Nasty Burning Smell !!

Hi from a newcomer.
I'm really after a bit of advice or just to share my tale of woe.
I have a 17" iMac G4 1.25GHz 80GB 256Mb (USB 2.0) that was (I repeat was) working fine unitil I tinkered!

I have to stand up and admit that I've always been a PC person until recently. Not because I didn't want a Mac (I've always loved the design ethic) but just because I couldn't afford one (or perhaps I should say the one I want!). Living with PCs I've been used to getting intimate with the innards and am quite at home taking one apart and putting it back together. Not from a great technical point of view but from a sort of trained monkey, tab A into slot B sort of way.

Anyway, I bit the bullet recently and bought a Mac Mini (it's just as lovely as I thought it would be) and by coincidence picked up for free a "Blueberry" at the around the same time. I decided to upgrade the hard drive in that and add extra memory to use as a music server and backup point, all went well.

Flushed with my new found "Appleness", I picked up a 17" flat panel iMac recently, it's always been my fav iMac design. I decided that it could do with a bit of an upgrade too (1.25GHz 256Mb 80GB). I read up on all the pitfalls and advice I could find bought my ingredients (250GB HDD + 1GB Internal DDR SDRAM + 1GB External SODIMM). All appeared to go well, I remembered the thermal paste, was well grounded, watched out for cable routes etc. etc. Put it all back together, and turned it on. Nothing exciting happened, got the file with the question mark on as expected and put the 1st restore CD in. At this point I decided to zero the disk before partitoning, took a while but worked. Then I went to partition it and create 4 volumes. That started OK and then the screen went black and I heard a feint buzzing and a "Nasty burning smell !!" (see I got there in the end).

Needless to say unplugged straight away and following a couple of expletives opened up the case again. With a bit of investigation I found that the culprit was on the underside of the main logic board underneath one of the heatsink pads (one of those that needs the thermal paste) where the screw goes through to hold the base on. I've cleared away the charring! and it seems as though the support of the pad is shorting against something. It's burnt through the board but apart from that on the board the only discolouration I can find is to the black plastic insulating circle underneath, I think this is just melted from the heat of the shorting. The only things I can see it might short to are the edge of the screw hole or something else on the board but can't find any evidence of it.

Here we get to the needy part. Can anyone with some electronics knowledge explain to me why this might have occurred? and if they would have expected it to fry the main board or not? There are no other items on the board near the short and no traces that I can see so I'm wondering if it could have effected anything else? Also any idea of anything I can reinsulate the pad support with? I assume it was originally embeded in the strata of the board itself. I had thought perhaps a dob of Araldite might do the trick?

I've been racking my brain to think how I could have caused this. I did not remove the logic board at all and am very very careful about static. The only thing I could come up with is that the support of the pad was slightly longer than it should be and was almost breaking through the board itself anyway. Then with the stress of my getting into the machine, maybe flexing the board slightly? I caused the board to fail around the support leaving it open to short against the screw or screw hole surround.......well it's a theory?

Phew! that was a long post! So I'm in your hands, if anyone has any ideas, suggestions or comments (clean please (or dirty and funny)) I'd be most grateful to hear them. As for the 20" mentioned in my profile, I'm afraid I'm an addict now and I'm in negotiations for it, hence the hopefully!

Thank you all and I await your responses.

PS. Excuse any spilling mistokes but I hav to rush ov to a meating know and dont have time to chick it

Hmm...sounds pretty bad. Can you make some pics and post them?
Would be easier to localize the problem, and suggest any "healing" measures... :dive:

Hi Avalon,

Thanks for the response. I've posted up three pictures at the following URLs:

http://www.onedog.co.uk/phantomdog/apple/full.jpg
http://www.onedog.co.uk/phantomdog/apple/closer.jpg
http://www.onedog.co.uk/phantomdog/apple/closest.jpg

Hope that makes it a bit clearer and thanks again.

looks to me like you bent the capacitor or slightly twisted it making a circuit and it melted. your going to have to find the part number and look at the specs and get a replacement one and solder it on. i would just take it to the apple store and see what they say.

