Dreamcast VGA to New Astro City

[FriedGoat]

So i've been looking at these 15khz mods and none of the boxes look like the VGA one I have.
What would happen if I took the RGB, Vsync and H Sync directly from the wires from the dreamcast (as they are labeled in the VGA box) and soldered to a jamma biscuit?


I think this box has a 15khz switch built in (between 31 (PC) and 15(TV)) ?
How am I supposed to do this?

pics

[Sp33dFr34k]

Not that easy. For one you can't just hook up h and v sync to a jamma loom, you have to use a sync combiner. Second, a jamma monitor needs a lot more voltage on the rgbs lines (think it's about 3v vs 0.7v). Besides that, how do you want to handle inputs?

[FriedGoat]

The inputs I am not worried about for now, I will hack a controller once I have the audio and video working. Following the scart guide and the vga box guide it says you can just combine the v and h sync wires by tying them together and attaching to the sync on the Jamma loom.

Scart method
http://pc2jamma.mameworld.info/arc_dc4.html

The main issue using the scart method is that the h/v sync are combined with video output so you have to build a stripper circuit with a lm1881 chip, which I really want to avoid.


Vga method

https://forum.arcadeotaku.com/viewtopic ... 70&start=0

The vga box method says that once you have a vga box you can snip a ground wire to tell the D.C. to output at 15khz and then you just solder the r g b and sync from a vga wire to the jamma loom.

As my box is different and I have clearly labelled outputs for v sync, h sync and the r g b , I wondered I could solder then straight on, but this would be before it passes through the vga box. obviously from what you are saying, power and other bits are combined and then send through the vga cable so this won’t work?

On top of that I have a nanao ms9 which people say they have issues with the h and v sync because they are both positive from the vga cable from the vga box.

It’s all going over my head.

[Sp33dFr34k]

I've never tried it myself but using only h sync or tying h and v together on my Naomi, the image kept scrolling vertically. I fixed this by making a loom to the JST connector directly on the MS9 chassis as that does take both h and v sync. Possibly you could try the same with your Dreamcast. If you would get a stable image from this, the low voltage issue remains which could be solved by a video amp. Have a read through this: http://forum.arcadecontrols.com/index.php?topic=92803.0

Personally, I didn't go this route and just bought a Jammacon made by TheLastBandit. Excellent piece of hardware and imo it's a very reasonable price. You can use it for all consoles that output RGB through SCART.

[yosai]

The vga box method says that once you have a vga box you can snip a ground wire to tell the D.C. to output at 15khz and then you just solder the r g b and sync from a vga wire to the jamma loom.

That 15KHz image is on a tri-sync monitor, and to run it on my low-res monitor I have a video amp that uses VGA in and plugs into a jamma fingerboard.

[FriedGoat]

That guy is using an amp, yes.
But the original guide he follows, here: http://arcadecontrols.com/arcade_consol ... #dc2arcade

doesn't mention an amp.

[Eddhorse]

I use the Jammacon for SNES, NES, XBOX, SATURN
Amazing piece of kit and you can add additional cards for different consoles.

https://forum.arcadeotaku.com/viewtopic ... t=jammacon

[FriedGoat]

Yeah, I am trying to avoid spending £100 for my dreamcast as I am stone broke.
I have already spoke to this guy though and in a few months will get one. However I wanted to see If i could get this done without spending so much.

I just cannot afford this hobby lol.

[FriedGoat]

So has anyone connected a dreamcast with scart or the VGA to a 15khz?
I've been trying today following this http://pc2jamma.mameworld.info/arc_dc1.html and pulling my damn hair out.

[FriedGoat]

Well, I think i've done it! We have picture!
Now to try and get the sound going, can I just solder the Left audio directly from the dreamcast AV wire to Speaker + on the jamma harness?

[FriedGoat]

Well, just for future reference if anyone comes across this thread, I connected the left and right audio and ground directly from the av cable to a stereo phono wire and connected it to a speaker that has a stereo jack input. Works like a charm temporarily until I put an audio amp into my Astro city.