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.