Model A1224 / Mid 2007 and Early 2008 / 2, 2.4, or 2.66 GHz Core 2 Duo processor

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Possible causes for memory failure and crashes, with any RAM boards?

My niece's iMac 7,1 (Mid 2007, 20 Inch, 2.0 GHz) is crashing and freezing, usually soon or immediately after startup. Sometimes it fails to get past the black screen at startup, and plays error beep sequences. At first these error beep sequences were short-short-short-short-long-short-long, which I haven't found documented anywhere, but all later error beep sequences were different: short-short-short-long-long-long-short-short-short, which is Morse code for SOS. Occasionally the Mac will run for 15-20 minutes, but not often; when it does, it often finally displays a kernel panic screen. The first test we did was to run the Apple Hardware Test CD (on this occasion, the Mac ran long enough to let us do this), but it didn't find any problems. Next, we tried different RAM boards (both in pairs and singly), including two new RAM boards, but that didn't help. Even though the problem didn't seem to be due to bad RAM, I ran the Memtest utility by Tony Scaminaci anyway, to see if the RAM controller circuitry might be bad. Memtest showed memory errors almost immediately (I tested several times with the same result). I also disconnected the internal hard drive, and tried to start up from an external drive, but that didn't help. Since these problems often occur immediately after powerup, it's unlikely they're due to overheating.

I think the problem is on the logic board, where the RAM controller circuitry is located. Does anyone have any other ideas? Could a bad processor (CPU or GPU) cause errors like this?

Edit April 25:

I bought a replacement logic board for this iMac, and I'm in the process of installing it today. Though I know I didn't get any suggestions here for my question, I'd still be interested in hearing some, but iFixit archived my question April 14 because, according to them, I "did not provide enough information to provide an answer." I thought the information I provided was pretty detailed, so I don't know what the "not enough detail" refers to.

Answered! View the answer I have this problem too

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More tests I recently did on this Mac, consisting of unplugging everything I didn't suspect before--Airport card, Bluetooth card, hard drive, etc., still point to the CPU chip. When I get a replacement CPU, I'll report the results.

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As I noted in the comment further below, but forgot to note in the comment above, the replacement logic board didn't solve this Mac's problems. So last week, I ordered a replacement CPU. That didn't help either--the iMac is displaying symptoms identical to when I first started working on it. The only part remaining from the iMac's original hardware that's likely to be the cause, is the GPU chip. I've never seen either a CPU or a GPU cause main memory errors in a Mac of the sort that Memtest would report, but there's a first time for a lot of things, but for all I know, this iMac's design might have such strange interacting circuitry, that something as simple as a bad sensor, bad audio board, bad display, etc. might trigger main RAM errors. At this point, I have to be open to even the most unlikely possibilities.

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I just finished the latest round of unplugging new suspects from the logic board that I hadn't unplugged before: display; ambient temp sensor; LCD temp sensor; audio board; infrared sensor; microphone; camera. None of this helped. My theory was that possibly one of these parts might be defective and putting weird values into its memory space in main RAM, which the logic board might be mis-identifying as bad RAM, since this has been seen with some defective peripherals like mice, keyboards, and USB hubs. I've also been noting that LED 4 hasn't been lighting up whenever the display remains black and the Mac plays error beeps, so maybe this is another clue that implies the video board is at fault. But I don't look forward to buying a video board and finding out that it's not the fix either.

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I'm still pondering whether it's possible that something on the video card may be causing the symptoms, but I still can't figure out how, because the two RAM testing utilities I used, Rember and Memtest (Rember being a GUI front end for Memtest), both test only main RAM, not the video RAM on the video card. I wonder if the video card somehow actually does interact with main RAM, and may be able to cause trouble for it, that RAM testing utils mistakenly show as main RAM failures.

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I've seen single RAM slots fail. To test for this, insert just one know good chip in slot #1 and try it, then try it in the #2 slot. How many beeps are you getting?

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That was one of the first tests I did--as I noted in my question, we tried different RAM boards (both in pairs and singly), and still got the RAM errors. Only rarely does the Mac play any error beeps--it either simply freezes, or if I can keep it running for a few minutes after powerup, I run Memtest, which displays RAM errors almost immediately.

I've just installed the replacement logic board, and there's no improvement, which greatly increases the chance that the problem is a bad CPU or GPU (these parts don't get replaced when you just do a logic board replacement). The fact that this Mac is one of the models that shares its RAM boards between the CPU and the GPU is another thing that makes diagnosing the source of a RAM error difficult. I suppose the next step is to try a known-good CPU, since it seems slightly more likely that's bad than the GPU, since a bad GPU will more often show video problems, either instead of or in addition to RAM errors.

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I just discovered that this iMac model (A1224, EMC 2133) is NOT one that shares its RAM boards between the CPU and GPU. That makes it much more likely that the problem is with the CPU, especially since I'm not seeing any graphics issues.

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I'm working on a 17" model of this type unit right now. Don't forget the thermal paste - I did and have to take it down again.

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johnsawyercjs will be eternally grateful.
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