The Mountain Topper 3 Bander, or MTR3b, is a famous radio and for good reason. It’s a compact, lightweight radio that gets you on 20m, 30m, and 40m. It sips power yet has a 5W output. It has a nice receiver. It’s ruggedly built. However, the power jack didn’t match any other piece of equipment that I had.
Shortly after building the 80m QCX-mini, I was frustrated that I had to take out two different sets of batteries if I wanted to use both radios. So, I began searching the internet for the proper adapter. It quickly dawned on me that taking additional fiddly bits into the woods is a recipe for losing said bits in the woods.
A better direction altogether: install a 2.1mm DC barrel jack into the MTR3b so that it uses the same connector as my TalentCell battery pack and my QCX fleet. It’s a simple modification that anyone with a soldering iron can accomplish.
The first board in the back lifts off of its header pins easily. 
Desolder the power leads then remove old jack. 
Install the new jack.
Luckily, even cheap DC barrel jacks are roughly standard.
Before reinstalling the power leads to the jack, use your multimeter to ensure that you’re connecting the positive red wire to the center lead. 
Verify that your soldering is done well. 
Reinstall the power board onto its header pins.
Standardizing all of the components in your station will come in handy in the long term. It is easier to hook up all components and repair equipment if there are only a few options to choose from. At this point, I am investigating how I can continue to use only 18650 batteries, 2.1mm DC barrel jacks, and BNC connectors across all radios and accessories. The list will continue to grow as I find new ways to streamline my station.
This TalentCell 12V 6Ah power bank will run this radio for 20 hours continuously.
Amateur Radio Station WB0ISG 