Ответ на "Do I need to buy a logic board?"FIXED! I finally found someone else who had fixed a dead iMac of the same vintage just by replacing the video card. And since I'd already replaced virtually every other part of consequence, it sounded like I might have a good chance of success. So I jumped... And it started right up! Never been so happy to hear a BootBong! It'll take me awhile to get a working system to my liking all loaded in, but so far, so good. Also found that the replacement motherboard was indeed new... no serial number! Found and implemented "Blank Board Serializer" (an Apple service utility), loaded the old serial number, and off to the races. So, now I have a virtually new 2011 iMac; only the LED Backlight Board, DVD Drive, and Display are left. Well, cabling, too. Plus, I have an original power supply, and the original Ci7/3.4 GHz motherboard as replacements if necessary. The new motherboard was the Ci5/3.1GHz model, and the replacement graphics card was the 6750 rather than the failure-prone 6970 that came with the machine new....