Bendix KT76C transponder requires major repair: Options?

[Peter R.]

A friend just let me know that his relatively new (four year old) Bendix
King KT76C transponder failed the mode C biennial test.

When the unit was sent in for a $250 troubleshooting (Duncan, I believe,
not Honeywell) analysis, he was told that his unit required a $1,700 repair
to replace the failed "tube" inside.

It looks as if a used KT76C is selling these days for US $1,700 to &1,900.
Does he really have no option but to throw this thing in the garbage?

--
Peter

[Marco Leon]

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


Peter R. wrote:
> A friend just let me know that his relatively new (four year old) Bendix
> King KT76C transponder failed the mode C biennial test.
>
> When the unit was sent in for a $250 troubleshooting (Duncan, I believe,
> not Honeywell) analysis, he was told that his unit required a $1,700 repair
> to replace the failed "tube" inside.
>
> It looks as if a used KT76C is selling these days for US $1,700 to &1,900.
> Does he really have no option but to throw this thing in the garbage?
>
> --
> Peter

[Marco Leon]

Oh crap, I just realized that the "KT76C" is an entirely different
animal so my previous post is null and void. Sorry Peter. Of course,
I'm thinking of the KT76A.

Marco

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
>
>
> Peter R. wrote:
> > A friend just let me know that his relatively new (four year old) Bendix
> > King KT76C transponder failed the mode C biennial test.
> >
> > When the unit was sent in for a $250 troubleshooting (Duncan, I believe,
> > not Honeywell) analysis, he was told that his unit required a $1,700 repair
> > to replace the failed "tube" inside.
> >
> > It looks as if a used KT76C is selling these days for US $1,700 to &1,900.
> > Does he really have no option but to throw this thing in the garbage?
> >
> > --
> > Peter

[Peter R.]

Marco Leon > wrote:

> Oh crap, I just realized that the "KT76C" is an entirely different
> animal so my previous post is null and void. Sorry Peter. Of course,
> I'm thinking of the KT76A.

No problem! :) With all of these acronyms, I am certainly not immune to
mixing them up, either.


--
Peter

[Ray Andraka]

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.

[ktbr]

Peter R. wrote:
> A friend just let me know that his relatively new (four year old) Bendix
> King KT76C transponder failed the mode C biennial test.
>
> When the unit was sent in for a $250 troubleshooting (Duncan, I believe,
> not Honeywell) analysis, he was told that his unit required a $1,700 repair
> to replace the failed "tube" inside.
>
> It looks as if a used KT76C is selling these days for US $1,700 to &1,900.
> Does he really have no option but to throw this thing in the garbage?
>

Garmin makes an adapter that will retrofit their transponders into
a KT-76(a/C) bay.

[Marco Leon]

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

[Ross Richardson[_2_]]

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

[[email protected]]

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

[Ross Richardson[_2_]]

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

[Ray Andraka]

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.

[[email protected]]

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 *
************************************************** ***********************

[Marco Leon]

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

[Marco Leon]

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 symptoms.

Marco

[RST Engineering]

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
>

[Ray Andraka]

Marco Leon wrote:
> 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 symptoms.
>
> Marco
>

Um, if it were the encoder, the transponder should still reply to
mode3/A interrogations unless the encoder ready is wired to the suppress
reply pin on the transponder. If that pin is not wired to the encoder
then you've got something else going on (I believe that pin is there to
allow a DME to suppress replies while the DME is expecting a response
from the ground station).

Also, I wasn't clear about the display on the Narco with the encoder not
ready. I think it displays dashes when the encoder is not ready..
Whatever it is, it is plainly not a valid altitude.

[Marco Leon]

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

[RST Engineering]

"Marco Leon" > wrote in message
oups.com...
>
> 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??

The encoder working or not working has absolutely nothing to do with the
reply light.


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

Why don't you consider running some tests before shotgunning parts that may
or may not be bad?

Jim

[Ray Andraka]

RST Engineering wrote:
> "Marco Leon" > wrote in message
> oups.com...
>
>>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??
>
>
> The encoder working or not working has absolutely nothing to do with the
> reply light.
>
>
>
>>If so, it sounds like I need to replace the antenna first before
>>thinking about the transponder.
>
>
> Why don't you consider running some tests before shotgunning parts that may
> or may not be bad?
>
> Jim
>
>
The reply light should come on whenever the transponder decodes a valid
interrogation and generates a reply. The reply can be suppressed by an
interrogation coming too soon after a previous one, by the box being put
in standby, or with an externally applied suppress signal. If the
encoder ready is hooked up to that suppress signal, it would prevent a
reply...but it isn't supposed to be connected that way. That input is
meant for other equipment to assert if a transponder reply would mess up
the equipment function. It is typically used for DME to suppress a
transponder transmission while the DME is waiting for a reply.

More likely, your transponder may be out of calibration. Older
transponders (I'm familiar with the guts of the Narco line) like the
Narco AT150 and earlier use resistor-capacitor combinations to set the
delays for the interogation decode. If the values drift or the trimming
pots move slightly, your transponder may no longer reliably decode
interrogations. The delay is also somewhat temperature sensitive to the
extent that temperature changes the values of the components slightly.
It could be your transponder needs to be recalibrated.

[Marco Leon]

RST Engineering wrote:
>
> The encoder working or not working has absolutely nothing to do with the
> reply light.

That's why I'm thinking it's the antenna.

> Why don't you consider running some tests before shotgunning parts that may
> or may not be bad?

Like what? I had the King KT76A benched-checked and it came through
with flying colors. They checked the impedance of the antenna and found
that it was low (high?) but within tolerances. Cleaning some contacts
improved it slightly. That's probably why they suggested I replace the
antenna.

Would you know if a bench check would tell them if the transponder tube
took too long to warm up? Since I had to fly the plane there, it would
have been "warm" already.

Marco

[Ross Richardson[_2_]]

Marco Leon wrote:

> RST Engineering wrote:
>
>>The encoder working or not working has absolutely nothing to do with the
>>reply light.
>
>
> That's why I'm thinking it's the antenna.
>
>
>>Why don't you consider running some tests before shotgunning parts that may
>>or may not be bad?
>
>
> Like what? I had the King KT76A benched-checked and it came through
> with flying colors. They checked the impedance of the antenna and found
> that it was low (high?) but within tolerances. Cleaning some contacts
> improved it slightly. That's probably why they suggested I replace the
> antenna.
>
> Would you know if a bench check would tell them if the transponder tube
> took too long to warm up? Since I had to fly the plane there, it would
> have been "warm" already.
>
> Marco
>
When I have had my transponder checked it was always in the the "field",
transponder in the plane.

--

Regards, Ross
C-172F 180HP
KSWI