Galaxian bad video

larrylemming

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I've been working away at a Galaxian cocktail for a while now. I finally got it to play (bad CPU and RAM were the culprit) but it's still having some issues.

Specifically, the video is a little bit funky. On first glance, it looks like I'm losing the least significant bit of the vertical count, but I know that I'm not because I checked it and it's fine. The starfield is fine, and oddly enough most of the invaders are fine. It's mainly in the characters where it's seen.



I replaced 2H-2L according to the Galaxian troubleshooting guide, but no such luck. ROMs both seem to check out fine as well (I recently replaced all the EPROMs.)

2 major questions:
1. Which way does the monitor scan (i.e. should I be looking for problems in the horizontal or the vertical control circuitry?)
2. Any suggestions on common culprits? The picture is stable, just not quite right.
 
Actually, I figured this out myself. Solution: Clip pin 6 to 5M. As far as I can make out, the tile map is generated via a vertical position plus some line-by-line shift. The lowest bit of the line-by-line shift was somehow getting mangled. By removing it, it all looks good again.

I started out by pulling the lowest bit of the vertical bit bus high or low. This affected the graphics that were already good, but didn't change the ones that were already bad, so the problem had to lie somewhere downstream of that. If the vertical bit bus is good, the only other factor (according to the schematic) is the horizontal position bus. Remove that from the equation, and you get a good picture.

Of course, this probably means that I lose a little granularity in the row-by-row shift, but hey. It looks good to me currently. More diagnostics to come.
 
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