Late-2006 iMac has horizontal lines on screen, freezes and shuts down

Sorry, but I can't buy your explanation about bad design since it basically stands a fundamental law of thermodynamics on its head: Heat flows from a hotter to a colder mass, never the other way. Heat pipes are very efficient heat conductors so there will be very little difference between its two ends, in this case the heat spreader over the chip at one end & the air radiator at the other. For the CPU radiator to heat the GPU radiator (& thus "pre-heat" the GPU), the air flowing over the GPU radiator would have to be heated by the CPU radiator to a hotter temperature than the GPU radiator.

That's just never going to happen under normal operating conditions.

I'm clearly no thermodynamics expert:) But if I understand you correctly:

- the single GPU & dual CPU heatpipes carry/transmit heat adequately enough such that both ends of all the pipes are roughly the same temperature. IOW they are sufficient for their design tasks. OK.

- the small GPU radiator fed heated air from the large CPU one does not degrade/derate its ability to dispel heat. The only way GPU heating would suffer would be for the CPU radiator to be hotter than the GPU one (the CPU radiator's air temp exceeding that of the GPU radiator). And this would only influence the bottom limit of the GPU temp.

So for example, in my IMac prior to installing 3rd party fan control, under descent graphics & CPU loads it was not unusual to see CPU & GPU temps over 70C AFAIR. Meaning the CPU radiator was coping/dispelling 70C temps into the GPU radiator & this did not represent a disadvantage. OK I get this, the only disadvantage then is temperate coupling of the two. IOW, neither would get hotter due to the coupling, the lower limit temp of each would be influenced by the other's current temp.

Then the only part of my proposal that remains is that it reduces longevity of these components by allowing them to reach temps of say 70C -> 90C (as was also the case in my mac mini). Instead of more aggressive fan speed / temp slope settings that would keep them always below say 55C -> 65C (at some expense of quietness). I still stand by this & the fact that it is an apple design decision that sacrifices some longevity for quietness (and provides no apple-supported way for the user to influence this balance).

And I still direct those reading this thread to make sure they take stock of your post a few back.

Message was edited by: pbcubed

You need to take into account one other thing: Air is a relatively poor conductor of heat. The CPU radiator isn't directly heating the GPU radiator, it is heating the air that flows through it. Same for the GPU radiator: they are both heating air that carries away the heat energy from the radiators.

Higher fan speeds achieve lower temperatures by moving more air through the radiators per amount of time, but this is a relatively inefficient process because the air won't heat up much during the brief time it is in contact with the radiator (because it is a poor heat conductor). There are techniques to improve the efficiency of this process (& Apple is as good as anyone at exploiting them) but the bottom line is for a fixed amount of space devoted to cooling, even hurricane air speeds won't keep the chips all that cool.

There are only two ways around this: greatly increase the size of the radiators or throttle the power (& thus the performance) of the chips. When necessary, Macs will do the second but for obvious reasons only when necessary.

So basically, as is always true in engineering, there is a compromise between mutually exclusive goals. Apple chose to offer high performance in a small package in all but its Mac Pro model lines. The bulk of the MP's & the size of their cooling systems should give you some idea of how much bigger iMacs & Mac Mini's would have to be to run their CPU's & GPU's at lower temperatures under heavy loads, & why the MP's are the appropriate choice for constant heavy duty use.

To decide if this is a bad design decision for the more compact model lines, look not just at the temperatures but also at the performance & longevity vs. the competition. From what I've seen, there are no PC's that do this as well. Most are both bulkier & noisier, yet don't control temperatures under heavy loads any better, or just throttle down performance to pathetically low levels when required by their often poorly designed cooling systems. Some just let things fly & don't last more than a few years under heavy use as a result.

So, if you think bumping up fan speeds will make your Mac last longer, go ahead & do it. But it probably won't do anything more than make it noisier than necessary.

First, thank you R C-R for taking the time to make such detailed responses in this thread... They've made me think deeper on the issue & I've learned from them.

I was going to query whether the reported graphics subsystem failures of circa 2006 iMacs might be unusual/unexpected, but instead find myself agreeing in large part with your assessment(s). It's possible its just the weakest link in the chain in an otherwise very attractive, powerful & superior package (esp considering stability, robustness & features of bundled OS & software). Perhaps we're just seeing expected rates of failure of the ATI X1600 or associated VRAM.

IOW we bought these for the advantages of the overall package, even if not aware of any possible compromise their desirable design goals may have presented (WRT Mac Pro lines).

Re: raising fan speeds - There seems to be a fair amount of anecdotal information that supports doing so reduces the frequency/intensity of graphic artifacts in affected units. With the benefit of hindsight, I still wonder if altering things to keep GPUs even just 10C cooler may have reduced their frequency of failure. Perhaps to the point that some other (replaceable) component became the weakest link (e.g.the PS).

Thanks again & I guess my view has somewhat changed on the issue & I feel done with it.

Message was edited by: pbcubed

This is just an injustice, when apple usually admit their system faults and failures not of the owners doing! Eg the macbook air hinges', earlier macbook pro card failures.
We have lived and struggled with this problem, it would be minor for them to repair and would keep our ongoing custom period! Never again after that 13 month failure will I never write off apple care (ever!)

Just wanted to add my name to the list of 24 in iMac W86x with the horizontal lines and freezing. I did hear of a guy locally that got his fixed after contacting Apple PR. I am looking into it and will follow up with any news.

20" imac with same # W86 - horizontal random lines (also some pixel schewing) - 3 months after warranty apple care ran out. I am going to try APPLE for help and will post what they tell me.

24" 2006 with Nvidia 7300GT, and this just started happening a couple of weeks ago. Today it got really bad - chunks of windows were "left behind" when they were moved, etc. Ramping fans up to 2000RPM with SMC fan control seems to have alleviated this, but I'm resigned to having to replace the video card.

Add me to the list. I have been having graphics issues for several months - horizontal lines, video glitches, occasional freezes, etc. I got the SMC fan utility software on the web - that seemed to slow down the negative effects a bit for a few months. Now the freezes and the the horizontal lines are more common. I cannot risk a machine meltdown, so I have purchased a new machine. Color me not happy! Given the number of complaints that describe the same symptoms, I would attribute this to a design flaw.

FWIW, I changed the RAM, scrubbed the drive, did a clean install of 10.5 and 10.6, ran disk permissions and disk repair, reset the everythingRAM, even peeked into the case looking for mega-dust bunnies.

Your only option is to replace the card. That is what I did,

http://discussions.apple.com/thread.jspa?threadID=2484564&tstart=0

This will help as well

http://www.techdc.com/imac-24-opened-and-upgraded