iMac shutting off shortly after boot
So I have been working on this iMac as a bit of a hobby at this point.
Long story short...
Bought a 27" iMac on Craigslist, wasn't working, so I bought some parts for it (Power Supply from iFixIt, random screws and brackets, etc. and eventually a replacement logic board). Nothing worked until I had the new (well, older) logic board in and it worked great...for about a day.
Then, randomly, I check on the computer and it's turned off. Odd...I figured it had a sleep issue, so I power up and turn off the sleep timer. That ought to fix it. Nope. Ten minutes later it's down again.
Now it won't boot up unless I give it several minutes and then it will hit the chime and then shut down again. If i give it a few hours, it will boot fully and I can putz around for a minute before it shuts down again.
I'm thinking it's heat related. I ordered new thermal paste as the stuff I used on the i5 I bought was purchased originally 2 years ago (when I replaced the CPU's in my Mac Pro 1.1 to Quad Cores). But I assume heat because:
1. It worked great for the first day...no issues whatsoever. Installed everything, etc.
2. It works longer when it's unplugged longer.
I guess it COULD be the logic board, but it seems unlikely since it did work really well for awhile there.
Anyone have any thoughts on this?
Try lowering the displays brightness to the lowest possible setting that you can still see it. Does it stay up longer?
I would also try installing a temperature monitoring software program to see if it reports anything getting hot.
As you didn't tell us what exact model & OS you have it will be tough going any further.
Please give us the last four digits of the systems serial number and the OS you are running on it. Also, can you tell us if the logic board you swapped out is the same or different from what it had in it. If it's different tell us what the CPU & GPU specs it has.