Chosen Solution
hi everyone, I am trying to fix my father in law iMac 27” 5k 2015. The mac doesn’t power on. The model is MK482LL/A, A 1419, EMC 2834. The Logic board is 661-03172 or 820-00134-A. According to my father in law, the iMac started having problems powering on, necessitating multiple tries and sometimes unplug- replug. The problem got slowly worse, to the point where the computer would not start at all. I disassembled the machine and looked at the diagnostic LEDs on the motherboard. Led #1 in on when the computer is plugged. When the power button is pressed, led #2 lights up for a second and turns off. No fan. no bip, no other sign of life. I ordered a new power supply unit, installed it, same problem. I followed the procedure ‘deep dive’ in a service manual I found online. I measured the logic board coin battery voltage, found that it was too low (~2.2 volts). I removed the logic board and replaced the coin battery. Issue was not resolved. No sign of logic board damage by visual inspection. I am trying the next step which is to force-reset the logic board real time clock, but I cannot find the reset pads that have to be short circuited, the picture of the logic board in the service manual is different from my logic board. I have a couple of questions: where are the reset pads on my model ?when resetting the Real Time Clock, must the procedure be done with the iMac plugged or not ?is there anything else I could try ? (except replacing the logic board). I am electronics litterate but not a technician. happy to post a picture of the logic board if useful. Any advice appreciated. Cheers
Sadly it does look like the logic board got damaged. Most likely a lighting or power surge found its way into the system. Often the power supply gets nailed as the AC cord is the typical way it find its way in. Other ways are via a wired network connection or using one of the I/O ports which has a device which is not protected at the outlet. Does your Father-in-law’s house often have lightning strikes near it? Do they often encounter power outages or spikes/drops? Does he have good surge suppressor or better yet a UPS to protect his gear? You may want to check the outlets for proper grounding as well as the breaker panel to the building ground either a metal water pipe leaving the basement or a one or more grounding rods.
