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#1
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transponder questions
My transponder can't be seen by ATC anymore and I'm
trying to narrow down the cause and plan for the repair. If it can be done simply by removing the radio and taking it somewhere to be bench tested, that'd be much easier for me. It's some vintage of a KT76. A couple of months ago, the altitude encoder was inspected and found to be mostly fine. The inspector did warn me about the lifespan of something but I don't remember if it was the radio or encoder. It seemed to run fine after this inspection but I think a couple towers mentioned it was weak. Recently, after a 13 minute flight, it still wasn't visible to ATC so I don't think it's a warm-up problem. Another symptom is that the light is constantly flickering (although it goes solid when I press ident.) If the encoder failed, would the transponder be completely silent ? Are there any simple tests I can perform myself ? Maybe a SWR meter on the antenna ? A certain voltage coming from the encoder ? If it is the radio, I'd like solid state this time around. Are there any transponders that have a built-in encoder ? That way, I figure I can take care of two birds with one stone. If not, I guess I should narrow it down before making a purchase. Is a transponder something that the owner can take care of ? I noticed that Aircraft Spruce will ship the low-end models but not the higher end ones, saying that they have to install them. Nevertheless, I have to believe that there has to be a sign-off at some point. Additionally, the encoder static system is supposed to be inspected each time... I forget the wording, something like, "any time there is a modification that may affect its operation." Thanks for any insight. |
#2
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transponder questions
If your plane is a certificated, production model, and not an amatuer-
built experimental, then only an avionics shop that is an FAA- certificated repair station can legally repair or replace a transponder system, according to my local FSDO (Houston, TX). Even though there are slide-in replacement units available from Honeywell/ Bendix-King and Narco that would slide right into your existing "legacy" KT-76 tray, it is not legal for the owner/pilot to swap out a transponder as "preventive maintenance" like it is legal to do so with a slide-in nav/com radio... all because the transponder and altitude encoder installation must be calibrated/certified when installed or whenever any repair work is done on them, and also every two years to ensure they're working accurately. The builder of a homebuilt experimental plane such as a Van's RV series can install and/or replace his own transponder and encoder, but cannot legally power it on in flight until he first gets it inspected and certified by a certificated repair station avionics shop, in fact the builder is technically legally supposed to placard the unit "INOPERATIVE" until the certification is done and if he builds the aircraft at one airport under a class B "mode-C ring" like in the Houston area and has to fly to another airport to get the transponder certified, he's supposed to call up ATC in advance to get special permission to make the flight without the transponder. That's what a friend of mine had to do after he built his RV-6. Calling ATC and obtaining advance permission to operate transponder-less for the flight to the avionics shop airport was no hassle at all. |
#3
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transponder questions
One's Too Many wrote:
If your plane is a certificated, production model, and not an amatuer- built experimental, then only an avionics shop that is an FAA- certificated repair station can legally repair or replace a transponder system, according to my local FSDO (Houston, TX). Thanks, that simplifies things greatly. Calling ATC and obtaining advance permission to operate transponder-less for the flight to the avionics shop airport was no hassle at all. I hope so, since you can't get an inoperative transponder there any other way. |
#4
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transponder questions
I could have written this post a few weeks ago. I just went through the same
thing over about 3 years. I spent a couple of hundred $ on various troubleshooting trips to the avionics shop and did not upgrade because various things worked for a while. When the transponder (a King KT-76A) started taking 15-minutes to half an hour to warm up, I upgraded to a Garmin GTX-320A. I've been told by a few shops that the "tube" in the old transponders have about a 7-year lifespan give or take a few. Having had this one since I bought the plane in 2000, I was on schedule and made the upgrade a no-brainer. Like you, I explored the digital xponder route and was told the same about having them installed. They can only send the GTX-320A through the mail. I bought the adapter for the Garmin that allowed me to plug-n-play the new one into the existing tray saving me $600+ in installation costs. Combine that with the higher cost of the higher model GTX-327, I didn't think the $1K price difference was worth the extra features. I did have to get a "data correspondence" check done at a shop to certify that everything was communicating with each other fine and they signed-off the installation as well. That ran two hours but I would need that done with any new transponder. I also had a pending trip through the Washington DC ADIZ that played into the decision but only slightly because the next opening I could find in the local (and trusted) shops were in July (and I did this in May). Having heard other stories, it seems that transponder issues can take a long and frustrating path to conclusion given its intermittent nature. If yours has not had its tube replaced in the last couple of years, I'd suggest bite the bullet and upgrade to a solid state. A word about the encoder. If the encoder was the culprit, I was told that the radio light would still blink but ATC would be getting erroneous altitudes or none at all while still getting the transponder code. Good luck, Marco "Road Dog" wrote in message ink.net... My transponder can't be seen by ATC anymore and I'm trying to narrow down the cause and plan for the repair. If it can be done simply by removing the radio and taking it somewhere to be bench tested, that'd be much easier for me. It's some vintage of a KT76. A