I am unable to change the aperture in manual mode. The power switch is in the correct position but although the shutter speed changes, the aperture will not budge from 6.3. Any ideas what I might have inadvertently altered?
As you wrote that “it works correctly in both AV and TV modes”, where only one value is changed at a time, the following might work:
In full manual mode (»M«), to change the shutter speed, you use the rotating dial on the top right of the camera whereas for the aperture, you use the large dial to the right of the display. To be able to use that dial, the on-off-switch has to be set to the position above “on” which has a pointing line on top of it which leads to the dial.
As you stated that “[t]he power switch is in the correct position”, the above should work. If it still doesn’t, please check whether the dial works in other scenarios such as flickering through your images. If it doesn’t—while the power switch is in the correct state—please contact Canon or others for repair.
You can also find an explanation of how to change the settings in the manual that can be found on Canon’s support page for the EOS 5D Mark 2