It’s the video daughtercard. A bad/faulty/dying video card will thwart boot. The RAM card thing makes sense because only one stick makes it easier for the EFI boot sequence to progress to a startup chime. Or, there’s a conflict between the two sticks that wasn’t apparent until now. Or both. The AHT does not detect a faulty video card; it only detects the presence or absence of that card. The only way to be certain it’s the video card is to replace it with a known good one. Good luck finding one of those today.
iMacs of that vintage are notorious for failing/failed video cards. It’s amazing yours lasted this long. The problem is basically the same one that affected G3 white iBooks and many G4 iBooks and PowerBooks. Many newer laptops made by Apple … and devices made by other manufacturers … also are affected. It’s a failing of the solder joints holding the video chip to the board. The technique for affixing the hundreds of tiny solder balls to the chip and thence to the board is known as BGA or Ball Grid Array. Thus sometimes a video problem can be corrected by reflowing the solder. But if the solder is bad, or the board flexes (G3 white iBooks, with their 50+ screws holding all the bits and pieces together, were notorious for this; the advent of aluminum and plastic Unibody cases helped), or too many heat up/cool down cycles are experienced, the chip will come loose again anyway. In the case of your 2008 iMac, in addition to all of the above, the cooling fins of the heat sink can be clogged by dust, etc. over time as the air intake vents directly below the card and heat sink assembly are hoovering up stuff from the desktop. This leads to overheating and eventual failure of the BGA. Reflowing the BGA doesn’t work because by the time the problem shows itself, too many of the microscopic traces inside the chip have been damaged by arcing. That’s why a more permanent repair involves replacing the chip with a new one. But new chips are hard if not impossible to find, and my favorite eBay bad chip repair/replace guy refuses to work on those old iMac video cards because he can’t guarantee a reflow and he can’t obtain new chips that will work with the card. Been there, done that a number of times. It’s time to ewaste that iMac, in my experience. Jim Scott, Eureka, CA > On Mar 27, 2018, at 6:11 PM, fishjoy...@gmail.com wrote: > > Thanks for the idea. However, this one (a 24" also) will not boot into Safe > mode. > In fact, I cannot get even a startup chime unless I pull one of the RAM cards > out. It doesn't matter whether the left one is in or the right one is in, if > I only have one RAM card in, it will produce the startup chime. However, I > still have a black screen. > > I did try to set up an appointment at an Apple store, but since it is 10 > years old, they would not look at it. > > A couple of questions: > • If it is the video card, does the RAM card thing make any sense? > • If the video card was going out, should the Apple Hardware Test have > told me that when I ran it last week? > • Is there any way to be certain that the problem is the video card? > Thanks! > > > On Tuesday, March 27, 2018 at 8:11:50 AM UTC-6, ValterV wrote: > Il giorno 27/03/18 03:17, "fishj...@gmail.com" ha scritto: > > > The machine worked fine for about a week, but today when I tried to wake > > it, I got a black and white checked screen. > Bummer. That screen makes me think about a damaged video card. > > I had a similar issue with an Early 2008 24" iMac: red vertical stripes > appeared all over the screen (bad video card), and the iMac froze on > startup. > > When I tried to boot in Safe mode, though (hold Shift key during startup), > it did boot up. So I thought the video drivers were conflicting with the > (now) bad video card, hence the freeze at boot. > I disabled all of the drivers, and the iMac would boot fine (although screen > redraw was sometimes slow). > > You can try booting in Safe mode and, if working, you may try my workaround. > > Here's what I did: > - Remove all AMD/ATI kexts in /System/Library/Extensions > (it depends on your video card maker and model; mine was ATI Radeon 2600) > - Terminal: Touch "System/Library/Extensions" > - Delete "System/Library/Caches/com.apple.kext.caches" (in Finder) > - Reboot > > I wrote more details here: > http://www.mac-forums.com/apple-desktops/341784-imac-vertical-lines-screen-f > rezees-boot.html#post1762659 > > > > > -- > You received this message because you are a member of the iMac Group, a group > for those using Apple iMacs and eMacs. > The list FAQ is at http://lowendmac.com/imac/list.shtml and our netiquette > guide is at http://www.lowendmac.com/lists/netiquette.shtml > To post to this group, send email to imaclist@googlegroups.com > To leave this group, send email to imaclist+unsubscr...@googlegroups.com > For more options, visit this group at http://groups.google.com/group/imaclist > > --- > You received this message because you are subscribed to the Google Groups > "iMac Group" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to imaclist+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are a member of the iMac Group, a group for those using Apple iMacs and eMacs. 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