A 3S LiPo hot off the charger can be 12.6 volts, and stays there a while, before slowly dropping. A NiMh may read high at 10v (or a bit more) initially, but it very, very quickly drops. Nominal voltage for the TX is 9.6v. If it doesn't produce a puff of smoke, the stress of the higher voltage will greatly shorten the life of the TX.

Use a LiFe cell instead. They have a lower initial voltage and are much closer to the nominal 9.6v. Many sources for a TX replacement pack. Some may even fit! You will need a charger that has an LiFe setting.
Still the 1 hr charge time, MUCH safer from fire, less need to reduce to storage voltage when not in use (a good idea anyway, but not as detrimental if you do not).

LiFe, beyond lower voltage, also may not have as high a "C" rating - so not favored for motors. But in this instance, where high current is not needed, is a better solution.