The Canon A1 uses a Horizontal travel focal plane shutter. There are two curtains wound on a drum each side of the film plane opening and spring loaded. The two curtains move independently of each other. The first curtain is just long enough to go from the take up drum to the set drum. The second curtain is long enough and taller to wind fully onto the set drum with an opening the distance of the film plane opening. When you wind the film the shutter curtains are wound onto the set drum with the first curtain blocking the film plane opening. When releasing the shutter on B the first curtain is wound onto the supply/take up drum by spring tension and the film plane is open to the lens and the mirror goes up. Upon releasing the shutter button the second curtain is wound onto the supply/take up drum by spring tension and blocks the film plane opening. As you increase the shutter speed the release time between the two curtains is changed resulting in a slit opening between the two curtains moving across the film plane opening. The width of the slit depends on the shutter time selected.
Now you have established that your shutter is in a state of malfunction as you cannot see through the film plane when the shutter is released. Even at 1/1000 shutter there would be a flicker of light on a properly operating shutter.
With the back open advance the film wind lever very slowly. When you see the curtain starting to move observe it closely, at some point you should see two curtains with a junction seal/overlap. If you see them then use a thin blunt object to see if they will separate slightly the full height of the film plane opening. The curtains are rubberized nylon or silk and can be damaged if poked too hard or poked with a sharp pointed object. If they are stuck from age and lack of use you might be able to get them separated otherwise a trip to the repair shop is in order or return it to the seller for a full refund.
Pictures are a Pentax, 1 is the shutter curtain junction, 2 is the film plane opening on B, shutter open.
These should give you an idea of what to look for.
Last edited by shutterfinger; 06-21-2014 at 04:30 PM. Click to view previous post history.