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TRANSPONDER SHOOT-OUT

Why would anyone do an article on transponders? After all, they've been out for years and they are all the same. Well, aren't they? To answer that question is easy, yes and no! All general aviation transponder supply ATC with "Mode A" which is secondary radar coverage along with one of the 4096 codes available on the transponder. The "Mode C" portion of the transponder shows ATC our altitude as related to pressure altitude. Just in case you forgot, you can get this altitude by setting the altimeter to 29.92" at anytime. This altitude is corrected on the ground so ATC will see you at the altitude your altimeter reads provided you have the correct altimeter setting. All modern transponders meet TSO-74c class 1 nowadays. They all have the same knobs  and numbers in the window so what has changed? 

Basically the new transponders built today are surface mount technology and non-tube (cavity) outputs which last for years without failing or even getting out of adjustment for the most part. No doubt, some are better than others, at least  that has been our experience at Avionics West. We included the S-Tec TDR-950, which in years prior was the Collins panel mounted unit, the new Garmin GTX 320 panel mount, the King KT-76A and the new KT-76C transponders. At the time of this evaluation transponders were hard to get due to the demand. S-Tec, King and Garmin bent over backwards to get me new units to evaluate and asked for nothing in return. My hat goes off to them for being so helpful!  All are great companies.  Narco and Trimble were'nt as willing to help out unfortunately.

 GARMIN GTX 320 TRANSPONDER. TSO C74, certified to 50,000 ft. This little transponder excited me early in testing because of the small size. Measuring 1.63 x 6.25 x 7.32 inches and weighing in at only 2.1 pounds, the GTX 320 made me think of the homebuilt market with often needs a transponder with a short depth. The Garmin was the smallest transponder we tested by far. One of the best features of the GTX 320 is, it's  11-33 volts and only draws 12 watts! Most transponders draw more than twice that amount of current. List price is $1,495.00 not including install kit and antenna. Our first real install of the Garmin GTX 320 involved using Garmin's install kit and connector. The tray fit poorly in the aircraft and the transponder seemed to wobble around in the tray. Clearly, the boys who designed the transponder weren't talking to the rack designers. We had to fiddle with it several hours to get the GTX 320 to look and fit well in the tray. Of the fit and finish, no doubt Garmin has some distance to go. Garmin has a "Optional" kit that will allow the GTX 320 to fit inside of a removed King KT-76A\78A  or the Narco AT-50A\150A. The adapter kits cost another $200.00 plus you still have to purchase the GTX 320. This totals up to $1,695.00. The adapter kits don't make economical sense to me. If your King KT-76A\78A fails, just purchase another one, slide it in the same rack and keep trucking! The new King KT-76A without the install kit lists for $1,725.00. You could just replace your sick King transponder and save at least $170.00 over the cost of the Garmin. The savings would have been greater if a Narco transponder was being replaced because the Narco is even less expensive  than the King is! I wish Garmin made a rack to replace boat anchors like the Genave and the KT-75 panel mounts. Now there's a real market!

WHAT'S INSIDE THE GTX 320? We opened up the GTX 320 and were somewhat disappointed. No doubt, the surface mount is excellent but you can see transistors soldered in over the surface mount. Probably there was a problem with the original PC board and it was modified to work properly. Some of the soldering around the high voltage area wasn't as good as it should have been. The only service one can perform in the field is setting the power and frequency. This is a good thing! After our installation, we found the frequency off by 5Mhz. I found adjusting the frequency on the bench a real chore. Basically what you are doing when you adjust the frequency output is bending the circuit board via turning a screw. Frequency and power adjustment is trial and error because when you tighten the lock nut, it throws the setting off! I called the product support line and they agreed that frequency setting is difficult at best. Garmin does have one of the best product support groups in the business. They were eager to help with my problem. They have the same great support on their GPS products. I've been inside many a Garmin panel mounted GPS units and they are top quality. I'd have a hard time believing the Garmin transponder is built at the same plant as the GPS units. I'm sure within time the PC board layout will improve. The GTX 320 pumps out 200 Watts and requires no cooling.

GTX 320 FACEPLATE: If you are familiar with the famous Garmin GPS 195, then you will know the buttons are on top of the unit, not below like most portable GPS's. Garmin designed their transponder so the on/off/alt switch was located on the far right. At first I didn't like the idea but after flying the GTX 320 I found it was a great place to place the control knob. The code knobs are very visible and easy to read with the backlighting. The knobs are hard to turn but that aids in putting in the code in turbulent air. I liked the way the GTX 320 felt and worked in flight. ATC picked up the transponder so everyone was happy.

KING KT-76A: This transponder has been out for over a decade using the "KT-76A" model number but everything inside has changed in the last couple years. Early models of this transponder were discrete components but now, it's all surface mount. The power out is still provided by a cavity like older style transponders. The KT76A meets TSO C74b and is tested up to 35,000 feet. Input power is 14Vdc but a dropping resistor can be used in 28vdc systems. We found output power to be 240 Watts on the bench. The KT-76A measures 1.63 x 6.25 x 10.00 inches. The King weighs 2.1 lbs. on my scales. List price of the KT-76A 14Vdc with installation kit was $1,860.00.

