I would put an enloop nimh pack in it from a reputable source such as electrodynamics.. I would not put a lithium battery in it. Depending on the version of dx7 it could shorten the life of the transmitter deck inside. I wouldn't send it in if it works properly.. more likely to cause more problems by having it banged around in shipping both ways plus having somebody crack it open.. A birdie told me that the older dx7 may have a bit more output power than than the current issue spektrum transmitters.. I know my old dx7 will outrange my dx18 by 2x the distance.