Transponders

[rvator9a]

Hi to all, I am just about to the point where I need to purchase a transponder and I have some questions before I head to sun&fun. I am building on a budget so price is a big factor, is buying used an option or are there any lower priced new ones available. I also am somewhat confused as to what is meant by gray code or serial encoding, any info or details that I should consider before I purchase will be appreciated.

[fmarino1976]

Hi there, it depends on the type of transponder. I would not buy a used King KT76/78, 76A/78A, or Narco AT-50 or AT-150. unless you are sure they are not too old. Those units have been around for a long time and the chance of you getting one with a weakened cavity (tube) is pretty high. The 76C has a tube also, but it is a newer model. Replacing a cavity would cost almost as much as a used Xpdr! If you can get your hands on a garmin GTX-320 or a 327 you'd be OK, since they use solid state components instead of the cavity. Terra made a little transponder, but I think that the little push switches used to change the sqwack code are discontinued and really hard to come by if they break. I really have no experience with the experimental units, so maybe someone else could pitch in some info here.

Serial vs. Gray code: Gray code is the most popular and older system. It can be said that it is parallel code. It uses 10 wires to provide altitude to the transponder. It is limited to 100 ft increments. The gray code only encoders are cheaper than the ones that provide RS-232.

Most of the new generation of transponders can use either gray or serial. With serial iformation you only need a single shielded wire to provide the ealtitude. Depending on the encoder (ie: SSD120-30-RS232) they can provide altitude in 10 ft increments. The transponder will only transmit 100 ft increments, but some of them will display the altitude on their screen. If you are installing a GPS, RS232 makes for a simpler and better install.

I hope I didn't bore you to death. If you have any more questions, I'll check the posts here again.

[Radomir]

They should have some refurbished Garmin units for sale.. you should probably be able to get a refurb 327 for the price of a new 320..

[Mel]

I had a King KT-76A that worked great for years. Once I started having problems. it went down hill pretty fast. I replaced it with a Garmin GTX-320A. The Garmin is all solid state, smaller, lighter, draws less current and comes on instantly. I highly recommend it. (Gray code input)

[dan]

I also like the GTX-320A, which is what I have in my panel. If memory serves, the -327 was gonna be just $100 more, but I still opted for the -320A. It's a full pound lighter, and it has knobs (which I like). Less stuff to fail, less money, less weight, me likey...

[Brian Vickers]

For a period of time I chased around on Ebay hunting a used Narco or King. I kept seeing the prices go pretty high for "as removed" units. I then started researching reconditioned units sold by Avionics dealers. The main tube is never reconditioned. All four avionics shops I talked with were persuasive in talking me into a new Garmin 320A. The recondition units are only $400 or so less than a brand new 320A. I am very budget minded and have incorporated a number of used components into my RV project. Bottom line: the solid state units really are an improvement. I am offsetting the higher cost of a new 320A by lowering my comm unit from an SL-40 to an Icom A200.

Just another opinion in a sea of many view points and values. Some builders just like the process of bargain hunting. That is certainly OK.

RV4 finishing

[RickWoodall]

With all the cool new glass and comms, audio panels, gps etc....why arent there some new transponders available?
I am ready to order all my avionics...and it seems the gtx327 is basically what I will get. With new cool glass, I cant put a 20 year old box in there. The old school transponder really looks crappy in the newer panels (sorry if that offended anyone) .

Why doesnt anyone go after this market with some new technology. (MGL, Dynon are you listening) Seems that for $1500 to $2k every plane there would be room for some competition? If anyone knows of a good quality unit that is priced better, lighter, and wont stand out like a record player in my panel...please advise!

[N941WR]

For me, it came down to my mission.

My -9 was built for day/night VFR flights with little consideration for an IFR upgrade. Thus, I went with the 320A and Icom A200. The A210 was not available at the time.

The 320A reports to ATC just fine and I seldom change it from 1200.

BTW, for the mode-C part of that, I have the Dynon converter. Don't forget to add in the price of that little unit to your cost calc.

[guccidude1]

I bought most of my center stack from the local avionics shop. I figured if they had problems, I could get it fixed very reasonably. I bought the used #2 GTX-327 out of a business jet for $1000. I used the serial output from the Dynon 180 and saved the cost of a blind encoder. Dan

[Brantel]

I bought a refirb Kt76a just because it was super low cost. It works great along with Dynon's converter....

Sure it looks retro but the user interface is hard to beat. The only thing I miss is the pressure alt display to ensure that the Dynon converter is working correctly.