Transponder Pondering

[Ironflight]

In prepping for an upcoming Instrument Check Ride, Louise realized that we needed to get Mikey?s Static and transponder checks done soon - and since the Val was just about due as well, we called our local itinerant avionics guy and arranged an appointment for him to come by the hangar. He?s always happy to come work on the glass cockpit airplanes because they have fewer problems than the old stuff he gets to certify, and this was his first chance to see the -6?s upgraded panel. We unpacked his gear and hooked up the plane for testing, and quickly discovered that while all the new stuff was dead on accurate, the only thing that we had left in the airplane from the previous panel was going to be a problem - the ancient Narco AT50A Transponder failed several of the tests, and wouldn?t respond to Mode C interrogation.

Just to make sure that the tech?s equipment was functioning properly, we hooked up the Val, and everything passed - rats! I was hoping that maybe it was a test box problem ! Ah well, out came the transponder, and the tech (A retired gentleman who works out of his home shop now) volunteered to take it home for a quick check on the bench to see if indeed the problem was there or in the tray, wiring, or encoder. I hate to throw good money after bad, or chase a problem in the wrong place!

Well, the word came back that what he found was ?interesting?. When he opened the unit (which hadn?t been touched since before Louise bought the plane, but still had a yellow tag pasted on the cover), the first thing he found was that the main power trace on the circuit board (between the connector and the switch) was charred and burned up. It had been that way for awhile obviously, because someone had soldered a 20 gauge wire to the board to bridge the bad part! Ahh, but they were obviously worried that the wire might short on something, so they had covered it over with Duct Tape to keep it insulated (!?)?.There was more duct tape on the bottom of the board - maybe it was rubbing on the case and shorting out? Obviously, what we have here is a Narco Boat Anchor - certainly not something that would ever be airworthy!

I gave a quick thought to finding a used AT50A to slide in, or one of the newer Narco upgrades, but after a quick call to Stein to find out the price of a Garmin 327, I decided that for a couple hundred extra, and the minor work of changing the tray and wiring, it made a whole lot of sense to go with a more modern, up-to-date, (and supported) unit. Transponders are easy to install, especially since you can use a Serial Data line from the EFIS for the encoder these days. Power, ground, antenna, and serial line - and that?s about it. Fortunately, it is also the bottom rack in the stack, so the mechanical stuff is fairly easy as well. And the guys at Stein came to the rescue with an offer of overnight shipping, going out immediately, so I can be installing it tomorrow and get it certified in time for Louise to not miss a day of training (she?s occupied with work this week). Yes Steinair's phone is often busy, and there?s a reason for it ? they treat customers very well!

The most amazing thing about this was that the unit has been working when talking to ATC right up until the current time! Maybe that is a testament to the damage-tolerance of the AT50A, so if you still have one and it?s working, I?d say you might stick with it - just watch out for what may be hidden ?inside the case? if you?re not the original owner. And be cautious of yellow tags that aren?t worth their weight in paper?.

Paul

 

[mtclay]

Somewhat Similar Situation

Paul,

My RV-8A has a not so ancient, but still old technology, Narco AT-150.

During my last pitot/static check in April it passed, but barely. The technician told me that it was on it's last legs, and to be alert for it to fail soon. Since then I've been nervously expecting it to fail at any time -- not a good confidence builder.

I too have zeroed in on the Garmin 327 as the most likely replacement. The guys at Gulf Coast Avionics (near me in Lakeland, FL) tell me that there is a module available to attach to the rear of the 327 that makes it a slide-in replacement for the AT-150 in the existing Narco tray. Apparently the Narco tray is deeper than a Garmin 327 tray.

Would you add the module and use the existing tray, or rewire and use the Garmin tray?

Thanks,

 

[Ironflight]

Would you add the module and use the existing tray, or rewire and use the Garmin tray?
,

I decided to go with a new tray because I am also going to get rid of the old discrete altitude encoder, and feed the transponder through the serial link from the EFIS. If I wanted to use the old encoder, saving the tray and existing wiring would be a lot more attractive. I would probably drive my decision with those considerations. In this case, i expect the encoder is probably pretty old, so how long will it last?

Paul

 

[SteinAir]

Throw the Narco tray away and put just install the Garmin tray. It's a quick, the Garmin tray is nicer all the way around, it's an easy re-wire job and it'll be cheaper. You'll be getting a tray with your new Txpdr anyway, and it's likely that you'll never use the old Narco tray again. Plus, the DSub pins are FAR superior to the Narco card pins....if you keep the Narco tray, now you have more connections (between 2 different sets of pins), and more expense for the little module - read places for Gremlins to live. If you have a Dynon or some other type of serial encoder, you're literally only talking about 3-5 wires to wire up the entire transponder....hardly worth keeping the old Narco tray.

My 2 cents as usual!

Cheers,
Stein

 

[mtclay]

Wow, you guys are fast!

Thanks for the good advice.

I'll order the Garmin 327 from Stein and install it in its own tray as recommended.

 

[Ironflight]

Transponder Transplant results

It is highly unusual when ANYTHING that we do on a homebuilt goes exactly according to plan, so I am a Little hesitant to say that this job is actually finished - but the installation went well, and the transponder is apparently working fine!

Here is Mikey's "final" stack:

A few notes:

1) The outside dimension (width) of the GTX 327 tray is slightly winder than the other trays in the panel, so I had to do a little bit of filing to make the slot wider so tat it would fit. The slot is edged with 3/4" aluminum angel, which meant I had a bit more material to remove, but it only took about 30 minutes to have the tray mounted nice and snug.

2) Power, Ground, and lighting were all I needed - plus a serial line for the altitude data, and the antenna. While the installation manual shows a right angle BNC connector (to which you solder the antenna wire) on the drawings, what was really there was a BNC bulkhead connector, so all I had to do was put a standard BNC male on the end of the existing cable, and it mated right up. Much nicer!

3) I took the altitude data off of the Dynon D10A, and low and behold, I had actually tucked the serial "out" wire that was installed in the harness connector into the wire bundles for safe keeping when we id the original install. It was a simple matter to find the end and splice it to th wire I ran out of the transponder rack. It worked the first time when power was applied!

4) I really like the GTX 327 because it displays the altitude it is getting from the encoder, so you know right away if it is working or not. A quick trip around the patch in the evening with a call to Houston Approach for a Mode C check confirmed the installation works. We'll finish up the IFR cert this week, but at least I now know it works!

5) It is really great to have a company like Steinair at our call when you need something in a hurry. They are homebuilders themselves and understand when things need to be a rush job, and when they don't - a real big advantage over the "big box" aviation stores who's employees might or might not fly, much less own airplanes!

While I certainly didn't expect the sudden expense of a transponder, and didn't really have anything against th old AT50A, it does feel mighty good to get that old beast and it's ancient encoder out of the panel - the last of the old stuff to go!

Paul