[HM] iMac flickering screen

[Florin Alexander Neumann]

On Wednesday, Sep 10, 2003, at 22:12 Canada/Eastern, Mary C. Youra 
wrote:

> My daughter, who is away at college, called today complaining that her 
> iMac's screen is flickering. She has an older iMac (with CRT) [...]

"Screen flickering" is a generic symptom that doesn't necessarily tell 
you much about the cause. If you have an experienced eye, you can 
distinguish between various kind of flickers and decide what is the 
probable cause.

Yes, it could be a problem with the iMac itself, in which case the 
repair shop is the only solution. But first you need to eliminate other 
causes.

Yes, it could also be a problem with incorrect settings. That's easily 
solved by going to System Preferences > Displays (and use the Show 
modes recommended by display). Test different settings and see if 
anything changes.

There is the issue of optical interference. The image on the screen is 
not constant; it is constantly "flickering" at a rate higher than the 
human eye can distinguish, so to us it appears as constant. Fluorescent 
lights also "flicker", and sometimes interference between the two 
different "flickers" occurs (the same phenomenon you notice when you 
see a computer monitor shown on TV).

But the most common problem for CRTs is magnetic interference, when 
magnetic fields generated by electric motors or transformers interfere 
with the monitor's image mechanism. Sometimes that's easy to figure out 
-- move the fan or power supply away, and the intereference goes away 
too -- but sometimes it's not. Changing surge supressors or plugging 
the devices in different power circuits doesn't help, because it 
doesn't address the basic issue, that of magnetic fields.

To test for this problem, remove any magnetic field generators from the 
proximity of the iMac (anything with an electric motor or a 
transformer, including stereo equipment, answering machines, 
loudspeakers, etc.) and test the iMac in different orientations and 
locations around the room. If the flicker changes in intensity or 
quality, then in all likelihood that's the problem.

It may not be something you can do anything about, by the way. The 
interference could be caused by something you can't see -- an 
electrical panel on the other side of the wall, for instance. I've seen 
this happen with a transformer installed by the phone company on the 
exterior wall of a student residence. The only solution was to locate 
the Mac as far away from the exterior wall as possible.

f