[WelMac] Troubleshooting - new Intel iMac 2.0GHz 20 inch LCD - screen flicker after coming out of screen-saver mode

[David Empson]

At 4:45 PM -0700 23/7/2006, Waiton wrote:
>When waking up the iMac from screen-saver mode, the screen fllickers 
>very fast (like on old CRT monitors but quite disturbingly) for a 
>period (sometimes a couple minutes, sometimes up to 20 minutes). A 
>full shut down followed by about 10 minutes staying off solves the 
>problem but it still occurs the next time it comes out of 
>screen-saver.

Does it make any difference which screen saver you use? There are 
several screen savers supplied with Mac OS X (e.g. the default 
"Flurry", series of beach photos, etc.)

>All programs function as they should, just the screen flickers... a 
>search on the Apple website forums results in a few threads that 
>mention this as well but with no solution.
>
>I've since disabled the screensaver and resorted to getting the 
>monitor to shut down after a short idle time instead. Waking up the 
>monitor from this doesn't give me the flicker so that's a 
>work-around I'm using.

So it doesn't seem to be related to switching off the display. The 
symptoms pretty much rule out a hardware fault with the screen itself 
or its power circuitry.

It could be some kind of problem with the video display hardware in 
the iMac (ATI Radeon X1600). Perhaps the screen saver is putting the 
video hardware into a slightly unusual mode and it isn't handling it 
correctly, or there is a VRAM fault which is being exposed by the 
screen saver. A software problem in this area (e.g. OpenGL bug) could 
be a factor.

Try some other "unusual" on-screen graphics. For example, get iTunes 
playing some music and activate its visualizer in full screen mode. 
Does that also trigger the problem? How about other software which 
makes intensive use of the video hardware, such as games?

Another thing to try: set the display to "Thousands" of colours 
rather than "Millions" and see if that makes the problem go away.

Are you running the latest version of Mac OS X (10.4.7)?

>However, I like to use the screensaver because I can set a password 
>request upon waking, but having the monitor shut down and wakeup 
>doesn't let me put a password on - it just starts straight into 
>whatever was on before going to sleep.

Does it happen if you use the screen saver but don't set it to ask 
for a password?

Is the computer doing anything active while you are letting the 
screen go to sleep, e.g. some kind of intense processing like MPEG-2 
encoding in iDVD, or are you just walking away from it while it is 
idle?

If the computer is just sittling idle, you could set the entire 
computer to go to sleep after a relatively short delay. There is an 
option to ask for a password when the computer wakes up again. (In 
fact, it is the same option as for the screen saver.)

Waking up the computer is quite fast (should be a few seconds at 
most, assuming the RAM contents have been retained).

>Anyone else with this issue?

Probably not a large number of people with iMac Intel models on this 
list. I've helped one person set up an iMac Intel (17") but don't 
think we used the screen saver. The machine was set to go to sleep 
before the screen saver was activated.

-- 
David Empson
dempson at actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand