hello, I just recently acquired a 500cm with 80 Planar and a A12 film back. The problem I am having is the the shutter release will not budge when the film is advanced and the shutter cocked.; not to worry, the darkslide has been removed. If I remove the film back, then the camera fires without a problem. Of course that is not very helpful since there is no film back. I suspect that there is a problem with the film back. The back does not default to 0 when empty and no numbers appear in the film counter window, once I start advancing the film (and not being able to fire the shutter). Needless to say, the red dot does not appear in the film holder once I fire the shutter, since to fire the shutter, I have to remove the film back. I have the suspicion that there is a problem with the film back. Any ideas as to what is going on???? Regards!