OFF COME THE COVERS: The KT-76A is nothing less than beautiful inside. The PC board is well laid out and everything seems to be professionally laid out on the surface mounted board. Quality is observed anywhere you look on the board. These things aren't designed to be repaired by old people like me. The components are so small I couldn't see to repair the unit should a failure happen. Best to ship this beast to the factory if it goes south and this is true with most surface mount equipment. In fairness, I've never seen a new style KT-76A fail. The supplied connector kit used gold flashed pins and so does the edge connector of the PC board. Again, this is just good building technique, which translates into quality. Adjustments are few inside unlike the old days where a bench tech could spend hours just tweaking the adjustments. King's factory support is excellent but factory repairs are much slower than in the past for some reason. Most shops have rental or loaner units around should a repair be needed. We see only a few KT-76A's come across our bench in a year's time.  

INSTALLATION: My biggest beef with the KT-76A is the dropping resistor to step the voltage down to 14Vdc if required. This is the only "modern" transponder that requires a step down converter. Fit and finish of the rack in the aircraft is excellent. The transponder fits tight just as it should. There's not much to say about the installation, it's straightforward and everything fits as it should. Backlighting can be either 14 or 28Vdc.  

DURING THE FLIGHT: I found the KT-76A knobs feel like knobs should feel, back lighting is excellent and the reply lamp is easy to see. We've probably installed  over 1000 of these great transponders. If any transponder could be called the "Industry Standard" the KT-76A would be the one.

KING KT-76C: This is King's fairly new digital transponder. With push-button entry for the code, bright gas discharge display and the display of your encoder output, no wonder the KT-76C is our best seller. The KT-76C measures 1.63 x 6.25 x 10.73 inches and weighs just 2.2 pounds. On the bench the digital KT-76C pumped out 250 watts and drew 18 watts at 28Vdc input. The code push buttons are backlighted . To enter the code you desire is as simple as pushing the correct buttons! Should you error, just press the clear button and enter the correct value. To squawk "1200" just press the VFR button. For "ident" press the "IDT" button. This thing is so easy to operate you just have to try it but be prepared to purchase one! List price is $2,173.00. 

INSTALLATION : The KT-76C installs just like the KT-76A as mentioned above. In fact, the King transponders are rack compatible, meaning they are wired the same and fit each others trays. If you do install a KT-76C in a 28Vdc aircraft that had a KT-76A prior, you have to remove the voltage-dropping resistor. The KT-76C fits snuggly into its tray and looks good in the radio stack. Once you turn the radio master on you will just love what you see. The display is easy to read and really stands out. 

FLIGHT TEST: I really enjoyed being able to verify my mode "C" was correct by just looking at the "FL" display on the KT-76C. I found this the easiest transponder of all to operate. The pilot can tell at a glance what code they have selected and whether or not the encoder output is correct. No doubt this is the "KING" of transponders. If you are thinking of upgrading in the future and have the space and budget, the KT-76C is what I would recommend.  

S-TEC TDR-950: What?!? An old technology transponder with discrete components. Why would you include a transponder like this in with all surface mount transponders? Easy, I wanted to see how older transponders would hold up with the newer units. The TDR-950 measures 1.63 x 6.25 x 7.25 inches and weighs 2.0 lbs. The S-Tec is 14Vdc only requiring the dropping resistor just like the KT-76A if used in a 28Vdc aircraft. This used to be called a Collins transponder but S-Tec purchased the Micro-Line from Collins, which included this transponder. It's a great box but that list price of a whopping $2,295.00 is more than most want to bite off for an dated transponder. S-Tec's factory support is one of the best around. I give them a call every now and then with an S-Tec autopilot problem. They are always glad to help out. I wish all avionics manufacturers had a customer service department like S-Tec. Those Texas boys are easy to work with.

INSIDE THE TDR-950: Discrete components everywhere but that's the way it was when this transponder was designed 25 years ago. On the good side is most avionics shops can repair them, parts are available and they seldom have a failure! Of the old technology transponders, this is the best on the market today. I'd like to see S-Tec update the TDR-950 but I doubt that they will. This transponder fits well in the aircraft and the rack is of high quality. Backlighting is excellent (14 or 28Vdc) and the reply lamp can be dimmed for night operation. In the aircraft the knobs feel like knobs should feel. The code is easy to read in all lighting conditions.

SUMMARY: If I were purchasing a new transponder or replacing my existing KT-76A, then I'd buy the King KT-76C if the bucks were there. It's just a well-built unit and the features are unsurpassed. With a high tech unit like this and good factory support, you can't go wrong. If my dollars were a little short, I'd go with the industry standard KT-76A. They are tens of thousands of them out there and that most customers really like them. The S-Tec TDR-950 works great but dated. For less money you can get any of the other transponder and get surface mount technology. This would be a great transponder in an aircraft that had space limitations. The Garmin has a ways to go yet. The few we've installed just haven't fit well and I've had a couple of problems, which the factory has helped out. I'm sure Garmin will solve the problems in time ahead.
This article by Tom Rogers originally
appeared on AvionicsWest.com and
is republished here with permission.