Biturbo V12 AMG said:

looks to me like you bent the capacitor or slightly twisted it making a circuit and it melted. your going to have to find the part number and look at the specs and get a replacement one and solder it on. i would just take it to the apple store and see what they say.

Click to expand...

Sorry to be a bit dim, but which capacitor? I can't see one at all in the area around the short.

Thanks for the help.

I believe that he is saying that there is a capacitor in the molten mess. Look at the largest picture. It appears that there is a post or can in the picture.

rman said:

I believe that he is saying that there is a capacitor in the molten mess. Look at the largest picture. It appears that there is a post or can in the picture.

Click to expand...

I see what you mean and thanks for the thought but perhaps the magnification of the picture is deceiving. That is actually a post from the heatsink on the other side of the board (the large silver thing sticking out from under the board) not a capacitor.

As a bit of an update I tried re-insulating it with some Araldite but to no avail, the short just burnt straight through it?

I think I am going to have to admit defeat and strip it down for parts neye: Unless anyone out there has a knackered 17" 1.25GHz they want to part with? Perhaps I'll post a new thread asking?

Sorry to say it, but that board doesn't look repairable to me...though I can't really imagine what has burned, cause there are no parts in that specific area. Did you probably squeeze one of the power cables when reassembling? Due to it's not very repairfriendly design, that happens easily on the iMac G4.

Avalon said:

Sorry to say it, but that board doesn't look repairable to me...though I can't really imagine what has burned, cause there are no parts in that specific area. Did you probably squeeze one of the power cables when reassembling? Due to it's not very repairfriendly design, that happens easily on the iMac G4.

Click to expand...

Thanks anyway Avalon. I tend to agree with you that it's shot.

I'm pretty certain I haven't caught any cables, but as you say there are no parts at all in the area. That makes me think that something on the high voltage side is making the faraday cage live and it's shorting back at that weak point, but as I don't know enough about that sort of thing I'm leaving well alone.

I might have the opportunity of getting hold of an old 700MHz logic board though but do you think I could use that instead? At least it would salvage something out of it? I haven't seen it yet but I assume that the IDE, power, video connectors etc. are all the same so whats the worse that could happen !!

Thanks again.

Apple used different video connectors with different board revisions.

So if you're lucky, the board you'll get will have the same connector than your old one, but I fear that it won't.

If I were you I'd wait until you had the money to get a 1.25GHz iMac G4 logic board personally, that way you get the right item and aren't forfeiting the extra speed of the G4 too. If you can just hold off until one comes up or you can afford one.

That's my 2p anyway :flower:

Vicky

lil said:

If I were you I'd wait until you had the money to get a 1.25GHz iMac G4 logic board personally, that way you get the right item and aren't forfeiting the extra speed of the G4 too. If you can just hold off until one comes up or you can afford one.

That's my 2p anyway :flower:

Vicky

Click to expand...

Hi Vicky,

Thanks for the idea but I contacted VIS about a new logic board and it's an unbelievable £845 !! I can go and buy a new G5 for that. I probably will just have to wait and see if one comes up 2nd hand. In the meantime the "snowball" makes an attractive paperweight

Oh that's what I meant, second hand - wouldn't suggest brand new, waaaay too much

You may find a spares machine that has a dead screen or something.

Vicky

onedog said:

I see what you mean and thanks for the thought but perhaps the magnification of the picture is deceiving. That is actually a post from the heatsink on the other side of the board (the large silver thing sticking out from under the board) not a capacitor.

As a bit of an update I tried re-insulating it with some Araldite but to no avail, the short just burnt straight through it?

I think I am going to have to admit defeat and strip it down for parts neye: Unless anyone out there has a knackered 17" 1.25GHz they want to part with? Perhaps I'll post a new thread asking?

Click to expand...

ok... just looked like a melted capacitor to me.