Problem with Vertical Hold control on 8liners.com 19" Chassis

[ranma]

First of all, sorry but I don't have the exact model and I don't have the chassis here to check, but maybe you can help me anyway.
I have a PC with a Matrox G400 Video Card and Linux and 15Khz resolution output via frame buffer and X.
The Image on the monitor looks pretty good but I couldn't really tell because of the problem I have. The problem is that I can't keep the image still with the control of v. hold
The image scrolls always and I have to adjust the v. hold control in a surgically precise position to keep the image in place (I mean turning the knob 0.5 degree to either side will make the image keep rolling), changing video modes makes the image to start scrolling again, and going back to the previous resolution doesn't solve the problem, I have to adjust again.

I don't know if the problem is understood. Now I'm not sure what's happening here, I guess that the normal behaviour would be to have a margin of adjustment as I have with the horizontal knob, but I'm totally clueless if this is something related to the chassis, the video bord or something else. So any help and ideas are welcome...

Thanks!

[Ken Layton]

This chassis accepts composite NEGATIVE sync only. Chances are your video card is outputting POSITIVE sync signals which is incompatible.

[MonMotha]

If that's the case, you can easily configure the G400 in Linux to use negative composite sync.  In the Monitor section of your xorg.conf, add "Flags -Csync" to your "Mode" specification, or, if using a "ModeLine", add "-Csync" to the Modeline.  Also make sure you have the "Composite" option specified on the modeline (or in flags), as otherwise the card will output separate sync, which would result in only horizontal sync being supplied to the monitor, which will certainly make the picture roll.

[ranma]

Ok!, this is good info! thank you, I'll have to try that. Now since I don't have the machine here I'd like to be sure that when I go I will be able to try everything I can.

About Composite sync, I forgot to tell that my video signal is passing through a J-PAC, wich creates the composite signal,  right?
In that case, what should I do with the separate sync signals?, should both of them be negative?

If that's the case, that's not my problem, since my modelines all have the "-Vsync -Hsync" options at the end.

Is there something I'm missing here?, should I try without the J-PAC and feeding the composite signal directly to the monitor?

I also have an original arcade board, I will try to turn it on and see if the problems happens with it too.

[MonMotha]

The normal method of creating composite sync from separate is to XOR them together.  It's cheap (one logic gate) and gives the correct results.  If that's all it's doing, you want -Hsync +Vsync.  You can also try -Csync and disconnect the vsync line, tieing it to ground.  That would allow the composite sync output from the video card to pass directly through.

Some devices include integrators to detect whether each signal is active high or low and they adjust accordingly (this requires about 4 XOR gates, which is the standard number you get on a 7486, plus some resistors and caps).  In this case, it will always output whatever it's set up to output, regardless of what you feed it.  I don't know how the JPAC is set up, so you'd have to consult the documentation.

[ranma]

Well, it worked!, thank you very much...