Bendix KT76C transponder requires major repair: Options?

Marco Leon wrote:

Peter,
How about replacing it with a Garmin GTX 320A with an adapter for
~$1300? I will do that if/when my tube goes.

I'm having an extended "warming" problem with my KT76C where it takes
sometimes 10-15 minutes for it to respond to ATC. I brought it in for a
$250 checkup and it passed with flying colors. When I inquired about
the tube maybe needing replacement my avionics guy said it was around
$500-$700 to replace the tube.
However, he said that the antenna was most likely the culprit and that
I should replace the small flagpole type with a blade first. Makes
sense but I'm not so sure an antenna problem will give me the same
symptoms so I'm prepping myself for a unit replacement.

Marco


Could be the encoder warm-up. The transponder won't reply mode C until
the encoder warms up. Some of them take 10 minutes or more to come up
to temperature.

Narco makes the AT165 in a version that is a slide-in replacement for
the KT-76. I've got an AT165 in my plane, and I like it. It is has a
solid state transmitter, so there are no worries about a tube wearing
out. The interogation decode and reply generation is all digitally
timed from a quartz crystal, so unlike older transponders it also will
not go out of calibration.

Ray Andraka wrote:

Could be the encoder warm-up. The transponder won't reply mode C until
the encoder warms up. Some of them take 10 minutes or more to come up
to temperature.

Now THAT's interesting. Does anyone know if there's a bench check for
the encoder itself? I'm surprised that the avionics shop didn't suggest
the encoder as being the culprit. Is it common?

Marco

Marco Leon wrote:
Ray Andraka wrote:

Could be the encoder warm-up. The transponder won't reply mode C until
the encoder warms up. Some of them take 10 minutes or more to come up
to temperature.


Now THAT's interesting. Does anyone know if there's a bench check for
the encoder itself? I'm surprised that the avionics shop didn't suggest
the encoder as being the culprit. Is it common?

Marco

I know it takes my encoder about 7 minutes to warm up. It is connected
to my GPS and I get "No Altitude" until it does. The transponder will
have its own "encoder" within it. It may also take awhile to warm. I
have the KT78. It would be nice if there was a little ready light.

--

Regards, Ross
C-172F 180HP
KSWI

The 76C has an altitude display so you can see when this
happens. Bill Hale

Ross Richardson wrote:
Marco Leon wrote:
Ray Andraka wrote:

Could be the encoder warm-up. The transponder won't reply mode C until
the encoder warms up. Some of them take 10 minutes or more to come up
to temperature.


Now THAT's interesting. Does anyone know if there's a bench check for
the encoder itself? I'm surprised that the avionics shop didn't suggest
the encoder as being the culprit. Is it common?

Marco

I know it takes my encoder about 7 minutes to warm up. It is connected
to my GPS and I get "No Altitude" until it does. The transponder will
have its own "encoder" within it. It may also take awhile to warm. I
have the KT78. It would be nice if there was a little ready light.

--

Regards, Ross
C-172F 180HP
KSWI

wrote:
The 76C has an altitude display so you can see when this
happens. Bill Hale

Ross Richardson wrote:

Marco Leon wrote:

Ray Andraka wrote:


Could be the encoder warm-up. The transponder won't reply mode C until
the encoder warms up. Some of them take 10 minutes or more to come up
to temperature.


Now THAT's interesting. Does anyone know if there's a bench check for
the encoder itself? I'm surprised that the avionics shop didn't suggest
the encoder as being the culprit. Is it common?

Marco


I know it takes my encoder about 7 minutes to warm up. It is connected
to my GPS and I get "No Altitude" until it does. The transponder will
have its own "encoder" within it. It may also take awhile to warm. I
have the KT78. It would be nice if there was a little ready light.

--

Regards, Ross
C-172F 180HP
KSWI


That's nice

--

Regards, Ross
C-172F 180HP
KSWI

Marco Leon wrote:
Ray Andraka wrote:

Could be the encoder warm-up. The transponder won't reply mode C until
the encoder warms up. Some of them take 10 minutes or more to come up
to temperature.


