I replaced my Narco AT50A two years ago with a Garmin GTX320A all solid state transponder. I bought mine from Aircraft Spruce and their current price is $1349 including the installation kit. I've been very pleased with the Garmin.

Narco's new AT165 is also 100% solid state (no cavity tube), and AircraftSpruce sells them for $1479.

thank you for the suggestions.
Well, I bought a king from a friend of mine but he didn't have the rack and plug, so I bought that too from someone else. Now  I dont have the wiring layout, does anyone know where I can get one from?

jaime
Happy flying.

Some transponders have the wire code on the inside cover next to the terminals. The encoders have their directions in the box when you buy them. What brand do you have? Using the same brand makes it a snap in replacement, otherwise it will take hours to wire.