When I first turned it on without a cartridge, it would run the tests and always show a yellow screen. I also got a yellow screen when trying to run games. The only game I could get to boot and go to the title screen was King of Monsters 2, but the graphics would be corrupted at the neo-geo logo, or rarely it would look ok until you started a game.
This is apparently a vram issue. I checked the traces to and from the vram and it looked fine. I reflowed the solder on the vram chips and it didn't really have much of an affect.
I don't have a logic analyzer to tell if there is anything wrong with the chips themselves, so I took a chance and just replaced them with this part from digikey: http://www.digikey.com/product-detail/e ... ND/2008361
I tested it out and now all the games will start up fine. I was quite excited this thing was fixed. Then I noticed after playing for a bit that the graphics would become corrupted again in a similar way. So I was hoping the cart was just not seated properly and reset the system. Now there was corruption at the neo-geo logo. Argh. The system is way more functional now, but I still get graphical corruptions. I can even play games for hours without it screwing up. It seems that as soon as I power cycle the system it will get flakey again. I even sometimes get the yellow screen from the self tests without a cart plugged in.
I found the best way to know its going to work is to power cycle it a couple of times and make sure I get the blue screen (all tests passed) every time. Then when I put a game in, and power cycle it, it will generally work fine.
One other thing I noticed is that when the graphics got corrupted in Magician Lord the character also fell through the world. I'm pretty sure VRAM wouldn't be used for collision detection, so I'm wondering if the issue is now with some other memory (work ram?).
Anyone have any ideas what to look at next?
Here is the picture of the main board. Appears to be first revision with the daughter board.
Here is a Video of the corruption:
http://youtu.be/xe_7OmMULnw
