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.

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.

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.

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.

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.

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

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.

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

On Jun 15, 8:30 am, 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.

It can. Most shops charge for a bench test/

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.

The cavity tube, probably


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.)

Sounds like the antenna's unplugged. Any chance it happened
accidentally when seat was removed for anual or something like that?
Noise will light the interrogation light if the antenna's unplugged.

Thanks for all the replies. I will have an avionics shop
look at it Monday.

If I do end up having to get a new transponder,
is there any advantage to the KT76C which seems
to cost more than the GTX327 ?

Thanks again