The Canon EF 20-35mm is a highly rated wide-angle lens that suffers from a common AF malfunction. Many users find the focus control stops working in both auto and manual mode. An excellent article by Piers Hendrie illustrates very clearly how to take apart the lens, and one possible fix for the issue.
The cause of this problem is that the USM ring motor (#4 below) is not applying enough pressure to the focus ring (5). Pressure is applied to the focus ring via a spacer (1), wave washer (2), and a felt ring(3). When you have this issue you can still hear the USM motor moving, but the focus ring does not move.
Photo credit: Piers Hendrie
The top case on the first generation MacBooks are dirt magnets. While the palm rests on the black models simply become polished with use, the white models gain a disgusting brown patina. Apple’s usual suggestion of a damp cloth does little to nothing to remove the tan stains on the palm rest. Dirt and sweat from your wrists seeps into the tiny pores on the plastic surface, rendering in nearly impossible to remove with traditional cleaning methods.
They key to cleaning the top case is melamine foam. Melamine foam is formed when melamine resin cures under certain conditions. Melamine foam looks like standard open cell foam and is comprised of tiny, extremely hard fibers. These characteristics allows it to scrape away material from extremely small cavities, which is exactly what is needed to clean the top case.
Melamine foam is marketed under the trade-name Basotect from BASF. Fortunately it is also available to the consumer in the form of the Mr. Clean Magic Eraser from Procter & Gamble.
As a general rule, I repair all my own out-of-warranty electronics. So when I bought, at a significant discount, a lens that had a broken plastic outer cover on it I performed the repair myself. This particular lens was made by Tamron, who happens to sell repair parts to the public.
Here’s the damaged part.