You may just have a slipping Main Dial. It can be fixed yourself if you are just a little "handy".

Fixing a Slipping Main Dial on a Canon DSLR

In recent months, the main dial on one of my Canon 50D DSLR cameras would slip when attempting to rotate it.

The slipping occurs more frequently in one direction than the other direction, and also appears to be weather-related, occurring more frequently in hotter weather.

This was particularly annoying, as most of my photography sees my camera mode set to aperture priority, and without a fully functional main dial, it was difficult to quickly adjust the aperture.

It turns out this appears to be a relatively common issue with some of the Canon DSLRs, including the 30D, 40D and 50D.

Getting the dial assembly replaced by Canon will apparently cost you about USD$150.

On closer inspection, it was clear that the rubber grip on the dial was slipping on the inner plastic wheel. Rather than sending the camera into Canon for repairs, I decided to try a DIY repair by applying some glue between the rubber grip and the plastic wheel.

I lifted up the rubber grip on the main dial with my finger (as shown in the photo below), and used a toothpick to apply some glue to the plastic wheel. The rubber grip was released back into position, I rotated the main dial by approximately 180 degrees, and repeated the process with some more glue. If necessary, use a toothpick or paperclip to assist in lifting up the rubber grip on the main dial.

I used some Tarzan's Grip General Purpose Glue, as I happened to have some handy, and the specifications indicate it will bond to rubber and plastic.

Be careful not to apply too much glue, as that could result in the glue squeezing out the sides when you release the rubber grip back onto the plastic wheel. That could result in the main dial being glued to the outer case of the camera, which would prevent it from being rotated!

The DIY glue repair has been successful, with no more splipping of the main dial, and was certainly a lot cheaper than sending the camera to Canon for repair.