Cordless Handisaw universal adapter/charger. How do I find out the polarity needed for the tip?

I got a Cordless Black & Decker Handisaw 3 years ago and the charger has been toast for a year so I threw it out. I have a universal adapter but can't figure out the polarity for the tip. The saw says 6 volts DC, then has a straight line on top of a dashed line, I don't know what the lines mean. I tried using my continuity tester to determine the ground but am lost. I'm ready to just toss it, so have it plugged in with a universal adapter at 6 v with + tip and pray it leaps to life in the morning.