The DOF preview button on the F90/N90 series is electronic not manual. It probably wouldn't be worth the cost of repair, but why not get an estimate. It could be that its a loose connection, but then again, the motor may need to be replaced.
Since you are in the US, you might try looking at an authorized repair facility here. But really any repair facility may be able to look at it for you. Nikon USA will not look at any non official import unless you can prove that it was purchased officially and was not gray market (your receipt should work).