Bendix KT76C transponder requires major repair: Options?

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

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

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

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.

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

"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

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.

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