Chosen Solution
Got messy image on my iMac (27” late 2012 model), as soon as it was turned on, can still boot up and login, get it connected with Thunderbolt Display through the thunderbolt port connection, image shows normal on the Thunderbolt Display, can I conclude that the display assembly from the iMac is out, and the logic board is still good? Is there any other possibility? Thanks,
If the external is woking then you’ve proved the issue is within the internal display assembly 27” iMac Display Assembly, Apple P/N 661-7169 Here’s the guide to replace it iMac Intel 27" EMC 2546 Display Replacement
