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Your allowed to modify up to five of the same transmitters for your own personal use but you can't sell them. But truth be told, its a paper tiger. When was the last time you saw an FAA agent coming to your, or anybody's door for that matter? Or even heard of one? I'm going on 45 years in R/C and even back when you had to apply for a license I've yet to see an actual FAA agent.
Here's a few pics of a very old Futaba I did a few years back. Pretty easy to do. Hook up Power (red), Ground (black) and Signal (yellow) then mount your antenna.
Here's where I mounted the FrSky in the side panels of the transmitter. Since this transmitter is just for me, I replaced the trainer switch with the FrSky mode selector. But in hind sight I probably should have just tucked the switch into the side panel and forgot about it.
I found good place to tap the PPM signal, the blue wire at the trainer port. I simply soldered the yellow wire from the FrSky there.
I also found a nice place to tap the Power and Ground on the back of the circuit board and again soldered the FrSky wires there. DON'T forget to remove your old transmitter crystal.
Mount the antenna and done!