The symptom: no AF function with any lens on the body. The lens tries to find the right focus, moved from one end to the other, then stopped.
Explanation: by the Canon AF system, the bodies AF sensor needs a brightness-change of the picture folling on one location of the AF-sensor. In fact, if we point the camera on a region of the object, where there is a change of the brightness (for ex a vertical edge an real object), then the image of this will appear twice on the same AF sensor element: symmetrical on the two sides reported to the sensor elements center. If the object was in focus, the two brightness changes will be exactly in the middle. The camera will measure how big the distance between the 2 points is. As bigger the distance, as more out of focus. The body will then send the information to the lens, how much it has to move the focus motor to go to the perfect focal plane.
If the body cannot see any evaluable brightness-change, then will send the lens to on end of the field, then will move it to the othe rend, to find the usable brightness change on the length of the AF sensor. If there was nothing usefull, the lens will stop without notifying of the sharp picture.
In our case it was clear, the body was not able find the focus.
– It could be a bad lens
– Lens with to high F-number (the AF uses the lightbeam from the outher regions of the lens)
– Low lighting
– Bad AF sensor
– No lightbeam from the object to the AF-sensor (dirty mirrors, bad mirror angle)
In our case it was the last one. I noticed that the submirror has a not very definite moving when the main mirror goes back to 45 degree position. I tried with my favorite tool (You know it from the denstist:. that spiked tiny needle on the end of a handle, with which they check the caries on Your teeths. Brrrr.) . With the mainmirror in basic (45 degree) position it was possible to move 2-3 mm the submirror towards the shutter… Ouuups! The submirror should lie on it’s bumpers in the mirrorbox.
I tried to focus with a lens after moving the submirror as much to the bumper as possible.
This time the AF was working!
Then I checked the spring with the dentist-tool. The spring was loose, it was broken!
Solution: To repair the spring I have to disassemble the whole body, PCB-s, base-plate, CMOS, shutter, the main mirror itself, then to change the spring.