Transponder replacement suggestions

skym wrote:
: I'll admit to some lack of knowledge here but, other than the Mode S
: ability of the Garmin 330, aren't xpdrs pretty much just a one trick
: pony? I.e., they just receive a radar signal and reply. Why would a
: Garmin 327 be any better than any other?

That's sorta my thought. Transponders are irritating because they do not do
anything that's *directly* useful to the pilot. They simply allow you to operate in
airspace or get services you otherwise wouldn't be able to. I have no doubt that any
of the new units will function just fine, but bells and whistles on a transponder just
seem kinda silly to me.

The new ones that show pressure altitude are nice, but I've got my encoder
rigged into my VFR GPS, so I can see it there just as well. Push-buttons are nice,
but not worth much extra dollars IMO.

I doubt that I'll be able to do a slide-in replacement because I will want to
add the DME suppression line... which is unavailable on the KT-76's connector.

I haven't heard much good about Narco stuff from lots of people who've had to
deal with service. Maybe that's changed? I do like the fact that they provide
installation information on many items on their webpage. Bendix/King really irritates
me in that respect.

My current thought is to either go with a bare-bones new unit like the Garmin
GTX-320, or maybe an older solid-state unit like the KT-79. That's about the same
vintage as the rest of the radios in my stack (KY-197, KN-53, KNS-80). Reworking the
tray isn't a big deal to me.

-Cory

--

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

On Thu, 11 May 2006 12:09:47 +0000 (UTC),
wrote:

skym wrote:
: I'll admit to some lack of knowledge here but, other than the Mode S
: ability of the Garmin 330, aren't xpdrs pretty much just a one trick
: pony? I.e., they just receive a radar signal and reply. Why would a
: Garmin 327 be any better than any other?

That's sorta my thought. Transponders are irritating because they do not do
anything that's *directly* useful to the pilot. They simply allow you to operate in
airspace or get services you otherwise wouldn't be able to. I have no doubt that any
of the new units will function just fine, but bells and whistles on a transponder just
seem kinda silly to me.

The new ones that show pressure altitude are nice, but I've got my encoder
rigged into my VFR GPS, so I can see it there just as well. Push-buttons are nice,
but not worth much extra dollars IMO.

I doubt that I'll be able to do a slide-in replacement because I will want to
add the DME suppression line... which is unavailable on the KT-76's connector.

I haven't heard much good about Narco stuff from lots of people who've had to
deal with service. Maybe that's changed? I do like the fact that they provide
installation information on many items on their webpage. Bendix/King really irritates
me in that respect.

My current thought is to either go with a bare-bones new unit like the Garmin
GTX-320, or maybe an older solid-state unit like the KT-79. That's about the same
vintage as the rest of the radios in my stack (KY-197, KN-53, KNS-80). Reworking the
tray isn't a big deal to me.

If you really don't care about Mode S get a KT76C. Check with the
shop to be sure but if memory serves it's pin and tray compatible with
the 73 and is just mode C. 5 minute swap.

wrote:

Hey all... just got official confirmation that my KT76's power output is way
low.... classic sign of weak/dying/dead cavity tube. Just looking for suggestions on
what might be a good, cost-effective replacement. I'm tired of futzing with
transponders so I'm pretty sure I want a solid-state replacement.

I just had a thought too (for any avionics techs reading)... perhaps the lack
of DME suppression lead to the premature demise of the KT76? I was going to wire it
up when I installed the DME, but the KT76 doesn't have that capability. Maybe the DME
cooked it?

-Cory

Nope, the external suppression, on transponders that have it, just
blocks off the transponder reply so that it doesn't send out a reply in
response to the DME interrogation pulses. Its only purpose is to cut
down on unsolicited replies going back to the ATC interrogator.

More likely, your cavity just got tired. The cavity is a tube used to
obtain the required transmit power. When these transponders were new,
it was the only reasonably priced technology available for generating
200W RF power in the frequency range. Being a tube, it eventually
weakens with age.

Most of the new transponders have semiconductor RF amplifiers in the
transmitter. The reply circuits are also completely digital, using
quartz crystals to derive the pulse timing. Transponders of the Narco
AT150 vintage which used resistor-capacitor combinations to generate the
pulse timing for both decode and reply generation, and therefore require
calibration and are prone to drifting out of calibration. Won't happen
with the newer transponders.