The PRC-104 is an interesting portable radio.  Some complain it is hard to repair, but fortunately they are pretty reliable if they are working, and extremely rugged.

There are two battery boxes, a small one which you can put your own cells or small pack into, and the larger pack which takes the standard BB-390 batteries.  It is designed with a charger circuit for those packs with an external connector.

The problem is, that while the case will take modern BB-2590 batteries, those have a higher fully charged voltage than the radio can work with and this could damage the RT power supply.

I decided to use a cheap Chinese buck-boost DC-DC converter to solve this problem and another one, the lack of ability to run the 104 from a 12 volt supply.  Here I removed the original (useless) BB-390 charger and have the DC-DC mounted on a small piece of fiberglass. 

This is a difficult install.  This DC-DC is just a tad too long to fit and the case needs some modification internally.  I think if you just decide to drill more holes in the aluminum plate you could dispense with the intermediate fiberglass board and it would work out better.

The switch has two functions:  one, to select batteries or external power as the source, and two, to disconnect the DC-DC from the batteries to eliminate the 32 mA idle drain when not using the set.  I set the DC-DC output to 26.0 volts.  The output is steady from 8 to 35 volts (the LTC3760 chip is rated for a max of 36 volts).

Here it is all finished:

I wired a cigarette lighter plug to the external connector.  It works very well, here it is running on 13.6 volts:

The drain is 422 mA with the RT in receive.

Here is the display during this test:

During transmit into a dummy load wattmeter the voice peaks look good at close to 20 Watts and the current (using the larger power supply below the digital one above due to current requirements) the peaks are showing at about 4.5 amps on the analog meter.

I tried the radio in my car and it works great.

The advantage of this DC-DC is that it is under $12 shipped and based on a great LTC part.

I checked for RFI and found I could hear weak birdies with the antenna terminated that varied with DC input voltage.  However, the levels are low enough that as soon as an antenna is connected there is no way they could be heard.