The PRC-350 is a nominal 15V supply, from a cassette of 14 1.2V NiCds (actually around 17V). Its clear that putting fourteen alkaline cells in there could damage the radio (21V).
Ideally, to allow either NiCd/NiMH or Alkaline cells, I would use dummy cells to fill the unused spaces. But I dont have any. Luckily, someone on one of the forums has worked out a way of adding a wire link to the terminal cards that effectively bypasses four cells.
The same applies to the PRC-349 (although I know that it can survive a brief run at 15V as ive just tried it!), this uses a 10 AA cell cassette to give 12V from 1.2V NiCds. Again, for alkalines, I really need to work out a scheme to allow me to bypass two cell positions. Perhaps here the easiest way might be some pieces of dowel with drawing pins in either end and an insulated wire between them!
Either way I now know how many batteries I need to acquire tomorrow! 16 AA's for the two PRC-349s, and 10 C's for the PRC-350
More on the PRC-349 tomorrow...