Quadra 700 video corrupted

[pff000]

I have a quadra 700 that was great until recently when the video became corrupted, with a pattern shown here. This happened 'all of a sudden' while the computer was running.
I have a new power supply, and the video is fine if I use a nubus video card. The problem is only with the internal video. I removed all the vram and ram simms, and the problem persists. I am guessing this could be related to the on-board video ram, either a chip is bad or an address line.

If I move the mouse around I can 'paint' the corrupted pixels with the wrong values. The corruption is limited to the apparent columns, and there is a doubling of sorts.

I hope someone might have an insight by looking at this picture. Any suggestions or ideas would be very appreciated.

Thanks!

 

[pff000]

I am happy to say I managed to fix this problem.
I replaced one of the video ram chips in the logic board with one I took of an unused video ram simm.
The Quadra is back to normal!
I guess I will save the other video ram chip in the simm in case another fails.

 

[Scorpio_ny]

How did you determine which one was the bad one?

 

[pff000]

After some time the screen had a stronger periodic pattern with black bands, suggesting that it should be the first or last of the 4 chips in the motherboard that was bad (assuming the video memory and placement in the board is relatively sane)

I took my 50% chance and removed the first chip. Nothing got worst confirming I had removed a bad chip.

it appears that the first band (8 colums) of pixels are stored in the the first chip, second band in the second chip… and after thr 4th band it wraps around. So now I can tell which chip holds each particular pixel in this video mode.

 

[Scorpio_ny]

That’s good to know! Thank you for sharing!