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video graphics and boot issues

Hi i have a 20" iMac G5 PowerPC processor 1.8Ghz that is having boot problems that I think are being caused by GPU malfunction.

When i power on i can hear the chime and then I can see noise on the black screen right before the white screen that has the loading icon on it. They are tiny multicolored specks that from afar look like vertical lines.

After this happens then the machine has a hard time booting. Sometimes it boots sometimes it doesn't. When I boot in safe mode it works every time and then occasionally I will see the noise while i am already logged on. When i boot in normal mode it will sometimes go to a black screen at the beginning of the boot and then the fans turn on at full blast "vacuum cleaner mode" i believe is what some of you guys call it.

I have searched everywhere for a solution and to pin point the problem. I have inspected the capacitors on the Logic Board and they are fine, not bulging or leaking. i have inspected the caps on the power supply and they are fine except that I missed the ones under the wiring harness. I will check on those asap.

Whenever I see someone with the same symptoms they almost always get the same response: Bad caps, bad power supply, bad logic board. I think it is the logic board but my question is: If in fact it is the logic board causing the problems, would the GPU be the cause of the problem? Has anyone tried to fix/ reflow these graphics chips?

I have seen it done on iBooks and laptops so I am wondering if anyone has tried it on iMacs. I have fixed many Xbox 360's this way and i have all the tools needed, but before I try it i want to make sure that a bad GPU would cause these symptoms and not some other part of the logic board. I would hate to buy a complete new logic board when all this one needs is a quick reflow.

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I have seen heat related issues that look a lot like this on these iMacs. Keep a close watch on your temps with the Temperature Monitor app.

Long term temp issues can damage chips, and give intermittent issues like this. Your solder concerns are another possible issue from high temps, so the two could be related even if it is a solder/connection issue.

Can you run the machine open? If you can, a small can of freeze spray and/or an adjustable heat gun (never above 60C!!!) might help with the diagnosis.

Yes I can run it with the the back cover off. I forgot to mention that when I run it with the cover off it improves a bit. I still get the video issues but it boots up with no stalling more frequently. I will try the freeze spray that is a great idea!! Thanks for the suggestion i will inspect to see if there is something that could be causing the heating issues.

So I have inspected the fans and the vents. I have found that the vents on the bottom near the heat sink are cluttered with dust and debris. I can see the internal heat sink vent has dust as well. I have noticed that the heat sink cover (with the G5 logo on it) is loose and upon inspection, i found that the cover under that has loose tape on the edges. I believe it is causing air to escape from the sides of the heat sink vent.

I have confirmed that the 2 big fans are working properly. I cannot tell if the small fan in the heat sink vent is working.

I inspected the vent connections and noticed that the gasket type sponge things are damaged. I think this would cause the cooling flow to be disturbed right? Like in a car when there is a bad gasket seal and causes leaks.

Do you suggest anything to use as a gasket? I have cork, would that work?

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I've found this is a recurring and extremely common problem in iMac G5 iSight systems. Everyone always says "capacitors!" when talking about G5's, but the caps in these machines are almost always nice and flat.

The issue is indeed in the GPU. In order to repair these machines, I've taken a similar approach to other BGA soldering related problems and gone with a heat gun solution. Oven/toaster/etc reflow will NOT work here due to the through hole electrolytic caps on the board which can NOT take solder reflow temp. Here are the steps I take:

1: Remove the logic board from the machine. iFixit of course has excellent instructions.

2: Remove the two small heatsink assemblies from the front and back of the board. They are held in by four spring-loaded plastic clips which can be released with a pair of needle nose pliers. Be careful! If you break the clips replacements are impossible to find, and those heatsinks are necessary.

3: Fold 3-4 layers of heavy-duty aluminum foil. Use this folded shield to cover the back of the board (the side where the power connector is). Fold around edges and capacitors. You do NOT want to heat the caps unless you want them to pop, which then requires more work to replace them. Press the foil down well and you can see the GPU create a nice intent in it. Then take a small hobby knife and cut away the foil ONLY around the GPU, giving maybe half an inch clearance on the sides.

4: Place a small piece of roll solder on the GPU, maybe 2mm. Make sure it sticks with a small dot of thermal grease. I use this as a temperature indicator . . . . a laser thermometer works too, but I find this method is quite foolproof.

