Dreamcast to JPAC

[pookycade]

I am trying to hook a hacked Dreamcast VGA box through the JPAC. By hacked I mean line 6 has been cut so the Dreamcast is outputting a "VGA" signal that is actually 15Khz. I verified that the output timing is correct on my VGA monitor which won't sync, but tells me that it is indeed 15Khz with a 60Hz refresh rate that is coming out of the VGA box. First noticed I have to have the computer on to even get the JPAC to light up. Second noticed that the yellow and green LEDs do light up, indicating I have a valid signal, but the screen is black (ie no picture). Adjusting brightness and contrast doesn't help. Any ideas ? Do you think this is a grounding issue or and amplifier issue or a voltage level issue.

[giantgonzolez]

Here's a little info about 15khz RGB from dreamcast.
http://www.gamesx.com/avpinouts/dricas.htm

[tom61]

The PC has to be on since the J-Pac gets power from the keyboard port. I suppose it could be a grounding issue between the two, but it'd have to be a pretty bad one, like an AC outlet being wired backwards.

Try getting a keyboard adapter for Dreamcast and pluging the J-Pac into that. The best one to get is the Smart Joy 2 or Total Control 2, as they would theoretically let you use the J-Pac to control one player in any game.

[pookycade]

Ok, a bit more experimenting and this looks like a different problem now.

I do in fact get a picture, and it is indeed a 15kHz 60 hz picture as expected. And I can turn on the Dreamcast and see a picture WITHOUT the PC also being turned on first.

BUT, over the course of about 30 seconds to 1 minute the picture slowly fades away and goes to black. After that I have to unplug everything, let it sit for a while, turn it all back on, and same thing happens.

Looks like a cap somewhere is charging up slowly and then squelching the signal. That or my ground is floating and slowly rises to the point that my effective RGB signals go to zero. No ideas beyond that. Anyone else have any pointers here ?

[pookycade]

Ok, last update as I give up for now. I have removed the PC from the equation by pulling off +5V and Ground from the Dreamcast VGA box and sending them in thru the keyboard connector on the JPAC to power up the JPAC from the Dreamcast. Problem is the same though. Turn on Dreamcast -> beautiful picture -> fades away to black over 45 seconds - > turn off Dreamcast, wait a couple of minutes, -> turn on Dreamcast-> beautiful picture -> etc.  No idea what the problem is here. I tried tying multiple grounds between the JPAC and the Dreamcast and that doesn't work. I give up !

[pookycade]

And finally problem solved. At least on my Dreamcast VGA box , you need to pull down the R,G,and B lines to ground through a 30K (or more I suppose) resistor.  I did this by soldering a 30 K resistor from the bottom output pins 1,2,3 on the VGA connector from the VGA box to ground. After that all is well

[Pyronious]

I've also been working on getting the Dreamcast hooked up to my arcade monitor using a VGA box and Andy's video amplifier (same as the JPAC).

Thanks to help from both Andy and Pookycade, I have it working now.

My VGA box also required the RGB outputs to be pulled to ground through resistors (I used 47k Ohms, works great!).  Otherwise the picture was completely white.

Anyway, here are a few pictures of the modifications necessary to connect my DC VGA box (manufactured by Madness Gameware) to a JPAC or Ultimarc Video Amp:

This image shows how the 15/31kHz select wire has been cut.



This image shows the resistors pulling the RGB outputs to ground (termination).

As you can see in the 2nd image it was much easier to solder the resistors where the traces to the VGA connector begin rather than end.

Hope this helps someone.