iMac Troubles

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davintosh's picture
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So last night I finally got around to swapping out the power/analog board on a buddy's iMac SE, and got the job done successfully (finishing by about 1 am; ugh. Talk about too little sleep.) So, I fire it up, adjust the brightness & focus, tear it all apart to adjust the placement of the degauss cable (nasty green hues where it should be blue), put it all together again, and all seems happy.

Then it locks up. Hard. Mouse is frozen. This is something I've only seen once or twice in OS X (this one is running 10.2.8.) I restart it, and it freezes as the Finder is coming up. I force another restart, and it doesn't even get that far.

By this time, I've about had it, so I shut it down & go to bed. After we've both had a good long rest, I hit the power button again, and it gets to the point of loading the finder again and throws up the "You must hold the power button to shut down" or whatever it says, in multiple languages, error window. Not good.

I didn't have time to mess with it any further -- gotta earn a living you know -- but it's been bugging me. I'm thinking it could be one of a few things:

Heat -- when reassembling the thing I noticed that the metal plate that the logic board attaches to serves as the heat sink for the processor; is it possible that there isn't sufficient contact between the two that is causing the thing to overheat? Since it booted up and ran fine at first, but even after sitting for several hours it won't boot now, is it possible that it did overheat and damage the processor (700MHz G3)?

OS Problems -- This seems less likely, since it did boot & run fine at first. When it locked up I was trying to fire up System Profiler. Or could a heat-related lockup cause an OS software problem?

Something shook loose inside -- possible I guess, but again it seems strange that it was working fine at first, but now it's not. Hmmm.

Anybody have any idears for me on this? Anything I'm maybe way off base on, or things I can try?

Thanks!

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Eudimorphodon's picture
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Usual Suspects

Bad RAM has always been the #1 source of freezes and kernel panics under OS X in my experience. However, I assume that this machine worked fine before your latest work? Ram usually doesn't just "go bad", so... I'd say heat's a good bet. Did the machine have heatsink goo on the CPU before you took it apart?

--Peace

davintosh's picture
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Re: Usual Suspects

I did notice it had some on it when I was putting it back together, but it likely ended up with less goo since I swapped out the whole power/analog board, metal plate, plastics assembly.

Can permanent, fatal damage occur from overheating? I wouldn't think that running it for 5-10 minutes with little to no processor load would result in something like this, but... I've never seen this sort of thing before.

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Eudimorphodon's picture
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Re: Usual Suspects

davintosh wrote:

Can permanent, fatal damage occur from overheating? I wouldn't think that running it for 5-10 minutes with little to no processor load would result in something like this, but... I've never seen this sort of thing before.

Well, for the most part with silicon you either "let the magic smoke out" or you don't. (You certainly can kill a chip from heat, but it would be unusual to "wound" it.) Have you tried disassembling it, cleaning up the surfaces, and putting some more goo on it? (And ensuring there's good physical contact between the sink and the plate?)

As for why it's crashing quicker now, you may have corrupted the OS or filesystem somewhat while it was suffering from heat exhaustion. I'd clean it up and try booting it from an install CD, and from there do an integrity check on the hard disk.

--Peace

davintosh's picture
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Re: Usual Suspects

Eudimorphodon wrote:

Well, for the most part with silicon you either "let the magic smoke out" or you don't. (You certainly can kill a chip from heat, but it would be unusual to "wound" it.) Have you tried disassembling it, cleaning up the surfaces, and putting some more goo on it? (And ensuring there's good physical contact between the sink and the plate?)

As for why it's crashing quicker now, you may have corrupted the OS or filesystem somewhat while it was suffering from heat exhaustion. I'd clean it up and try booting it from an install CD, and from there do an integrity check on the hard disk.

--Peace

"Magic Smoke"; that's great!

I haven't had a chance to tear it apart since this morning (work interferes with life), but that'll be the first thing on tonight's agenda (provided the wife & man-cubs don't have anything major going on.) Booting from a CD is also on my list.

Thanks for the reassurance. I've just been fretting about all of the dire possibilities whilst I'm here at work today. Since I've never really toasted a processor (knock on wood) I wasn't sure if what I was experiencing was symptomatic or not.

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davintosh's picture
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Things are better...

I tore into the machine tonight and discovered that the metal plate that the logic board mounts to was quite different from the original; the original had a square dimple in the plate that made contact with the processor by way of some thermal transfer tape. The new plate (pulled from an older 350MHz iMac) had an aluminum block that transferred heat from the processor, but it was in the wrong place altogether.

I didn't feel up to tearing the thing all the way back down again to swap out the metal plates, so I relocated the aluminum block to make contact with the processor. I put it all together and it booted right up. I ran a few apps and all seemed OK, but then I ran chess for a little bit and it crashed & locked up the whole system.

I downloaded Temperature Monitor to keep tabs on the chip, and at idle it was hovering around 85 to 90 degrees F. But the temp would shoot up with any little bit of load. I fired up iTunes, and it would run at about 110 while playing music. I turned on the Visualizer, and it zoomed up to 150, then on up to 167 and iTunes crashed. That took less than 30 seconds. It didn't lock up the system, but I wasn't going to take any more chances so off to sleep it went.

The way the temp was swinging so wildly I'm thinking that there isn't very good contact between the processor and the heat sink. There's no way it should've got that hot that quickly. Next step I guess is one of two things; pull the logic board and put some thermal grease in there and try again, or tear it back down and put the original metal plate back in it.

Anybody have any other thoughts?

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davintosh's picture
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What Is Too Hot For A 700MHz G3?

I got to thinking about the heat problem on this iMac, and I guess I'm not sure what the normal heat range is for it. My G4 PowerBook seems to run happily at 120-140F, but the way the iMac's apps crashed at 160 makes me wonder what the upper limit is for a fast G3.

Anybody have any references regarding normal operating temperatures for a 700MHz G3?

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davintosh's picture
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Success!

Well, I tore the iMac apart tonight, swapped out the metal divider panels, put it all back together and all appears well. It's been sitting on the bench happily running iTunes with full-screen visualizer for about 45 minutes now; last night it would run for about 30 seconds with the medium visualizer running.

One wierd thing is that when I run Temperature Monitor on it now it tells me it can't find any temp sensors; before I swapped things out tonight TM ran fine, showing one sensor on the processor. Why would it work one night and not the next? I know I didn't forget any connectors -- besides, isn't the sensor something that's built into the logic board?

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