[galarneau]

I bought this iMac off eBay for parts, but it actually seemed to work after some fiddling around when I got it.

It's worked for about a week, and now today, it started having some weird video artifacts on the screen:

(click the thumnails for a larger picture)



Windows would get a pink tint to part of them, and as I moved a window around the screen, it would leave trails of pink horizontal lines behind it. If I moved another window over the video artifacts, they would be erased, like the window I was moving scrubbed them off the screen.

After rebooting, the video was totally scrambled. I took a screenshot, which I figured would tell me if the actual VRAM was messed up or if the monitor just wasn't working properly. Since the screenshot is messed up, I think the problem must be in the VRAM itself.



The weird thing is that I ran TechTool to check the VRAM and it says that it's alright, even when there were clearly artifacts on the screen.

To torture test the machine, I ran a Quake 3 demo in a repeating loop for 8 hours one night and it worked fine.

I'm stumped as to what might be going on here. Anybody have a clue?

[Detrius]

The fact that the screenshot was identical to what was on your screen says that your display is not goofing on you--that in fact what you see on the screen is what is sent to the screen.

As far as tech tool pro... the only thing it's good for is checking hard drives.

The only possibility other than the logic board is software, but that's a real stretch. I'm leaning towards bad logic board myself.

Of course, make sure the thing has the latest firmware (4.1.9f on the slot loaders), but that's not likely what's going on here.

[galarneau]

Well, I swapped out the downconverter board with a know good iMac, and the problem still occurs.

Guess I'll throw in another logic board and see if that fixes it. It's either that or the Power-Analog-Video board, I think.

Anyone ever see anything like this before?

[galarneau]

Hey Detrius,

Do you know how to differentiate between a bad PAV board and a bad logic board? This iMac runs for a long time with no problems, but then gets the weird video problems, so the logic board seems to work fine most of the time.

I tried swapping logic boards with my old iMac DV SE, but the processors are in different positions on the logic board, so they don't make contact with the heatsink and it quickly overheats and shuts down.

[Kilbey]

Have you tried the VGA out port?

[galarneau]

Originally posted by Kilbey:
Have you tried the VGA out port?

Yep, the video is equally scrambled on the external monitor.

[Detrius]

Originally posted by galarneau:
Yep, the video is equally scrambled on the external monitor.

That's how you tell it's not the PAV board.

Down converter boards virtually never go bad.

As far as video issues, about the only things the PAV boards do are geometry failures and universal flickering colors--i.e. the green gun is only firing at half. The PAV boards more often cause power related issues. The key thing to remember about the Analog/Video portion of the PAV board is that this is the part that controls the CRT. You can associate video failures with this board with what you would see in a failing monitor.

I suspect that if you ran the Apple Hardware Test CD, you would get VRAM failures. However, since you bought the machine off eBay, you may not have gotten those CDs.

[galarneau]

See, that's what's weird:

The TechTool Pro CD I have tests VRAM by writing a pattern into VRAM and then reading it back, and it passed just fine while the screen was acting goofy (see first screen shot).

I think you're right, and it probably is the logic board, but I hope it's not. PAV boards are cheaper than logic boards.

[galarneau]

Well, I switched the PAV boards, and the problem persists, so I guess it is the logic board.

Since the monitor had some serious screen burn-in issues, I'm not so bummed out.

Now I have a guinee pig for my iMac DV -to- ATX conversion.

<rubs hands like an evil genius>

[Detrius]

Originally posted by galarneau:
See, that's what's weird:

The TechTool Pro CD I have tests VRAM by writing a pattern into VRAM and then reading it back, and it passed just fine while the screen was acting goofy (see first screen shot).

I think you're right, and it probably is the logic board, but I hope it's not. PAV boards are cheaper than logic boards.

TTP will run a VRAM test even when there's no video card visible on either the AGP or PCI bus... and it still passes. The only thing TTP is good for is hard drive hardware tests.

[Detrius]

Originally posted by galarneau:
Well, I switched the PAV boards, and the problem persists, so I guess it is the logic board.

The PAV board cannot affect external video (except if the PAV board makes the machine not boot at all).

If you have warm air coming out of all of your vents in your car when the AC is on, replacing the vent cover on all of the vents is not going to make it cold--the air is already warm by the time it gets there. The thing that proves this is the fact that warm air is coming out of ALL of the vents--even the ones at your feet that don't have covers. The fix is to add freon (or turn the temperature down).

[galarneau]

I understand your logic. What was confusing me was that the machine would run fine for days at a time and that zapping the pram would often get the machine to work again. I was hoping that a bad PAV board might have been sending some weird voltages to the logic board that was messing up the VRAM, and that the logic board was essentially good.

I had no real technical basis for this theory, as I am not in any way trained in this, as you are. I just wanted the logic board to be ok, as it's the most expensive part for me to replace.

Thanks for all your input, though.

[tooki]

Originally posted by Detrius:
I suspect that if you ran the Apple Hardware Test CD, you would get VRAM failures. However, since you bought the machine off eBay, you may not have gotten those CDs.

The only problem being that the Hardware Test software was not introduced until mid-2000, so all tray-load G3 iMacs and many slot-load models never included a disc, and may not be compatible with the disc from future models.

tooki

[ApeInTheShell]

So what tool can you use to accurately test VRAM?

[Detrius]

Originally posted by ApeInTheShell:
So what tool can you use to accurately test VRAM?

I use MacTest Pro and Apple Service Diagnostics. However, these are only available through Service Source. If I remember correctly, you can get access to all of Service Source by signing up for the $300/year technician training.