iMac Mid 2012 starting problem

[Povilas1988]

Hello,
Few days ago i have got Imac which some times wont start up. I made fresh install and it started to work. iMac was opened (screen separated) so i cleaned and changed thermal paste. but when putting all together didin't saw that CPU Slides from its place and i bend a little bit of its corner (w/o pins). but Imac starts and works good. for few days. Yesterday i started Computer and it starts realy slowly around 10 mins. After start the computer freezes and needs to Force turn off. After that i cant turn Computer for one hour. ir just start fan spins black screen. so i left ir for 10 mins ant it starts, but progress bar stops at around 75 Percent. Forced turn off one more time and P ram reset helped. Today macbook started progress bar goes to 90% and IMac just shut down, black screen. Now it starts after 15 mins. after standing with power cable removed. Loads to 50% restarts and message logo appears that Imac is restarted of the problem. After pressing any button loads to 50% and black screen appears. Imac restarts and with black screen. I have Mouse and keyboard with LED lights when Imac starts forst mouse turn White LED then purple When imac won't to start the mouse lights only whote and nothing happens no chimney no nothing.

Specs iMac 2012 27 I7 Nviadia 680x 2 gb ram 16 gb ram Aple ssd and 3 tb HHD. OS installed on apple ssd .

The person who gave me that mac told that some times it just wont start and have black screen. no apple logo no chimney appears. He decided not to repair it. One technitian told start to look CPU may be the issue.

 

[Povilas1988]

After some experiment found that if LCD back light is turned in Motherboard the Mac wont start. some time backlight work. but then after 50% progress bar it shuts down. Wit baclight cable off Mac starts. i will write all info with photos after repair. mabye this Thread will help someone else.

 

[Povilas1988]

Hello So i Managed to repair iMac. The Problem was really bad CPU connection. I just take out CPU changes thermal paste again. And saw that cpu is bent a little bit. Straighten cpu, reasembled back. And iMac started to work as new. It came to me with random shutdown problem. After just reasembling, making good cpu connection, Random shut down problem gone (Do not know for how long. Looking in foward to buy one more CPU, in case that bended cpu is bad and problem may come back again.) iMac after just 2 years is started to randomly shut down and the iDeal team didin't find anything wrong. Using it for 2 days now on free time after work . Minimum 3 hour on heavy Load. only one time freezes. Found Old solution SMC fan control after that all problems gone at all. Maybe it will help for someone with similar problems.

As i think. Bad CPU connection is taking more power from PSU and in some time period it breaks up (in my case i had all signs that PSU failing and i need to change PSU). After i Did as i mentioned earlier. it starts to work file. Will update more after week. Month if that helped me.

 

[Povilas1988]

The mani problem i CPU. And cooling. For now it is wot not bad, but when CPU gets around 76 degrees it Starts to freeze. SMC Fan control helps but after 2 hours of heavy load i must give about 30 mins to cool. It is not critical for this moment. I it usable.