RGB to NTSC encoder question

arfink · Post by **arfink** » Mon Nov 05, 2007 7:34 pm

Just wondering if any of you arcade people know this:

I have a jamma to TV converter that goes from RGB to NTSC. BTW, it's a homemade supergun. (made by me, of course) I only use my Neo Geo with it, so I only need one voltage, 5 volts. I have been using a power supply from an old AT computer case to run the thing, which was working fine, but I wanted to use a smaller power supply. So, I went to my local Axe Man surplus store and found a power supply that had the 5 volts at 3 amps, which was plenty of power, but when I hooked up the new power supply my encoder went all crazy. After about a minute the colors would start to screw up, eventually fading until everything turns a mucky green color, and then the image would collapse and start rolling crazily.

Has anyone heard of this happening before? My guess is that since my new power supply does not have a true ground that might be the problem. After all, the AT one worked fine. I'll give the old AT power supply another run when I get home this weekend and see if that fixes it.

edit: I think it need the ground because the IC that produces the NTSC signal needs to have a steady ground as a baseline, and if there is any noise (non-isolated power supply anyone?) then the SYNC would mess up, producing this bad behavior.

BTW, if I can get the power supply fixed I'll write a nice how-to for my project. (Neo Geo MVS console) After all, the Neo-Geo.com forum people won't even let you see the insides of their consolized neo's, (disgusting fanboyism) let's bust this one wide open!

Krepticor · Post by **Krepticor** » Sat Nov 10, 2007 1:57 pm

best thing I've seen out there is the ability to use one chip and create a RGB-> VGA converter (actually I plan to do this myself)

All you need is to get a LM 1881 IC and follow

this

and this (PS the composite in would be the sync in for RGB and the sync out would be horizontal out for VGA)

good luck and god speed!
~Krepticor

PS learn Japanese

arfink · Post by **arfink** » Mon Nov 12, 2007 5:48 pm

Well, my RGB to NTSC converter uses this same chip, if I recall. It has some caps to filter the signal, some resistors to make the color levels just right, and a crystal for the timing, but that's about it. It's the one that's sold by Neo Bitz, I think it's the Neo Bitz-S converter.

Krepticor · Post by **Krepticor** » Tue Nov 13, 2007 9:05 pm

well whatever works for you the VGA cable idea is high quality and for the cost on one chip *cough* free samples *cough* and a few components that add up to like $5 on Mouser, I thought it was very cost effective and good *enough*

arfink · Post by **arfink** » Fri Nov 16, 2007 10:32 am

Well, I guess the final thing is that the arcade boards don't use VGA output. It's actually at a CGA frequency. AKA, Standard frequency, it's like 15kHtz or something like that, nothing close to VGA.

michael93304 · Post by **michael93304** » Fri Jul 25, 2008 9:26 pm

Arfink you might want to check the voltage that it is 5 volts. If it is under 5 volts that could be the problem. I designed my own rgb converter and added audio to it. http://www.arcadeplaying.com/rgbntsc/index.php I am also selling them!

arfink · Post by **arfink** » Sun Aug 31, 2008 9:54 pm

Yay! I finally figured out what made it mess up- flux. Yup. Flux. I happened to just smear a bunch on there one time while soldering and forgot to clean it off. Turns out flux is somewhat conductive, just enough to make the sync stop working. A little rubbing alcohol cleaned it all up, works good as new! Now to finish my stereo mod, and then a complete writeup will follow.

michael93304 · Post by **michael93304** » Mon Sep 01, 2008 4:37 pm

Flux I didn't know that it was conductive! Thanks for letting us all know what happen.

arfink · Post by **arfink** » Mon Sep 01, 2008 5:55 pm

Yeah, some flux is more conductive than others. The stuff I use is a liquid gel flux- not the best for electronics, it's what I had around though. It's flows really nice, but it's also more conductive. If you use enough flux it is conductive- not much, but enough that circuits which are susceptible to interference and signal induction get messed up by it. Like RGB to NTSC converters, it would seem.

arfink · Post by **arfink** » Mon Sep 22, 2008 2:30 pm

One other thing I discovered- your power supply has a lot to do with the picture quality- a noisy power supply=noisy picture. Also if it's +5 volts is over 5 volts by even the slightest bit you can wind up overdriving the RGB lines, causing bleeding colors and bad sync problems. The solution to overdriving is to use some trim potentiometers to put all the levels back where they belong.