I've always wanted to get into medium format............so a couple of weeks ago I bought an RB-67 system from KEH. So far so good except that I don't see horizontal lines when I try to compose a shot. Just have the black vertical lines on either side. I thought that I might have the wrong focusing screen, so I sent for another one........same problem. I can't find much information when I google, except that there should be red lines. What am I doing wrong?