Fix or replace contrast/brightness sliders on 030 PowerBooks

I've collected a number of early PowerBooks and all but one suffers from quirky contrast and/or brightness controls. I've been told that the slide potentiometers on these models were prone to failure and that they weren't repairable once they went south. I de-soldered one of the units and found it was made by Alps Electric. So far I've been unable to find replacement parts. Any ideas?