The various DIY "retro bright" recipes works very well on old 70s and 80s plastic. It's a bit messy to work with, if you use arrowroot or wall paper glue as thickener in stead of corn starch you don't have to stick it in the microwave. I've done it a few times and I think the yellowing largley is a film of dirt and grime. In cases where you can take the plastic cover completely off and run it in the dishwasher once or twice it turns out just as white and clean again. Be careful with the dishwasher, not all plastics can take the heat, but a lot of hard plastic do.