iMac not turning on

Hi,

So I bought my brother in laws iMac January 2020, all working fine until In February 2020 I didn’t realise it was in sleep mode and pulled out UK 3 pin power plug from an extension socket, as I pulled it out it sparked and at that point I realised it was actually on. I moved the iMac to a different room and when powering it back up it had a box with a question mark on screen. I eventually managed to reinstall the OS that came with the iMac from new. When it started up the boot chime could be heard and then would load into the OS, but noticed sound wasn’t working properly, speakers would just crackle? So I went to update the system to Mojave, before unplugging it it was running Mojave.

This is where the problems started, it wouldn’t let me update to Mojave, as soon as you get towards the end, a window would pop up saying unable to install ( can’t fully remember exactly what it said but something along them lines) . I tried many times to install it but each time failed in the same place with same message, only the OS that came with it would install. After weeks of trying I put it in the home office and left unplugged.

With covid lockdowns etc I thought I’d try again, so in October 2020 I thought I would replace HD and see if that would make any difference. Removed screen carefully, swapped HD to a known working one, connected screen, powered up and nothing! Thinking the screen wasn’t connected correctly checked but fine. I did some research and looked fir the LED lights on logic board, only led 1&2 light up and no boot chime.

After i put in in the office when I gave up with it in March, it wasn’t plugged in, and before changing hard drive I didn’t power it up, so I don’t know if it was powering up or not, but surely it should of if it was working in March?

I replaced the battery on the logic board for a new one, but still not powering up. The fan works and hard drive can be heard doing something, but no boot chime.

Before I skip the iMac is there anything I can try to get it working? Put the Logic board in the oven? To reseat gpu if that’s the cause?

I’ve read a few posts of the same thing happening when replacing a HD on a iMac 27” mid 2015. I not sure if the initial unplugging when powered has done damage to the logic board?

Any help appreciated

Gaz

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Do you have a second Mac system? If you do let's prep up an external bootable drive, using it boot up your iMac. Press the Option (⌥) key to gain access to the Startup Manager to select the external drive.

Reference: Mac startup key combinations

I do have another Mac, but only led 1&2 are lit on logic board, the Mac is not booting no screen display, so unfortunately I can’t gain access to startup manager.

What else did you do to your system? Did you remove the logic board to gain access to the back?

So initially I unplugged the Mac when it was in sleep mode, plug sparked in extension socket, when turning back on a box with a question mark in was displayed on the screen. Installed original OS and managed to get it booting into OS, but sound no longer worked just crackled, boot chime was fine. Unable to update OS, kept failing. So removed screen to replace hard drive, from doing that it no longer powers up, led 1&2 on & no boot chime. I then removed logic board to replace battery. Put it all back together and still only led 1&2. It’s not detecting gpu which seams to be a common fault from what I’ve read about my model of iMac.

Potentially reflowing the solder pads on the gpu may fix it? Who knows!

@burrows1980 - Reflowing solder joints won't fix a logic board failure. I'm still not sure if thats your issue here. If it was why did it boot up after the fact.

I'm more likely to believe you damaged the display or the display cables.

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