So, I received the replacement logic board today, installed it, and now the iMac starts up as expected. I still have a video problem, but that is because the replacement LCD screen I ordered is bad. At least the old one still works now, multi lines and all. So somehow I must have injured the logic board when I originally installed the replacement LCD screen. I am definitely more careful handling all the parts now. Learned a bit about troubleshooting iMacs, so thanks again for all the communications. Guess I'll close this post. Thanks. Ron