Chosen Solution
Hello! My iMac turns on normally, and on a screen I see some strange repeating stairlike pattern of almost black blocks of pixels. I type my password and press enter. After that the pc tries to complete the log in, it fails and reboots itself. Please help) Update (02/20/2018)
This is the pattern
Ouch! The herringbone pattern you are seeing is a GPU board failure. To repair it you’ll need to replace the part ATI Radeon HD 2600 Pro, 256 MB - Apple P/N 661-4426. It’s a lot of work given the age of the system you might want to consider getting a newer system as this system can only support OS-X 10.11.x El Capitan and no higher and many apps today likewise won’t support it. Here’s a similar model’s IFIXIT guide to replace the GPU board: iMac Intel 24" EMC 2111 GPU Assembly Replacement