Now THAT's interesting. Does anyone know if there's a bench check for
the encoder itself? I'm surprised that the avionics shop didn't suggest
the encoder as being the culprit. Is it common?

Marco


Yes, the older encoders are designed that way. They use a heater to get
the temperature to a known value to stabilize the pressure reading.
Mine is a Trans-Cal encoder, and it takes about 8 minutes on a 60 degree
day. Takes longer on a cold day. The transponder (a Narco AT165) has
an altitude readout, which shows 0 until the encoder comes alive.

Ray Andraka wrote:
: Yes, the older encoders are designed that way. They use a heater to get
: the temperature to a known value to stabilize the pressure reading.
: Mine is a Trans-Cal encoder, and it takes about 8 minutes on a 60 degree
: day. Takes longer on a cold day. The transponder (a Narco AT165) has
: an altitude readout, which shows 0 until the encoder comes alive.

Reads '0'? IIRC, the grey code doesn't allow all high or all low signals, so
the transponder should know it's bogus information. Seems like a bad design decision
to read valid info with an invalid signal. My newly-installed KT-79 and
oldly-installed KX-135 read '---' and 'No Altitude', respectively until the Narco
AR-850 encoder warms up... 2-3 minutes on a standard day. Probably 3-4 at 20 degrees
F.

-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

Ray Andraka wrote:


Yes, the older encoders are designed that way. They use a heater to get
the temperature to a known value to stabilize the pressure reading.
Mine is a Trans-Cal encoder, and it takes about 8 minutes on a 60 degree
day. Takes longer on a cold day. The transponder (a Narco AT165) has
an altitude readout, which shows 0 until the encoder comes alive.

This process starts when the electrical/avionics are turned on correct?
I was thinking about putting the transponder (in my case a King KT76A)
in "ALT" instead of "STANDBY" at startup to help speed up the process
but now I'm thinking that it wouldn't make a difference.

Over the last 4-5 flights, New York TRACON has been complaining that
they only get a primary target during the early parts of my flight and
thus have refused radar service. Of course, on my way back to the
airport, it would start working. Given that the transoponder was given
a clean bill of health from a bench check, the encoder seems to be the
next logical troubleshooting step. Replacing my pole antenna with a
blade-type (at the recommendation of the shop) does not seem like it
would be the solution to the problem given the symtoms.

Marco

A primary target is one that is not giving any transponder reply; they are
painting you strictly from skin reflection. Why would you think that the
altitude encoder has anything to do with that problem?

A bench check tells you that the box is working. It says nothing about the
antenna or feedline.

Jim



Over the last 4-5 flights, New York TRACON has been complaining that
they only get a primary target during the early parts of my flight and
thus have refused radar service. Of course, on my way back to the
airport, it would start working. Given that the transoponder was given
a clean bill of health from a bench check, the encoder seems to be the
next logical troubleshooting step. Replacing my pole antenna with a
blade-type (at the recommendation of the shop) does not seem like it
would be the solution to the problem given the symtoms.

Marco

RST Engineering wrote:
A primary target is one that is not giving any transponder reply; they are
painting you strictly from skin reflection. Why would you think that the
altitude encoder has anything to do with that problem?

A bench check tells you that the box is working. It says nothing about the
antenna or feedline.

Hmmm. I'm not sure if the controller said that I have no "mode C" or if
they were just getting a "primary target." Being under a Mode C Veil,
they deny service just the same. I thought it may be the encoder
because of the significant delay I experience in getting radar contact
with ATC seemed to make sense with others' experiences in their encoder
warm-up times.

Another point of info, when ATC claims that they don't see me, I do
notice that the Reply Light is not flashing. When it does, ATC sees me
a minute or two later after their radar sweeps. So, if it *was* the
encoder and not the transponder, I should still see the Reply Light
flashing, correct??

If so, it sounds like I need to replace the antenna first before
thinking about the transponder.

Marco