Refurbishing a Canon 30D
16. Feb 2009 
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I recently picked up a Canon 30D for a great price. Two things were wrong with it: its shutter release was unreliable and its focus was unreliable. It turned out that both were easy fixes: the shutter release just required a bit of cleaning and the focus was unreliable due to a dirty mirror.
Cleaning the shutter release involved opening up the body, disassembling the two-stage switch and using isopropyl alcohol to clean the metal contacts. This is described in detail here. There is also a video of the procedure here. The procedure is pretty easy and I was able to do it without using my soldering iron. There are quite a few screws of different dimensions to keep track of, that was the only tricky bit. I was very impressed with the quality of manufacture of the body.
The AF problem was simply a dirty mirror which I solved (carefully) using the eraser head of a pencil and some lint free lens cleaning paper.
During my research on tuning AF, when I thought that it was backfocusing, I scanned various forums and pages on how to calibrate the focus points of the 30D. It turns out that the AF mirror rests on an eliptical 1.3mm bolt inside the mirror box. By turning this bolt, one can play with the offset of the mirror with respect to the AF sensors. The offset adjusts the effective distance from the AF sensor to the centre of the lens, which should as close as possible to the distance from the centre of the lens to the CMOS sensor as well as the viewfinder. The procedure is described in detail here, here and here. There is a caveat though: the offset bolt will change this distance in one direction for the sensors at the top of the image and in the other direction for the sensors at the bottom of the image. By adjusting the offset, you might be throwing (at least) those sensors off. (This is also explained here but the author refers to the sensors on the sides which on the 30D should not be affected as there are actually two elliptical bolts.)
One more thing, if you have a problematic EF 50mm f1.8 that is severely back- or front-focusing, you might be able to bridge some solder points inside the lens body to correct it. This informative post details the procedure with images. More information on this thread, which also links a collection of lens exploded diagrams here.