5: Heat, SLOWLY, with a heat gun on its lowest setting. You do NOT want it blowing so hard it is going to fry things or blast away tiny components. Mine is a two-speed $10 model from Harbor Freight, and it's always served me well. Hold first 3-4 inches away for at least a minute to preheat the surrounding board and chips evenly, then close to about an inch away to really heat up the GPU. Use a circular, even motion and be sure to heat the whole chip package evenly. When the solder you put there earlier becomes a nice pretty shiny molten ball KEEP heating for an additional 60 seconds or so (your solder is leaded unless you're buying from some odd source, and the GPU's is lead-free which melts at a slightly higher temperature).

6: Turn off your heat gun and STEP AWAY. Do NOT jolt or otherwise molest the board! Allow it to cool for a minimum of 30 minutes, then make sure it is COMPLETELY cool to touch. Only then should you remove the tinfoil. Closely inspect the surrounding capacitors and insure that NONE have popped or developed a domed top from the heat. They should all be perfectly flat at the top. If not they will need to be replaced as well, but if the tin foil was positioned properly and you didn't wave the heat gun too much none should be.

6: Thoroughly clean the GPU surface and that of the large northbridge chip on the other side of the board, do the same with the heatsinks. Remove all the old thermal material. I use Artic Silver's Arcticlean two-part cleaner and I highly recommend it, but 91% isopropyl alcohol will work in a pinch with a bit more labor. Then reapply thermal grease to the GPU and northbridge chips in a thin, even layer. USE THE GOOD STUFF. Arctic Silver 5 is my preference, but AS3 is okay too. Poor quality heatsink grease and uneven application of it is a large reason these problems happened in the first place!

7: Reattach the heatsinks. Be mindful of the small piece of tape on the GPU heatsink, it goes where the two power handling chips are on the top left side of the GPU. Else the metal heatsink might short out the SMD capacitors located there. Reassemble according to iFixit's manuals for Logic Board replacement.

I've repaired about a dozen iMac G5 iSight machines in this manner. It has worked, without fail, EVERY TIME. I have yet to have a customer return one.

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Thanks so much fregac!! Using my iMac G5 right now, and video performance is back!!!!

I disagree with the fact that you are not instructing people to use a good quality flux in the reflow process.

Flux is extremely imprtant to remove oxidization.

No mention of a, "TAL" time, ( time above liquidus).

Heat:20c above the liquidus temperature of the solder.

For leaded solder, the liquidus temperature is betwee 179c and 186c.

Thesolder you suggest sitting atop the GPU, could cause massive damage if it ran off, (bridging circuits).

If the board is leanign to one side, (even sligtly), this can cause defects in the BGA, such as, "tombstoning". (Keep it level as possible).

The alternative to using a piece of solder to guage the temperature, is to buy an infrared thermometer.

Or a K-type temperature monitor.

Both are relatively inexpensive on ebay.

Better to spend a few extra pounds for the right tools and equipment, than having to replace the logic board after a botched, DIY reflow.

Flux, accurate temperatures and timings are imperative!!

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stop searching - the problem is of course the GPU, no dust, no debris - "only" the graphics processor has bad solderings under it. unless you have the right tools and knowledge - you can't repair it on your own.

the best fix would be to change/reball the GPU, the other possibility would be a reflow - but thats not easy either

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If the solder is an issue, a reflow (without replacing the solder) is a very temporary solution, right?

that depends - some work without any problems - others not, it's not only to put warm air on it. if it's done right, you can save many of them

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Since this gets hidden as a comment, I moved it here.

Get all that dust & junk out of there! Yea!

Use the freeze spray sparingly to lower the temp of a hot part 20F-30F degrees over a few seconds- you can do thermal damage blasting a very hot part with -30C (check the can for actual temp). One part at a time.

I would steer you away from the cork. Silicon Sealer should work well. Give it a few days to cure, it outgasses bad. Working with liquid gasket is hard and messy. I suggest building a gasket using the loose part, a tube of silicon gasket (shower caulk is probably OK even, as long as it is silicon), a very flat surface, and wax paper. Prop the cover up so it does not crush the silicon. Make sure you use the waxy side of the wax paper - the wax stays on the silicon, the paper comes off, and you have a gasket. Clean the wax off before you glue it down or place it on the PC board.

Try to make the new seal the same thickness as the old seal.

If you need an adhesive, use a bit more silicon - it will come off, but it hangs on good, and is impervious to heat up to 1000+F. Press down to make a good seal when you put the cover back and apply pressure for a day (clamp, light weight objects, etc.)

How do we find out if the small fan is working? That should be your next step IMHO.

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Additional comments to fregac's write up for a 17" iMac G5 first gen.:

1. It helps to thicken up the foil by crumpling it in straight pieces or oblong shapes to fit in and around the capacitors. On a 17" iMac G5, first generation, there are capacitors extremely close to the GPU, building a thick shield of crumpled foil helps.

