Hello, I just receive my first film camera, the Mamiya 645 AF from KEH and I have an issue. When I attached the 80mm 1.9 lens, and the back I was able to view find in the viewfinder, then something happen when I tried to test the camera out without film per the manual. I removed the dark slide, opened the back, and there were a series of clicks after I pressed the shutter button. I took the back off I there's a black curtain that is down...the mirror is also down as well. I'm not able to view anything in the viewfinder--it's dark. What am I doing wrong. I looked over the manual and it doesn't provide an answer. Any help is appreciated. Thanks!