In my 16 years of flying, I've had 0 transmitter crystal failures, so I see no need to replace them, plus most consumers replace the radio within 5 years due to wear and tear, batteries get old, antenna's wear out and the pots as well. But never a trans crystal failure, the receivers and crystals have died time to time but they are under extreme abuse all the time. I read the FCC guidelines and sport pilot is correct, and everybody is correct as well. Yes you can repair your transmitter by yourself as long as it does not manipulate or alter the modulation of the frequency it was tuned for. If your switch dies, you can change it, if the antenna breaks, you can change it, ect,ect.... But you may not replace the crystal in the transmitter unless it is retuned to the proper frequency. If FCC was going to hand out liscenses for radio tuning then there would be a trade school course on the subject. I pay for my HVAC liscense every year and there are thousands of hours required to obtain it along with schooling, you just can't apply for the liscense, so the FCC has thier guidelines to qualify any such repair facility to do repairs and they follow the FCC guidelines, they don't send the FCC money to renew a liscense, they just pay a fine if found in violation of the regulations.