Hi,
I have a 27” 5k iMac (late 2014, i7 4.0, 295x 4gb). Tonight, the unit froze so, I rebooted it. After I power cycled it, the power would click off after only a few seconds. I did this cycle a few times. Sometimes if I let is sit unplugged for a while, after reconnecting power, I could get it to progress to booting up into macOS. But is locks up shortly after that. When plugged in, the usb is outputting power ( I have led light that I use, powered from the iMac ). I am guessing it’s either a bad power supply or logic board.
Any advice?