transponder questions

Road Dog wrote:

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 ?

The KT76C is a drop in replacement that could be
why.

Grumman 236 wrote:

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.

Interestingly, when I flew the plane to the airport with the
avionics shop today, the transponder worked perfectly. No
tower complained, and the reply indicator was well behaved.
That would seem to imply an intermittent antenna problem
(unless there's something in the radio that will do that.)

I took a look at the antenna connection before the flight.
It looked (to me) like a hastily done coax connection. I
know almost nothing about avionics, so I don't even know
if it is coax, but the mesh sleeve of the cable was exposed.
I thought it was supposed to be covered by the metal sleeve
of the connector itself... Maybe the installer took off too
much plastic, or maybe it got pulled out when the guy did
the altitude encoder inspection...

Anyways, I'll let the shop sort it out.

I had an intermittent problem(kt79a) one time, a capacitor stopped
working when it got to a certain temperature. It was expensiv to trouble
shot but eventually after lots of hot air, they found the part.




Road Dog wrote:

Grumman 236 wrote:

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.


Interestingly, when I flew the plane to the airport with the
avionics shop today, the transponder worked perfectly. No
tower complained, and the reply indicator was well behaved.
That would seem to imply an intermittent antenna problem
(unless there's something in the radio that will do that.)

I took a look at the antenna connection before the flight.
It looked (to me) like a hastily done coax connection. I
know almost nothing about avionics, so I don't even know
if it is coax, but the mesh sleeve of the cable was exposed.
I thought it was supposed to be covered by the metal sleeve
of the connector itself... Maybe the installer took off too
much plastic, or maybe it got pulled out when the guy did
the altitude encoder inspection...

Anyways, I'll let the shop sort it out.

Yup. I had little info on this before I bought the adapter. The GTX-320A is
shorter length-wise so when you screw in the adapter, it becomes the same
length as the King KT-76A. So if your King was flush with the other
avionics, the GTX-320 will sit flush as well.

The adapter attaches with two simple screws to the back of the transponder.
When I slid it into the old tray, I had to reach behind and push up the tray
to get the connectors to seat. For some reason, one of the tray's guiding
rods in the back that goes into one of the adapter holes was not lining up
correctly but a little push fixed that.

The transponder fired-up right away and I was amazed at the light flashes.
The King would flash slowly and almost steadily when hit with multiple calls
(sweeps?) but the Garmin would flash quickly for each call. Being in the New
York area and all the TCAS-equipped jet traffic overhead, the flashes were
almost strobe-like. Oh, and it started flashing the moment I turned it
on--something I've never witnessed since I owned the plane.

Like I posted previously, I could not justify the walk-away cost of the
GTX-327 no matter how cool it looked. I'd much rather put that $1K in the
bank towards an autopilot.

Marco


"tjd" wrote in message
ups.com...
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.

Road Dog wrote:

Anyways, I'll let the shop sort it out.

Be sure to keep the antenna clean, they can get covered with oil on the
belly and that can cause issues...

On Jun 17, 7:42 am, kontiki wrote:
Road Dog wrote:

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 ?

The KT76C is a drop in replacement that could be
why.

The KT-76C is also still a cavity-tube type of transponder, and will
be subject to the tube-related problems just like the rest of the
older technology transponders, despite the fact that it's got a nice
digital display and interface.

That's why we opted to bite the bullet and pay for a new GTX-320A plus
installation instead when the old King transponder bit the dust only a
few months after buying the 172. Wish now that we'd have paid the
extra
~ $500 and got the pushbutton GTX-327 instead.

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



Oil/dirt on the antenna can de-tune the cavity too. Clean the antenna
well and test it again before paying someone to tear into your transponder.

If ATC isn't getting any reply, then it isn't likely to be the encoder.
The transponder will still reply without an encoder attached at all:
it will just have zeros in the 12 altitude bits.

Since the reply light is blinking, I can infer that the receiver is
receiving the interrogations and formulating replies. That generally
means that it is decoding the interrogation pulses OK. That precludes a
problem with the decoder that is severe enough to prevent a reply
(earlier models used resistors and capacitors to measure the timing of
the interrogation pulses to decode whether to reply and what kind of
reply to send. They occasionally need calibration to adjust the timing.
Newer digital units derive the timing from a quartz crystal and are
far less susceptable to getting out of alignment).

I concur with Jim that the most likely culprit is the cavity is
de-tuned. Like I said, make sure the antenna is clean because a dirty
antenna can detune the circuit.