Notes on the Vietnam-era AN-PRT4A and AN-PRR9 6m helmet/handheld radio

Years ago, when I bought these, I was much more involved in the ham radio community and wanted to be able to communicate on 6m as well as the other bands I was equipped for. I happened to find these long-obsolete radios on someone's table at NEARFest for a song, so I grabbed them and then promptly forgot they existed as the batteries were nearly dead. There are plenty of pages about these radios that go into much detail about everything except the batteries, so I'll not expound on them much, but I wasn't too happy with the battery availability. The transmitter unit takes a BA-399/U 15V dry cell and the receiver uses a BA-505/U 6V dry cell. Most people seem to run the transmitter on a pair of 9Vs and the receiver on a single 9V with a dropping resistor or reverse protection diode in series with it (since the radio doesn't have one built in!) but I hate the cost of 9V batteries, so I decided I could do this better by making Vietnam-era milsurp radios MODERN! That means... yup. Time for a lithium battery conversion! So far I've determined that I can probably buy cheap rechargeable CR123A lithium batteries and chargers for them on Amazon and they'll fit nicely in the form factor of the original batteries-ish, so my rough plan is to design a custom PCB for each one, use CR123A battery holders, and maybe have a friend 3D print a housing for the whole thing so it fits nicely. This way I should stop losing money on these as soon as the cost of building the adapters is eclipsed by the number of times I've been able to recharge instead of buying 3 more 9V batteries at 2-3 dollars apiece.

Current parts list: transmitter adapter will use 2x Keystone Electronics #1079 dual CR123A holders. receiver adapter will use 2x Eagle Plastic Devices 12BH23P-GR single CR123A holders. All are available on Mouser for decent prices.