Ah. Well, you will have to get another wall-wort, of course. The control is probably the level control for the tweeter.
In principle, all you have to do is plug in the PS, connect the terminals up to an amp, and listen. I urge caution since these are old and may be faulty. (I have had on and off problems with old Janszen tweeters.) So, first plug the wall-wort into the AC by itself and see what is on the output and if it is stable. If so, connect it to the speakers and listen for noises. Good els elements will be quiet but may make very, very soft crinkly noises as they power up. After that, use your voltmeter to insure there is no DC or AC on the main speaker terminals before you connect it to a good amp.
Good luck.
Kal