Galaga vid board help -lines on screen pic included

makamusement

New member
Joined
Jul 6, 2009
Messages
156
Reaction score
1
Location
New York
Galaga vid board help -lines on screen pic included

I have a Galaga Video Board with a strange issue. The game works and plays, but has these 2 lines about a half inch thick that run the width of the screen during the entire game. See attached picture.
Here is what I know for sure.
1. Not a monitor issue - plugged in a working boardset and no issues.
2. Not a CPU board problem, swapped the CPU with a know working vid board and no issues.
3. Not an issue with any of the custom chips. Swapped the custom chips from known working video board and same issue remains.
4. TTL chips all measure OK with Logic Probe when compared to the working video board.
5. Not an issue with the connector between the 2 boards - swapped this one into known working boardset and no issues.

Any help at all is appreciated. Thanks in advance
 

Attachments

  • GEDC3064.jpg
    GEDC3064.jpg
    92.5 KB · Views: 64
4. TTL chips all measure OK with Logic Probe when compared to the working video board.

You probed every pin of every TTL IC on the video board, on both bad & working video boards? Damn, that's a lot of probing.
 
Just because you may have poked every TTL pin and noticed some sort of activity does not mean it is working correctly. Without a logic analyzer that lets you see the signals in parallel with respect to one another, you can't tell if things are really working correctly. Timing is critical in these games, and you can't test that with a logic probe.

Since you said the game plays without problems, then you can probably assume the memory and processing parts of the circuit are working. Also, since the sprites appear to be displayed properly, the bars are probably on the tile layer. I would look in the output video circuits after the memory ICs. There are a few buffers and TTL logic that multiplex and select the pixel to display (sprites vs tiles), as well as the CPU access to the color RAM.

Anything in that part of the circuit could fail without affecting game play. If you had a bad RAM, ROM, CPU, or video timing then more than likely it would affect the game.
 
Bad resistor pack... bad gate on a chip.

Separate the boards and touch your hands/fingers on the bottom side of the video board until you see where it goes nuts on the screen when you touch it.
 
Back
Top Bottom