OK. Forgive the lengthy explanation, but you need to understand how the system works in order to understand the problem.
First, look at the back of the lens, in the area of the photo:
Note the slot in the shaft. When the lens is cocked, that shaft points at the red dot to its left, roughly at the 9 o'clock position.
When released, the shaft rotates counterclockwise* about 300 degrees and the slot points to 10 o'clock.
If your shaft rotates farther than that, something is broken. it definitely does not rotate 'more than one [full] turn'.
Now look at the little Lock Pin in the hole to the right of the shutter drive shaft. This is important.
When the lens is cocked and off the camera, this pin should be slightly above center in its enclosure.
If you move the pin down (toward the outside of the lens), it will release the shutter.
When you cock the shutter manually, this pin should return to its centered position. If not, there's a broken spring.
If it doesn't return on its own, you can check it by moving it up with a needle or tiny screwdriver with the lens cocked.
To see how the system works, look at the front of the camera body:
The flat stub on the front of the shaft should point to the red dot when the body is cocked. If not, cock the body.
There are two fixed pins pressed into the body. Pin 1 releases the Lock Pin on the lens when the lens is fully seated, thus transferring shutter control to the camera body.
Pin 2 guarantees that the lens is cocked when it is mounted or dismounted. The slot on the lens shaft must pass over this pin when the lens is changed.
This is the cause of the jam. If the lens is released at the wrong instant, the Lock Pin on the lens has not yet engaged to hold the shutter open, and the shutter cycle will start, resulting in the control shaft rotating from its cocked position. When this happens, the lens won't pass pin 2 to come off the camera.
The solution is to return the lens to its fully-seated position, if possible. Remove the magazine from the camera, then open the rear curtains. Use a screwdriver to rotate the control shaft inside the body in a clockwise direction until the shutter is cocked again. The shaft has a slot specifically for this purpose.
What I've described is the simplest of the jam scenarios, and the easiest to cure.
It's also possible for the lens shaft to rotate around pin 2. The only solution to this is a complete disassembly of the camera by a qualified technician, and even then some parts of the lens mount may have to be destroyed to remove it.
This second type of jam can occur when someone is trying to clear the first type. There's a tendency to try to remove the lens if at all possible by twisting it in the removal direction while rotating the shaft inside the camera body. This can result in the lens shifting far enough to wrap its shaft around pin 2, because the Lock Pin was disengaged and the shutter was not fully cocked.
To avoid this problem, whenever clearing a jam, return the lens to its fully seated position before turning the shaft inside the body. Rotate the screw driver as far as it will go clockwise (don't force it). Remove the screwdriver and confirm that the shutter is cocked. Press the lens release (not the shutter release) and turn the lens very slightly in the removal direction, confirming that the shutter remains open. If it does, it's properly cocked and can be removed normally.
- Leigh
* that's 'anticlockwise' for our British friends.
