Mac desktops forum: Imac rebooting itself and stuck on black screen with chime?

it gets caught in a boot loop. screen is black and chime plays over and over. sometimes after I unplug the power for awhile and plug it back in I can get it to fully boot up but it's just a matter of time before it restarts on its own and gets stuck in the boot loop again. Thanks!

Then that's not good, and sounds like you've got a bad MLB. Not sure why they would have replaced the PSU if the symptoms were the same last time, but Apple is a very strange company, and hasn't had any more success in weeding out the morons who don't know what they're doing than any other retailer with a service department.

If I were to venture a guess, I'd say the SMC chip is bad, and keeps getting a corrupted state. When you unplug it for a time, the capacitors maintaining that state will drain and it resets when they deplete. The fact that it keeps getting corrupted means that there is something very wrong.

So unless you'd like to add to the story that you have a program like smcFanControl installed, or anything that might alter the fan speeds on the iMac, or just interact with the low level system functions... You're likely looking at a new MLB, so I hope you have some kind of warranty on the unit still. If you have some program like smcFanControl, then I would start by getting rid of that and hoping the damage it did isn't permanent.

Thanks for the help Jimmy. I do not have any apps installed for fan speed. I don't have warranty on this machine so I'm screwed.

Someone recommended blowing the dust out because it may be over heating and possibly a fan going bad. I did that and it booted up no problem. However, I had it unplugged while I drove to the store so it is probably what you mentioned where the capacitors reset.

I stupidly misplaced the directions to reset whatever boot up file I reset last time. This same issue happened a year ago and once I deleted , whatever the file was that I can no longer remember, the issue went away for almost a year, until recently starting up. I can't remember what folder I found it in but it had to do with some sort of system start up. Basically it was described that this file could become corrupt and when you delete it and restart your machine it automatically creates a new one. I'm a moron for not saving the directions for this process.

Thank you very much for looking into this for me. Greatly appreciated.

I would wager it was purely coincidental that deleting some file made the issue go away. I highly doubt that would have anything to do with it, because the problem you're describing is happening at the EFI level. The OS hasn't even started loading yet, so the issue is on the MLB if you do not have any programs that tinker with the SMC or any other EFI level components. If it's not the SMC, then it's the EEPROM chip that holds the EFI which is bad, either way, it's a new MLB to fix.

The whole blowing out fans thing is kind of the tech version of a doctor telling you to drink lots of fluids and get plenty of rest. Yes, it's good advice, but the odds of it having anything to do with your issue seems highly unlikely.

For now, what you may consider doing, is just not shutting the thing off once you get it booted up and running. You can see if putting it into sleep mode will allow you to keep it going more or less indefinitely. It may also be worth looking to see if there are any SMC updates for your unit. There's always a possibility that laying down a fresh copy will clear things up. Not a very good chance, but what exactly do you have to lose at this point? You're probably looking at $500US minimum to get the thing fixed, so you may as well give some of the more unorthodox ideas a shot before laying down that kind of money.

I see this post was last year but just tonight, I took a brand new iMac (iMac13, Late 2012) and got the chime over and over however, this was its first start and Apple said they had no idea and want me to send it back. I believe that in Mt. Lion when there is a kernel panic, you get NO four language dialog box and that the machine just restarts itself, could that be it? Nowhere on the web have I seen an answer and in my 18 years in repair have never had that happen.

Send that machine back to Apple, like they asked you to, and they will ship you a new one by return.

Should cost you nothing to send it, they pay all the shipping charges

P