I have 3 viewports at 20 scale and 1 viewport at 100 scale.
I want to turn off certain layers in the 100 scale viewport.
I think it's something about freezing in one viewport only??
Any ideas?
Switch to/activate the viewport whose layers you want to freeze and then explore the "VP" (view port) settings of the Layer Properties Manager:

Switch to one of your paper space tabs, then toggle the paper/model button at the bottom to model, then open layer manager. Toward the right side of the row for each layer is a column called "current VP freeze", this allows you to freeze layers in individual Viewports.
Thanks Guys
I used to know how to do it but it's been years since I needed it.
if you have expresstools loaded, you can choose the freeze layer command in an active viewport to freeze it there.
i envy you for not needing to freeze layers in a VP...