I think it would be easier to just buy some transistors which can handle the voltage and solder them in instead
OK. I thought about that, but the transistors are on the receiver board and supplying any 'more suitable' transistors with the 6.5V would surely mean supplying the board with the same 6.5V - and I don't really know enough to understand what that would do to the other components and the IC on the board, but my gut feeling tells me it is probably not a good idea.
Expire ? That's a new expression for me regarding transistors.
Also Magic dust ? It's usually Magic Smoke that powers electronics, maybe yours runs on dust... interesting.
Your transistors can handle 25V, it's the Emitter-to-Base voltage that must not be over 6V.
Expire / self-destruct / terminate / perish / give up the ghost / die / fail / pack up / break / melt / implode. Call it what you want, it just don't work
Magic Dust : well, you see, here in Africa things are always slightly different and unusual. I could consult the local witch doctor, but am not certain that I would get a definitive answer, and it would likely cost me a goat, or at least 3 chickens in fees. And since the 'dust' vanished almost as quickly as it appeared, I suppose that one could imagine that it had been smoke.
Again, I think that the problem is that the transistors are on the receiver board and therefore limited to the restrictions and limits of the boards components.