couple of months ago, the altitude encoder was inspected and found to be mostly fine. The inspector did warn me about the lifespan of something but I don't remember if it was the radio or encoder. It seemed to run fine after this inspection but I think a couple towers mentioned it was weak. Recently, after a 13 minute flight, it still wasn't visible to ATC so I don't think it's a warm-up problem. Another symptom is that the light is constantly flickering (although it goes solid when I press ident.) If the encoder failed, would the transponder be completely silent ? Are there any simple tests I can perform myself ? Maybe a SWR meter on the antenna ? A certain voltage coming from the encoder ? If it is the radio, I'd like solid state this time around. Are there any transponders that have a built-in encoder ? That way, I figure I can take care of two birds with one stone. If not, I guess I should narrow it down before making a purchase. Is a transponder something that the owner can take care of ? I noticed that Aircraft Spruce will ship the low-end models but not the higher end ones, saying that they have to install them. Nevertheless, I have to believe that there has to be a sign-off at some point. Additionally, the encoder static system is supposed to be inspected each time... I forget the wording, something like, "any time there is a modification that may affect its operation." Thanks for any insight. |
#5
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transponder questions
On Jun 15, 1:41 pm, "Marco Leon" wrote:
I bought the adapter for the Garmin that allowed me to plug-n-play the new one into the existing tray saving me $600+ in installation costs. I was just wondering about this since our transponder has been acting a little "funny" lately as well. So, the 320a with adapter really does just slide in - i.e. zero install cost aside from recertification? Also looking at the AT-165 for a couple hundred more... Would really like a 330 but... man, close to $5k for a transponder??? ugggggh. todd. |
#6
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transponder questions
Yup. I had little info on this before I bought the adapter. The GTX-320A is
shorter length-wise so when you screw in the adapter, it becomes the same length as the King KT-76A. So if your King was flush with the other avionics, the GTX-320 will sit flush as well. The adapter attaches with two simple screws to the back of the transponder. When I slid it into the old tray, I had to reach behind and push up the tray to get the connectors to seat. For some reason, one of the tray's guiding rods in the back that goes into one of the adapter holes was not lining up correctly but a little push fixed that. The transponder fired-up right away and I was amazed at the light flashes. The King would flash slowly and almost steadily when hit with multiple calls (sweeps?) but the Garmin would flash quickly for each call. Being in the New York area and all the TCAS-equipped jet traffic overhead, the flashes were almost strobe-like. Oh, and it started flashing the moment I turned it on--something I've never witnessed since I owned the plane. Like I posted previously, I could not justify the walk-away cost of the GTX-327 no matter how cool it looked. I'd much rather put that $1K in the bank towards an autopilot. Marco "tjd" wrote in message ups.com... On Jun 15, 1:41 pm, "Marco Leon" wrote: I bought the adapter for the Garmin that allowed me to plug-n-play the new one into the existing tray saving me $600+ in installation costs. I was just wondering about this since our transponder has been acting a little "funny" lately as well. So, the 320a with adapter really does just slide in - i.e. zero install cost aside from recertification? Also looking at the AT-165 for a couple hundred more... Would really like a 330 but... man, close to $5k for a transponder??? ugggggh. todd. |
#7
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transponder questions
I would try to swap in a known good KT76 of the type you have. I think
there's a significant wiring difference between the 76 and 76A, so be careful. Paperwork may be required to make it legit, but I'm sure you've taken greater risks in your life. It doesn't sound to me like the encoder is the issue. I don't think a malfunctioning encoder can stop the transponder from sending replies. Has ATC ever told you that to stop squawking altitude? If not, then that's further evidence that the encoder is not at fault. I think if you switch to the "On" position, the encoder is completely irrelevant. You might want to look at http://www.public-action.com/911/transpon/ and http://www.gtwn.net/~keith.peshak/AltitudeNag.htm for useful tutorials. The transponder transmits at 1090 MHz, which is well below the band of most police radars, but you might find a radar detector that is bad enough that its front end will respond to a 1090 MHz signal. Older radar detectors had only a crystal detector, so they'd respond to almost any microwave signal. Or possibly, if the you hold any radar detector close enough the signal will be strong enough to make the detector buzz if the transponder is radiating. This assumes you don't have a broadband RF/microwave power meter. They are available at Radio Shack. If the transponder tube is dead or nearly so, it may not draw much current. You could perhaps wire in an ammeter to determine how much current your transponder is drawing. Compare that to the max current rating. Does the transponder get warm to the touch? That is, after the flight, is the box warm from self-generated heat? If not, then nobody is home inside. A broken wire or broken tube could explain that. You shouldn't overlook aircraft power problems, but that's also unlikely. It's getting enough current to turn on the reply light. It takes more current to power the tube, so it's conceivable that your connection to aircraft power is tight enough to supply a low current, but loose enough to prevent a larger current. However, that's somewhat unlikely. Also, don't forget that ATC equipment doesn't always work correctly, either. I fly in busy airspace and I prefer the transponders with digital entry, rather than rotary knobs, but they do cost money. Good luck... -John Road Dog wrote: My transponder can't be seen by ATC anymore and I'm trying to narrow down the cause and plan for the repair. If it can be done simply by removing the radio and taking it somewhere to be bench tested, that'd be much easier for me. It's some