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Bad colors with VGA ISA card running DOS programs?

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First post, by Mauricio

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Hey folks,

Long time reader, first time posting!

I have recently acquired an Epson Equity II+ desktop. This is a 286/12 AT clone. It came with a CGA multifunction card, which I have replaced with a Paradise VGA Plus 16 ISA card. I wanted to run the the popular "tunnel" program (sometimes called a screen saver) as a test. However, the palette seems somewhat garbled. See this video clip:

https://www.dropbox.com/s/ojggy8camr2s4ou/tunnel.mov?dl=0

Is that familiar to anyone? I have tried different DIP switch settings and two different monitors, but no luck. In case you haven't seen it, the gradient is supposed to be smooth, and it runs fine (smooth) in DOSBox.

Thanks for any help!

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Last edited by Mauricio on 2018-03-31, 22:31. Edited 4 times in total.

Reply 1 of 4, by wbc

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Hello!

This can be caused by either bugs in program (as I heard some RAMDACs don't like palette updates via "rep outsb" instruction instead of "out dx, al") or by issues with video controller or RAMDAC itself. Did it work on other videocards properly? Does it experience similar bugs in other applications?

--wbcbz7

Reply 2 of 4, by Mauricio

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Thanks for the reply. I took my Windows 98 PC out of storage, as it has a (single) ISA slot. Sure enough, I see the same problem on that machine with that graphics card, while the GeForce graphics card in the same machine is fine. I also got a video sample from Scorched Earth (video link and image below), which clearly has incorrect colors and some flickering pixels. Strangely, text and EGA is fine, it is just VGA that has issues.

So this is almost certainly a problem with the graphics card, right? If so, anything else I should do to diagnose? And anything that can be done to fix it? Thanks!

https://www.dropbox.com/s/lnx6hr74eui48xq/scorch.MOV?dl=0

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  • scorch.jpg
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    scorch.jpg
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    677 views
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Reply 3 of 4, by Mauricio

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Looking up possible RAMDAC issues, I found this:

http://www.vcfed.org/forum/showthread.php?451 … 00AX-bad-colors

... which could be related to what I am seeing. Do you think it is worth trying to replace the RAMDAC? The model appears to be ADV476KN50 and is (oddly) still sold on eBay. This would involve desoldering the old one, though, something I have not done before.

Reply 4 of 4, by Mauricio

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Yet another update... I found and modified a Turbo Pascal program to test VGA palette handling. I have attached the program here. Here are the results with my Paradise card and another (working) card. You can probably guess which one is which.

https://www.dropbox.com/s/eoswpi1hdg8ms5q/vga_bad.mov?dl=0
https://www.dropbox.com/s/07ngruarb78x3ne/vga_good.mov?dl=0

Having learned what the RAMDAC does, this really feels like a problem with that chip. I ordered a replacement and will attempt to desolder the current one to replace it.

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    VGA_PAL.zip
    File size
    3.88 KiB
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    51 downloads
    File comment
    VGA palette test program
    File license
    Fair use/fair dealing exception
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