Chosen Solution
Has been running reliably since @dan helped me last year with thermal paste issues. Suddenly the system will hang partially through a status bar or if it goes to user menu, it will let you log in but it will either hang within a few minutes or begin a restart cycle. Tried to go into safe mode and it hangs there also. Tried recovery mode, started disk utility to check ssd and on pass 3 of 7, it froze! A few times, a crackling quick pop was heard briefly…what should I focus on power supply? logic board? Thanks!
Sadly, the logic board is the failure here. Inspecting the onboard diagnostics would help aim you, even still probing the power rails referencing schematics and boardview drawings would be the next thing I would do. But you need the deeper skills to follow them and a good DVM. As far as swapping out the logic board, you are limited to what this series used. Apple produced two versions one with just the Intel Iris Pro intergraded graphics and the other with the additional NVIDIA GeForce GT 750M dedicated GPU 21.5” iMac Core i5 2.7GHz (IG), soldered CPU, iMac14,121.5” iMac (IG), soldered CPU, iMac14,221.5” iMac (DG), socketed CPU, iMac14,3 You can’t use the 4K display models logic board as the board to display connection is different. But if you are planning on just getting a new system either 2017 or 2019 offer the same ability for upgrading upto 32 GB of RAM, socketed CPU & NVMe/PCIe blade SSD connection.
