iMacs have a particular cable in the CRT area that tends to fail and take out one of the colors with it. It's a little goofy that an internal cable would fail, internal cables are one of the least likely to fail parts.
At my daughter's school where I help out the single biggest failure mode of our 250 or so iMacs is a screen tinting problem. The techs who fix them know this failure well and it's usually a particular cable. I don't know which cable it is but I could find out.
Kidding apart, following is what I have observed in trying to repair some PC CRT monitors. This may explain some of the responses.
Whenever there was an intermittant color problem like the one you describe, it could be traced to a bad connection on the VGA connector/cable. Once or twice, I just had to straighten a bent pin in the connector.
Last Friday afternoon, when I first fired up the iMac at home, I read the Apple service manual for DV slot-loaders. In the process of working through as many of the troubleshooting tips in the manual as I could, I removed the bottom cover and the EFI shield. All cables seemed to be connected tightly, but I did push on the video board to make sure it was firmly connected to the CRT. It was. Later on that day, I did the firmware update prior to OSX installation, and the video was still tinted yellow. The color reverted to normal as I was doing OSX updates. Strangely, it stopped doing yellow/normal cycles Friday night, and the yellow tint hasn't returned. I've been running the machine for hours every day, with nary a video/screen problem. But now that I read the above, perhaps all my poking around caused a delayed reaction.
For starters I would create a color test pattern, something like an AppleWorks Draw document with large red, green and blue objects on it. You should create it on another machine so you can set the colors correctly and have a comparison. Look at the doc on the much in question. It would probably be good to include each intermediate color, red-blue (purple), blue-green and green-red. If any of these blocks are black or blend with the white background that will tell you which color is messed up and something about what is wrong. If a primary color (R, G, or B) is black then that color signal is out. If one of the intermediate color blocks is white or nearly white then the opposite color (the one that isn't in the block) has an adjustment problem.
Thanks for the tip. Since the screen tint went away, I've been able to obtain excellent screen colors by using the built-in color calibration program, and by adjusting the focus and screen brightness. But if the screen goes goofy again, I'll use this tip.
About something in the seller's work area affecting the CRT, it is highly unlikely to cause an overall tinting. Magnetic fields typically cause shaky pictures and add horrizontal lines in the picture. Permanent magnets (like speaker magnets) can cause localized discoloration. This is however fixed automatically when the CRT is powered off & then on - as it goes through a degaussing cycle (sort of wipes the CRT off of residual magnetic effects). Some times it may take a few degaussing cycles if the affecting magnets are too strong.
If you don't hear the decaying bong (that's what degaussing sounds like, at least to me) the control circuit hasn't reset. This is typically a thermal device (thermisistor) and it takes a while to cool off. So turn it off and wait 10 minutes and try again. If it still doesn't, give it more time. Some monitors don't run a degauss cycle on soft power up, only on plug in. So if it doesn't degauss on power up try un plugging it, waiting 10 minutes then plug it in again. I ran into this on an Apple AV model that had some real bad magnetization problem.
I get the normal start-up degaussing sound, and have since the first time I turned it on at the original owner's home. Thanks for everyone's input. I'm still puzzled about why this machine's working perfectly, but I'm not complaining, mind you.
Jim
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