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Why won't my iMac power on?

Posted on 2009-12-28
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Last Modified: 2013-12-11
I have a 20" iMac (2006) that won't boot up.  There's a little bit of history to it, when I replaced the internal power assembly after it was dead in the water after being recovered from a pawn shop post office break-in.  This fixed the issue for nearly a year.  Two months ago, it wouldn't start.  Plugged in, the power button did nothing.  I unplugged it, put it in the corner, and forgot about it until two weeks ago.

I plugged it in to mess with it, and it powered on fine.  Ran it for two weeks with no issues.  I came back from Xmas break to find it off.  Again, the power button won't do anything.  I've tried different plugs, different power cables, etc.  No dice.  I'm reluctant to replace the internal power assembly (it cost $300+) to fix the issue again, but I'd like to know what the issue could be, and how I can better diagnose it.

Any ideas or suggestions will be greatly appreciated.
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Question by:wukovits
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by:
Mike_Carroll earned 860 total points
ID: 26134291
Disconnect from mains power for a hour or two. Reconnect and try it again.
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by:wukovits
ID: 26134358
I'll try that, any reasoning behind that, like what might be at fault, or why it did it in the first place?
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by:jhyiesla
jhyiesla earned 288 total points
ID: 26134382
What the other expert has suggested may very well work as it will reset the SMC, but you may not have to wait that long.  Also, you should probably unplug the keyboard and mouse as well as an external monitor or other device.  In other words isolate the iMac completely.  Probably 5 minutes would be adequate.
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by:Mike_Carroll
Mike_Carroll earned 860 total points
ID: 26134572
Discharges the power supply.

I've been in the computer business for a long long long time and to me the best tip ever is to disconnect for an hour or so before trying anything else when they won't switch on. Believe me when I say that it will work in many cases but in my experience, it needs to be for more than one hour. I'm only going from experience. Strictly speaking, I would agree that it should not need that long but it quite often does.
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by:wukovits
ID: 26134622
Would this indicate a need for a new power supply, or just an intermittent issue that will need to be addressed every so often?  What can cause the problem, a surge or something?
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by:Mike_Carroll
ID: 26134649
Generally speaking a surge although it's hard to tell what comes up the power line sometimes. I could tell you stories about power problems that you wouldn't believe. They'll all be in the book, one day when I get time (c;
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by:jhyiesla
ID: 26135059
Not sure that sitting here we can say for sure whether it's the P/S itself or something in the motherboard's circuitry that utilizes the P/S to fire up the Mac. If the Mac is worth keeping, it might be worth paying Apple tech support to take a look. At least you should get some answer on what the problem is.
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by:PCBONEZ
PCBONEZ earned 852 total points
ID: 26135288
The power supplies in those are problematic. [The capacitors fail from heat.]
Sometimes they damage the logic board on their way out.
I know of at least two people that rebuild those PSU's and logic boards.
They can be found here several days a week. www.badcaps.net [In the forum!]
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by:wukovits
ID: 26140688
PCBONEZ:  Any advice on who to use?  Do I just post a topic and see who replies?  I've never heard of replacing the capacitors on a MB, have you had any degree of success with this type of repair?
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by:PCBONEZ
PCBONEZ earned 852 total points
ID: 26141005
The member that goes by Toasty is very into those PSU's.
He even knows the different revisions of the unit.
There are a few others that know MAC parts, I just don't recall their names.

The best thing to do is just post a topic like "Need help with ____" and the people you need [and some you don't] will show up.
It usually doesn't take very long.
Techs there are the kind that regularly solder and replace transistors and things.
Just a different scope of repair than most folks here are familiar with.

I do PSU's but I don't know the MAC units anywhere near as well as they do.

~~
If the board still at least boots to a BIOS screen and there is evidence [either by bloated caps or known problem brands/series] of capacitor problems the success rate of restoring a board to problem free service is up over 95%.
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If the board doesn't boot at all and there is evidence [either by bloated caps or known problem brands/series] of capacitor problems the success rate is maybe 50%-80%. - It's hard to call that one without knowing a number of details about the it's history. - A power surge, bad power supply, or bad caps -may- have damaged the chipset or burned out MOSFETs or other voltage regulators on the board. If the board's history is clean and there are questionable caps probably 85-90%  success for those.

~~
Motherboards, PSUs, LCD screens, LCD TV's, and Networking gear [routers, switches, ect] are all frequent visitors to the soldering table over blown capacitors. - It's actually very common but most people just say "it's bad" and replace things without even trying a repair.
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by:Mike_Carroll
ID: 26141813
Have the caps on the board actually failed? You can tell from the capacitors... failed ones bulge and leak. In extreme cases, they can also explode. I've replaced capacitors on boards and mobos. It's a tricky job but far from impossible. Caps failure will eventually cause power on problems but generally speaking, the machine becomes highly unstable before this happens.

Presumably, you've tried the disconnect for a couple of hours?

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Author Comment

by:wukovits
ID: 26141823
The disconnect worked.  It turned on this morning, but when I went to wake it from sleep by hitting the space bar, it powered off, and would not turn back on.  I've got it waiting now, we'll try it again in the morning, and we've now plugged it into a UPS to try to avert any surges, but I'm still worried with the frequency of the power outages.
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Assisted Solution

by:Mike_Carroll
Mike_Carroll earned 860 total points
ID: 26142145
Having to disconnect to get a power on is actually pretty rare.

If you're having that much trouble keeping the machine on, there's obviously something else going on. You should not be seeing that quantity of surges unless there is something seriously wrong with the power and that is unlikely.

You should really get the machine and the PSU checked out. Even a simple visual check on the PSU caps to see if they are the problem. You can find pics here http://www.badcaps.net/ that will show you what failed ones look like. Just be careful if you are trying it yourself as you can get a nasty shock if you aren't careful.

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by:Mike_Carroll
ID: 26142150
Oops... sorry, I see somebody else has already suggested badcaps.net (c:

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by:PCBONEZ
PCBONEZ earned 852 total points
ID: 26142651
>>I went to wake it from sleep by hitting the space bar, it powered off, and would not turn back on.<<

When the system is sleeping only the Standby power is supplied to the motherboard.
When you 'wake it' the other Rails come on and power up.

It sounds like when the PSU first energizes the other Rails it is tripping back off due to an overload.
[Too much current.]

~~
Having to disconnect from AC to get a PSU to 'power on' is very common when the control circuit in the PSU is defective. There are too many different circuits used to be specific about what the problem is, but, usually it amounts to a defect 'faking' the IC chip in the control circuit into 'thinking' the PSU is already on.
- Since it 'thinks' it's already 'on', it won't start.
The defect does that by 'latching' a pin on the IC chip high or low [depending on the circuit] rather than applying a high or low to that pin momentarily when the PSU starts.
By unplugging AC you deenergize that circuit which resets the latch allowing the PSU to start.
Culprit is usually a bad cap or a burned resistor near the IC chip.
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