| The original Boss PS was not regulated, so it puts out 12v when only one pedal is atttached, but less when you put more pedals on the chain. The old Boss pedals internally drop the power down from 12v to 9v. If you plug a modern 9v power supply into the old pedals, they'll drop the power down lower than the 9v they're designed to run on, so they won't work properly.
If you don't want to use the old ACA adapter, you could use a 12v tap from a pedal power supply that has such a tap. Or, you could have the pedal modified to work on 9v, but since I think there's some collector value for the older OC2, you might not want to do that. Personally I'd sell the old one and get a newer one that runs on a standard supply. There's no difference in the audio circuit so it's not like the old ones will sound better. | 
