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15 inch "Desk Lamp" iMac G4 700MHz M6498 PowerMac 4,2

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iMac g4 won't boot up and weird screen when it cans

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Hello, recently a school gave me an imac g4 that was used for educational purpose and the school said that the mac was dead. So i first pluuged the power outlet and it booted but with a weird screen changing to completely white. A few weeks ago and yesterday, i just replugged it to test the mac but now, the mac won’t boot up, no fan nothing. Only a fast clicking sound coming from the power supply that stops sometimes after a few seconds. I dissasembled all the imac and look for any damage on the mac. With a multimeter i checked the voltage coming from the first power supply board and i got 400 v continue but on the second board, i have no voltage. Have you got any tips to look for maybe shorted parts or capacitors that usually break up in these machines ? Thanks for help

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G’Day Correia ;

You are on the right track , HOWEVER , to be really successful one should purchase a decent ESR meter to check any electrolytic caps in the power supplies .

An ESR tool (Effective Series Resistance , I could be wrong , been a long time for that definition ) will test capacitors ‘in circuit’ . No need to de-solder . HOWEVER there can be a gang of caps in parallel and some meters will give a pass . If any have swollen tops or stuff leaking they are bad FOR sure . If any are in a parallel gang I would swap them all . Hope you have a heavy duty motorized solder sucker to get caps out of multi layer boards ( heat from most hand irons will get pulled away into large copper ground planes )

Sorry I have not had any G4 experience , but Apple makes such good quality I doubt caps are bad , but , I have done so many other devices and brands that I would not be surprised .

That ‘clicking’ you mention could now be a shorted MOSFET causing switching power supply to constant reboot . Use DVM on semiconductor junction test to look for bad power MOSFET or transistors .

Less likely is you are hearing the hard drive smacking the heads against the frame . Shed the hard disk for boot test maybe .

Just some thoughts , Good luck

Hugh

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Ok, i will check that. The clicking sound comes from the first power supply (hot part) so i will look for mosfets. Thanks for your answer :)

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Hello, i saw on YouTube that on these models, people were using an hair dryer to boot their mac and this worked for my mac but i can’t find the component that react to heat :/

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ahoy there...sure sounds like a bad cap to me . quite often many restart attempts will warm up a flaky capacitor's inards and device will start . Beg or borrow an ESR meter maybe ?

Luck

Hugh

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Hi, i just changed the three capacitors and now it works ! But the problem is that i have weird multicolor lines on the screen i don’t know if the lcd is dead or the gpu

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I posted above pictures of the problem

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