I have a problem with the display of an iMac G5 20". It shows up as vertical lines of small squares.

If I change screen resolution, the display will look ok for about half a second, and then it comes back again. Anyone had seen this before? (This is does not look like the usual problem with iMac and thin vertical lines.)

Any ideas?

http://support.apple.com/kb/HT2390?viewlocale=en_US

Maybe?

Thank you, I've checked that and it's more than 3 years old and not in the series (my starts with CK536xxxxxx).

Is there anyone who has seen this kind of pattern like the attached image? Actually I've replaced all 1800uF capacitors cause one looked suspect, but still the same pattern.Might it be the GPU that’s broken or any other part? The computer works fine besides the strange graphics pattern.

Provided there is no AC leakage at the power supply (because AC leakage gives that kind of banding) it would appear to be the GPU.

Checked with a VGA Adapter to external monitor today, the same pattern shows up... :-( Probably GPU then... or might it still be some other component?

Either:
1) use a decent multi-meter and check your power supply for AC leakage.
2) eyeball the capacitors in the power supply for "bad" ones (if you had to replace on the mainboard I'd bet there are one, or more, bad in the power supply)

Loosen the 3 screws on the bottom and pop the back off. Take a look at the capacitors. Many of the iMac G5's have problems with capacitors bulging/leaking and fail after doing so. They will look like small cylinders on the board. The bad batch had an "X" marking engraved on the top, the newer replacements (the good ones) have a "K" on the top instead.

http://en.wikipedia.org/wiki/Image:Badcaps-tayeh-4.jpg

The tops of the capacitors should be flat and shiny, not bulging up or cracked/corroded in any way.

When my video RAM went bad, I also saw similar problems, but luckily it was repaired under AppleCare. Try booting from the diagnostic CD (hold either C or D down when booting, forget which, different models vary) and see if you can make out the screen enough to run a test. My bad video RAM showed up as that (from what I could read) on the diagnostics.

If you have the bulging capacitors with the "X" I'd call up AppleCare and raise hell. These were a well-known problem and tell them that you feel that yours should have been recalled as well. Personally, my iMac G5 went through:

3 logic boards (1 bad video RAM, 1 bulging capacitors, and 1 wouldn't power on)
2 optical drives
1 front bezel (the place where the screws attach it to the back broke)
1 power supply (would not wake from sleep, then would not power back on for several hours)

A combination of A LOT of heat from the G5 in the small encosure and cheap logic board electronics made this model very problematic for a lot of people.