Please forgive my English.
I will try to explain haw to do it, I will do this in a couple of weeks because I’m having too much work now. If yow have a radio like futaba attack4 that have one of this chips, CD8081 or NE5054 you can take out the signal from it. The chip can be found in the board and it’s simple to identify it. You must solder a 3.3k resistor to the pin 10 and then solder it to a jack so you can connect the interface there. Any of the interfaces will work. I’ll use a stereo jack so I’ll connect an external source to feed the transmitter.
In a Couple of weeks I’ll make these and post the article fell free to e-mail me for any question. I’ll respond in my poor English.
Good luck, Gustavo from Sevilla Spain
