The 50d is a camera with not too may problems. But once happened, that the camera of a friend crashed, gave the error code ERR99. Like construction, the 50d is very similar to the 40d.
Excepting the fact, that the CMOS is fixed to the body not with washers and screws, but with springs and screws.
This means, that the shutter in the 50d can be replaced ONLY using a measuring gauge for the original displacement of the cmos.
So, before You take out the cmos, You have to mark 3-4 points on the base plate of the cmos. Measure the distance of this points from the plane of the bayonet. After disassembling and assembling of the shutter, You will have to readjust the noticed distances with a screwdriver. After this operation the cmos will be practically in the same position.
The screw and the spring.
The screw has a T6 (Torx) key opening – this kind of screw permits to transmit relative big torques, without axial force.
The measuring bench with the camera.
Measuring the position before the repair – under the pin there is a marking point made by me.
Here You can see, how the gauge used by me has high resolution (=0,001 mm) but only 12,5 mm small track. Therefore the zero-setting must be with a quality etalon – in my case 40 mm.