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, "[email protected] <javascript:>" 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 
> <http://www.mac-forums.com/apple-desktops/341784-imac-vertical-lines-screen-frezees-boot.html#post1762659>
>  
>
>
>
>

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