late 2012 iMac, random shutdown

[Sill]

Today I experienced a very abrupt shutdown from my iMac. More accurately, it was like a power outage, not a shutdown. Just <snap> and the machine was off. It wouldn't respond to repeated presses of the power button. I unplugged it from the power strip and then tested the strip, because the power light in the rocker had failed too. The strip tested OK so I plugged the iMac into it and it into the wall, and another push of the power button brought the machine back to life. I worked with it for another hour, and then it shut off again. There it sits, until I figure this out.

What should I look at, besides getting another machine?

 

[ImDanTheBoxMan]

Could be something on the internal PSU starting to fail as it is now 10 years old. If you feel comfortable replacing it yourself I can't imagine a new one costing much, if not try to find an independent repair shop that will do it. Just a small disclaimer that working on a power supply is highly dangerous and should only be done by someone who knows what they are doing due to the high voltage going through it (even if its unplugged it can still have power stored in capacitors). Guides to replacing the power supply: 21.5 inch, https://www.ifixit.com/Guide/iMac+Intel+21.5-Inch+EMC+2544+Power+Supply+Replacement/15863 27 inch, https://www.ifixit.com/Guide/iMac+Intel+27-Inch+EMC+2546+Power+Supply+Replacement/15722

 

[avz]

Is it a 27 inch iMac? Did you recently upgraded the RAM? If not, than it is possible that the components failing in iMac's power supply. My blu-ray player from 2009 recently failed in a similar way: sudden shutdown. I was able to get the disc out as it turns on and after 30 sec turns off again. Highly likely one of the capacitors decided to call it a day.

 

[Sill]

Yes it is the 27. Did those have weak power supplies?
I haven’t upgraded the RAM in it but I’ve been meaning to. The only thing I’ve done to it so far is replace the hard drive with a new 1TB drive from Larry. If I have to repair the power supply I’ll get more RAM for it too.

I was looking at a pic of the power supply and it seems very straightforward. Looks like 10 electrolytic capacitors to replace, all the others are film and/or ceramic. Those don’t fail unless they take a surge. I could probably redo that supply for about $20 plus the rope seal. Or I could buy the supply new for about $100, assuming those are not counterfeit.
Either way this thing gets fixed. I was planning on turning it into a Linux machine.

 

[avz]

Yes it is the 27. Did those have weak power supplies?
I haven’t upgraded the RAM in it but I’ve been meaning to. The only thing I’ve done to it so far is replace the hard drive with a new 1TB drive from Larry. If I have to repair the power supply I’ll get more RAM for it too.

I was looking at a pic of the power supply and it seems very straightforward. Looks like 10 electrolytic capacitors to replace, all the others are film and/or ceramic. Those don’t fail unless they take a surge. I could probably redo that supply for about $20 plus the rope seal. Or I could buy the supply new for about $100, assuming those are not counterfeit.
Either way this thing gets fixed. I was planning on turning it into a Linux machine.

From my experience with 27” 2012 iMac, they are very picky when it comes to RAM, getting the specs right is not enough, some modules are just seems to be thinner than others and they don’t make a good contact resulting in random shutdowns and restarts. MacBook Pro Mid 2012 has a different placement of the RAM modules that is less critical to the modules being thin. Luckily I was able to use 2x8GB of RAM that I bought for my iMac in my MBP, otherwise would be a waste of money.

 

[Sill]

From my experience with 27” 2012 iMac, they are very picky when it comes to RAM, getting the specs right is not enough, some modules are just seems to be thinner than others and they don’t make a good contact resulting in random shutdowns and restarts. MacBook Pro Mid 2012 has a different placement of the RAM modules that is less critical to the modules being thin. Luckily I was able to use 2x8GB of RAM that I bought for my iMac in my MBP, otherwise would be a waste of money.

That must have been frustrating! I haven't bought generic RAM for decades so I'm glad I've never run into that issue. When I had a 386DX in 1992 almost all of the DRAM came out of Taiwan, they were at least good quality, and counterfeits weren't an issue. Toward the late 90s when I was in the business I used TransIntl. For the past 20 years, MacSales.

 

[Steve121178]

I've had a few random shutdowns with my late 2012 21" iMac also over the last month or two.

I've put it down to old age.

 

[Sill]

Yep. Electrolytic capacitors age out. Assuming I find some bad ones on this board, I can get this computer working in a day and for about $30. The potential downside is that there are no schematics available for the board in this machine, so any further diagnostics would be in the dark, and I'm not about to risk heating up SMD devices to check them out-of-circuit. I'll probably order the display gasket and a few RAM modules, pull the screen and start testing those caps. If I can find some failed parts, great, if not I'll stop there and just order the new power supply for $100.

Assuming it isn't counterfeit.

 

[Adarna]

Whatever spare parts you think you need you better buy it soon as Apple declared this iMac obsolete already so they ceased making any parts for it fuhrer.

 

[Fishrrman]

Unless you are sure you can do the component-level repair, it's probably worth spending a little more for a new/used/refurbished power supply and just swap it out "as a unit".

What kind of drive is inside?
If it's easily accessible (when changing the power supply), might be worth putting a low-cost SSD in there...

 

[Sill]

Unless you are sure you can do the component-level repair, it's probably worth spending a little more for a new/used/refurbished power supply and just swap it out "as a unit".

What kind of drive is inside?
If it's easily accessible (when changing the power supply), might be worth putting a low-cost SSD in there...

I'm positive I can change out any of the board-level passive components - its a hobby of mine - but I run away from the SMD work. I don't have the proper tools for the job, and that includes patience.

The internal HD on this machine was a 1TB Fusion Drive, which failed about 3 years ago. The replacement was OWC's version of the Fusion, but in a 2TB capacity. IIRC the late-2012 machines in the 27" form had both the space and the logic to support a second internal drive, so I'm considering the addition of a 256GB SSD to run the system files from. That plus a healthy dose of added memory should really speed the machine up.