iMac G3 video problem

[tmtomh]

Swapped in a new mobo to an iMac G3. The problem: video artifacts. A series of dots and lines, in a grid-like pattern, across the entire screen. When the initial grey bootup screen shows, the artifacts are red and white. When the blue desktop comes up, the artifacts are reddish and much more subtle - and the menubar, though white, appears to have no artifacts.

The arrow cursor also is accompanied by a tall, thin line (about 6 pixels wide and 40 pixels high, give or take) just to its right. This line moves whereever the cursor does - although, strangely, when the cursor changes to the spinning beachball, the line disappears.

I've reinstalled the OS, swapped RAM, reset the motherboard via the CUDA button, deleted the extension and kernel caches - no help.

I've also attached an external monitor via the VGA mirroring port on the back, and the external monitor shows the same problem - which means it's not the flyback transformer or any other part of the analogue circuitry.

So does this mean the new motherboard has a defective video chip or defective video RAM? Or is there something else I might be able to try?

TIA for any advice/info.

Best,

Matt

 

[Sosai X]

Does the logic board have an expansion VRAM chip installed? Maybe it came loose during the swap. Or maybe the video cable connection on the bottom of the iMac isn't properly connected, or maybe on of the pins got bent when you reconnected it?