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Helpful answers
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Dec 27, 2015 11:32 AM in response to Stanley Streicherby Jeff,I assume that the power strip or surge protected receptacle that it's connected to wasn't damaged from any resulting spike or surge, when power was restored. It's very likely that during the power outage, any remaining charge in the 3.6-v, ½AA lithium battery on the iMac's motherboard may have been completely depleted. A very weak or dead internal battery can cause startup problems, so in your case, the solution could be as simple as replacing the battery. To do so, you'll need to tilt the iMac back so that it's resting on the edge of its round base, but you must support the back of the screen and the neck with something that's firm. The first step in disassembly is removal of the user access plate from the bottom of the base. Next, you'll need to remove the (4) Torx machine screws that secure the bottom housing. Because the lower housing is connected with wires and cable to the upper housing, you'll just tilt it open at 90º from the upper housing. The battery will be visible on the logic board. Before snapping in the new one, observe the correct polarity when positioning it above its socket. Reassembly of the bottom cover first requires using a small plastic (non-abrasive) tool to scrape off the old thermal paste residue on the two contact points on the lower housing, as well as the two in the upper housing that directly mate against them. This step is critical, to prevent CPU overheating. After all four contact points are clean, apply a very small amount of new thermal paste to the two points on the lower housing. Carefully raise/tilt the lower housing, until it closes against the upper housing. The (4) Torx machine screws that secure the bottom housing should be evenly tightened by hand, and not over-tightened. For this reason, I wouldn't use a ratcheting screwdriver. By manually rotating a Torx screwdriver, you'll have a better feel for the amount of the torque that you're giving it. The lower housing must be firmly attached, for proper thermal dissipation.
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Dec 27, 2015 12:08 PM in response to Jeffby rccharles,Some models of the iMac g4 have a PMU reset button on the logic board.
https://www.ifixit.com/Answers/View/103073/Where+is+PMU+reset+button
might help too.
http://www.macexpertguide.com/reset-pram-mac/
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Feb 1, 2016 6:28 PM in response to Stanley Streicherby Zepple,I have had the same problem. The power supply is probably damaged but your G4 can be restarted using a hair blow dryer. Just point the dryer down the vent holes. It has taken up to 10 minutes of heating for mine to restart again and the cooling fan will be on full blast when it restarts.
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Feb 1, 2016 8:30 PM in response to Zeppleby rccharles,That likely means the solder is cracking. Some people have put the logic board in the oven and heated it. Please do more research on this before trying.
R
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Feb 5, 2016 1:38 PM in response to rccharlesby K Shaffer,• Resetting the iMac (Flat Panel G4) Power Management Unit (PMU) - Apple Support
The later G5 PPC iMac model, pre-dating early Intel-iMac, uses a different idem
because their chip is not called a PMU. Some of the links to various power chip
reset articles may no longer work from this page; the chip names are still correct.
•Resetting the power-management chip in your Mac:
http://www.thexlab.com/faqs/resetpmchip.html
There is a tiny reset button next to the test ports under the metal disc bottom plate
cover in the iMac G4 series. Usually the service support documentation covers this.
{I have an original Apple Service Manual in PDF I used to self-diagnose my iMac G4
under warranty because the only shop that would see it under AppleCare refused to
& so I had to do most of the troubleshooting myself. And Apple told them what to do.}
(The reflow of solder for graphic chip in G5 iMac + some intel iMac was more common
as was attempts to do the same with some white iBook G3/G4 models, than iMac G4.)
Oh: The battery is in the bottom on the logic board, the power supplies are in the top.
Heating from the top may be slow to help the logic board directly if solder is bad.
These models usually require a known-good PRAM (clock) Battery installed; so
if the unit has sat around a few months unplugged, by now it probably is dead.
This battery does not re-charge while in use: a 1/2AA 3.6V Lithium. Cost, to $15.
• example: 1/2AA 3.6V Lithium: for some models, incl. iMac G4: $3.95:
http://eshop.macsales.com/item/NewerTech/BAA36VPRAM/
The PRAM and NVRAM reset sequence uses the same keys; recent article, here:
• How to Reset NVRAM on your Mac - Apple Support
...Anyway, should someone reference this site, in this year, above links may still
be of some help. Resurrected ancient threads with dead links are of little help...
Yet some vintage ones - that should be archived - may re-appear & serve to confuse.