Reviewed April 2002
Several of the older aircraft I've bought have come with a Narco AT50A or AT150 transponder installed. This unit may have been state of the art in the early 80s, but for me it has proven to be highly unreliable. I have had two AT50As bite the dust and the repair was more than the replacement value. Foolishly I tried to save some money by replacing one of the AT50As in 1998 with a new tray-compatible AT150. The AT150 has proven to be equally unreliable in insidious ways: it will work fine on the bench for hours on end, but in my Bellanca the transponder will mysteriously fail about 30 minutes into a flight. A friend's known good unit works fine in the plane, so it is something in the box, not the installation.
After being griped at by Renton Tower for not having a functional transponder for about the tenth time, after replacing most of the wiring, the connector, and after having had the Narco fail during a transponder certification check even though it was happily replying on the flight in to the avionics shop, I decided to throw the Narco out and start over.
For replacements I evaluated the "standard" panel mounted units: the Bendix-King KT-76A (reconditioned), the Apollo SL70, and the Garmin GTX-320A. The Bendix-King has the benefit of field-installed experience and it is a reliable unit. It is readily available in either new or reconditioned form for about $1200 or $900 respectively. It is also designed around older discrete electronics and it uses a cavity-tube transmitter design that requires warmup and uses a fair amount of power to run.
Both the Apollo and the Garmin are relatively new designs dating from the late 90s that use a solid-state transmitter. The advantages of the solid-state transmitter are no warmup time and no need to worry about replacing the cavity tube which will eventually fry itself when its service life is up.
The Apollo SL70 is the most advanced and is the smallest height box. It has an electronic code selector and display, and it displays the reported altitude right on the front panel. The Apollo has a one-button "squawk VFR" feature. It is also the thinnest and lightest unit, although it is a full-depth unit. Unfortunately the Apollo is also the most expensive unit at about $1800.
The Garmin GTX-320A is a more traditional 4-dial selector unit without any frills. It is a "standard" height box but the depth requirement is shorter than the norm, about 9" on the Garmin. This could be very handy on restricted-space panel installations. The Garmin's advantages are price (about $1200) and the availability of tray adapters for another $100 or so which can adapt the Garmin into either a King KT-76A or Narco AT150 mounting tray. If you are simply looking for a low-cost, zero-installation replacement for either of these units, the Garmin GTX-320A is a new technology design with good value.
Even though the other radio in my Bellanca panel is an Apollo unit, I went with the Garmin for reasons of price and because the front-panel height is similar to the old Narco unit. If I had installed the thinner Apollo I would have ended up with a open gap in my panel which would have been hard to fill.
The fit and finish of the GTX-320A is quite good. The switch action is tight and snappy. The reply lamp is a small, low key LED on the Garmin, which I appreciate because it won't be a distraction in flight. The backlighting appears to be accomplished via LEDs.
For a small box, the GTX-320A is surprisingly heavy weighing in at 2.9 pounds. This is heavier than the Narco is replaced by 0.7 pounds. The connectors on the Garmin are a standard BNC-style RF connector in the center and a DB-25 data and power connector.
The one area where I found the GTX-320A to be less than ideal is the mounting tray. The tray is folded up aluminum which is pop-riveted together. To my eye the tray is on the flimsy side, especially when compared to the Apollo radio trays which are made out of much thicker (and stiffer) plate. I had a secondary and more annoying problem which was that in my Bellanca the avionics stack is supported by two vertical U channel pieces on either side of the trays. These provide great support, but they're fixed in position. The GTX-320A tray was about 0.040 too wide to fit between these supports, and just slightly too wide for the front panel cutout. Both my Apollo SL40 radio tray and the old Narco AT150 tray fit between the brackets without interference.
Since the dimension was close, I filed on the brackets and widened out the front panel cutout so that the GTX-320A could be squeezed in with a tight interference fit. I also had to drill out the front pop rivets holding the front of the tray together and replaced them with flushed rivets on the outside of the tray to avoid interference. Needless to say this was not an ideal installation.
Another possible difficulty to keep in mind with the GTX-320A is because the unit is shorter than a "standard" avionics box, you may need to add extra supports for the rear of the shorter tray. This would not be an issue in an all-new installation, but might require some extra work when replacing an older box with the GTX-320A.
A positive feature of the GTX-320A tray is that the rear of the tray which supports the connectors is a removable casting. It is rather heavy, but having the rear of the tray removable makes installation of the RF connector (with a snap-ring) and the data/power connector quite easy. Four hold-down screws tapped into rear casting, reachable with a long screwdriver through the front of the tray, secure the rear casting to the tray itself.
The wiring for the GTX-320A is a standard blind encoder 10 wire interface, along with power, ground, and a few frills like DME suppression and external IDENT lines. For a simple standalone installation it is simple to connect to any encoder type. I used an Ameri-King AK350 since it was already in the aircraft from the previous installation. The tray DB-25 connector is a high-quality crimp-pin unit, although you can solder the pins if you don't have the rather expensive crimp tool. The right-angle RF connector, along with the data/power connector backshell and securing screws are all supplied with the GTX-320A. Everything fits together in the rear tray connector and centers up nicely. Overall there were no surprises with the electrical connections.
I am pleased with the GTX-320A. It operates as expected for a no-frills transponder, and the installation was relatively painless. I would like to see the tray improved, but the unit itself and installation accessories were of high quality. The GTX-230A is price-competitive with the older generation of King and Narco transponders and should offer increased reliability.
Given a fresh avionics stack installation, though, I would go with the Apollo because of the smaller height and next-generation feature set such as altitude display.