vintage of a KT76. A couple of months ago, the altitude encoder was inspected and found to be mostly fine. The inspector did warn me about the lifespan of something but I don't remember if it was the radio or encoder. It seemed to run fine after this inspection but I think a couple towers mentioned it was weak. Recently, after a 13 minute flight, it still wasn't visible to ATC so I don't think it's a warm-up problem. Another symptom is that the light is constantly flickering (although it goes solid when I press ident.) If the encoder failed, would the transponder be completely silent ? Are there any simple tests I can perform myself ? Maybe a SWR meter on the antenna ? A certain voltage coming from the encoder ? If it is the radio, I'd like solid state this time around. Are there any transponders that have a built-in encoder ? That way, I figure I can take care of two birds with one stone. If not, I guess I should narrow it down before making a purchase. Is a transponder something that the owner can take care of ? I noticed that Aircraft Spruce will ship the low-end models but not the higher end ones, saying that they have to install them. Nevertheless, I have to believe that there has to be a sign-off at some point. Additionally, the encoder static system is supposed to be inspected each time... I forget the wording, something like, "any time there is a modification that may affect its operation." Thanks for any insight. |
#8
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transponder questions
The transponder transmits at 1090 MHz, which is well below the band of most police radars, but you might find a radar detector that is bad enough that its front end will respond to a 1090 MHz signal. Older radar detectors had only a crystal detector, so they'd respond to almost any microwave signal. Or possibly, if the you hold any radar detector close enough the signal will be strong enough to make the detector buzz if the transponder is radiating. This assumes you don't have a broadband RF/microwave power meter. They are available at Radio Shack. But the transmitter doesn't put out any power unless it is being interrogated. Doing this on the ground (unless your home airport has an interrogator on the airport) isn't going to tell you much and in the air (in a metal airplane) you have a very effective shield between you and the antenna. My best bet is that the tube/cavity resonator has detuned with time and is not responding on-frequency. It is POSSIBLE that the bottle has given up, but detuning is the bane of vacuum tube transponders. Jim |
#9
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transponder questions
If your in the Dallas area, I can come to your hangar and validate if
your transponder is totally TU or just off frequency. If it is off freq, there can be multiple reasons for it including a crappy antenna connection. A new transponder won't fix that. It appears to be receiving properly by the reply light working, but not enough or the right rf leaving the antenna. Dave www.craigmileaviaton.com Mobile pitot/Static transponder certification Dallas RST Engineering wrote: The transponder transmits at 1090 MHz, which is well below the band of most police radars, but you might find a radar detector that is bad enough that its front end will respond to a 1090 MHz signal. Older radar detectors had only a crystal detector, so they'd respond to almost any microwave signal. Or possibly, if the you hold any radar detector close enough the signal will be strong enough to make the detector buzz if the transponder is radiating. This assumes you don't have a broadband RF/microwave power meter. They are available at Radio Shack. But the transmitter doesn't put out any power unless it is being interrogated. Doing this on the ground (unless your home airport has an interrogator on the airport) isn't going to tell you much and in the air (in a metal airplane) you have a very effective shield between you and the antenna. My best bet is that the tube/cavity resonator has detuned with time and is not responding on-frequency. It is POSSIBLE that the bottle has given up, but detuning is the bane of vacuum tube transponders. Jim |
#10
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transponder questions
RST Engineering wrote:
The transponder transmits at 1090 MHz, which is well below the band of most police radars, but you might find a radar detector that is bad enough that its front end will respond to a 1090 MHz signal. Older radar detectors had only a crystal detector, so they'd respond to almost any microwave signal. Or possibly, if the you hold any radar detector close enough the signal will be strong enough to make the detector buzz if the transponder is radiating. This assumes you don't have a broadband RF/microwave power meter. They are available at Radio Shack. But the transmitter doesn't put out any power unless it is being interrogated. Doing this on the ground (unless your home airport has an interrogator on the airport) isn't going to tell you much and in the air (in a metal airplane) you have a very effective shield between you and the antenna. My best bet is that the tube/cavity resonator has detuned with time and is not responding on-frequency. It is POSSIBLE that the bottle has given up, but detuning is the bane of vacuum tube transponders. Jim Oil/dirt on the antenna can de-tune the cavity too. Clean the antenna well and test it again before paying someone to tear into your transponder. If ATC isn't getting any reply, then it isn't likely to be the encoder. The transponder will still reply without an encoder attached at all: it will just have zeros in the 12 altitude bits. Since the reply light is blinking, I can infer that the receiver is receiving the interrogations and formulating replies. That generally means that it is decoding the interrogation pulses OK. That precludes a problem with the decoder that is severe enough to prevent a reply (earlier models used resistors and capacitors to measure the timing of the interrogation pulses to decode whether to reply and what kind of reply to send. They occasionally need calibration to adjust the timing. Newer digital units derive the timing from a quartz crystal and are far less susceptable to getting out of alignment). I concur with Jim that the most likely culprit is the cavity is de-tuned. Like I said, make sure the antenna is clean because a dirty antenna can detune the circuit. |
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