation and oxidation respectively) can gum up the connector works quite readily.

Thanks,

Jim
 
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