2. You will need a T8 star bit (Torx?) to get the heat sink off on a 17" iMac G5 (maybe others too). I could not get my assortment of allen wrenches to work.

3. Use a spudger or tiny plastic blade to scrape off old thermal grease.

4. The GPU is the chip labeled "nVidia GeForce FX".

5. The northbridge is the one with plastic wrap, small capacitors, and a few IC's on it. It's on the same side as the GPU.

6. The PPC CPU is on the backside of logic board and labeled "IBM" and "U3"

7. Remove battery for added safety before installing foil (foil is conductive!!)

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Sorry it took me so long to get back to you guys, I finally had time to attemp this. I went ahead and did the re flow folowing fregac's instructions above. I now have a working iMac G5!!! :D Before i started I booted up using techtool pro and ran the graphics test, sure enough it reported a GPU problem. After the re flow I performed the same test and no problems were reported. The machine booted up just fine the very first try!! I left the mac on for about 3 hours while monitoring temps using the iStat pro widget and no overheating issues!! I also did some photoshop work to see how it would perform in a demanding environment. Everything is working just fine, it has been about 4 days now and everything seems fine! Big thanks to fregac!!! Thanks man!!

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Hi there, I have the same issue and I am now attempting the solution given by fregac. I would like to be sure I am not doing foolish things so I post here before heating anything: I don't understand point 2:

«2: Remove the two small heatsink assemblies from the front and back of the board. They are held in by four spring-loaded plastic clips which can be released with a pair of needle nose pliers. Be careful! If you break the clips replacements are impossible to find, and those heatsinks are necessary.»

I don't see these heatsink assemblies. I gather that one has to unscrew the large ventirad copper radiator where the G5 letters are printed, held by screws mounted on springs. It was very dusty. Then I only see a sticky pink patch on the nVIDIA GForce chip. I gather this is the GPU I have to heat, right? I don't have to mess with the two other large chips, right?

There are two plastic foils on the mother board, one close to the GPU, the other around the CPU and far away so I won't mess with it. I understand I have to take the one near the GPU off.

Thanks for advice before I attempt to heat the chip. Best regards, Christian


Hi there, without feedback, I attempted the repairc on my own, and... IT WORKED! Thanks a lot. I am writing you from my born again iMac G5.

Best regards, Ch.M.


Well... sad news, it didn't last for long... I am back with the same old wiggly screen just before the brutal shutdown when macOSX is launching... I am selling it away for parts... I gather there is no easy way to use this beautiful display as an external monitor is there?

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This is what happens when you don't ask your own question. Nobody sees it when it's stuck onto an 7 month old question with an Accepted answer.

WOW! I just now saw your question after all this time lol. Sorry for not replying but I am really glad that you got your G5 up and running again! Since I posted this I have repaired about a dozen G5s using this method and it has worked EVERY TIME!! Definitely beats dropping a bunch of cash on a new logic board.

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Any way Xnriqux or fregac can make a youtube video and post it so us mechanically challenged can see it and not just read it...


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Bill Gander : The instructions I posted were specifically meant for the later generation iSight models of the iMac G5. If you have an older iMac without the iSight camera the issue probably lies with bad electrolytic capacitors. Those caps are found both on the logic board and inside the power supply, and sometimes can be faulty even without appearing deformed. The first thing I do with those is always replace all suspect capacitors and that solves every problem present 90% of the time or so.

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fregac, if it were the capacitors why would doing your procedure fix the issue I had then....? It seemed to work fine for about 5 weeks and I feel I didn't get the temperature hot enough (wussing out) for the GPU to final the solder. I will let you know this time what I did to fix it, once it get at it again.

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I've done GPU reballing ith a hot air gun on G3 laptops.

I bought a replacement G5 as my regular G5 died from faulty ram sockets.

the new G5 had garbled graphics. slightly usable but garbled.

Yesterday I decided to reflow it.

followed all the fregac steps as above and as I have previously done with laptops (fixed 5 in the past).

waited an hour then added thermal silverpaste.

red LED came up!!!

added more thermal paste, again 4th red LED.

this morning I tried pmu reset and change battery and still red LED.

I took the heatsink off and the little bit of plasticy stuff that goes betweeen the heatsink and the gpu and I flipped it over.

replaced heatsink, and a touch more thermal paste and fired up the machine and it booted to GRAPHICAL PERFECTION!!!!

very stoked with the outcome.

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