I have just fixed my Canon 50D using a method published on martybugs.net. For some time the top dial next to the shutter release button of my camera had been slipping. This appears go be a known issue with both the Canon 50D and 40D models and could be an expensive repair, or (with a great deal of care), a fairly straight forward fix taking less than 15 minutes. It does involve the use of glue and is a fiddly process. Just make sure that you do not use too much glue which could prevent the wheel from revolving at all if it leaks into the casing. The process involves securing the rubber ring onto a plastic wheel using a suitable glue. It is best explained by visiting the following link. Note : Use a match or cocktail stick to apply the glue, 😱 DO NOT squirt it straight from the tube onto the plastic wheel 😭
http://martybugs.net/blog/blog.cgi/gear/bodies/Fixing-Slipping-Main-Dial-On-A-Canon-DSLR.html
It worked for me 😎