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Gyruss - No Moving Sprites


SNESNESCUBE64

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First time that I've ever worked on a Gyruss. I have to say, the fact that it had any custom ICs at all kind of intimidated me a little bit. I am not the best when it comes to video issues, but is something I am working on getting better. But anyway, this was a pretty straightforward repair. So the problem at hand was that there were no moving sprites at all. Text would appear, but nothing that moved.

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This is great because it really helps narrow down the circuit. After doing a bit of research online, I found out that there was actually a test rom, which ended up making the repair even easier. What was nice about this, is that it was only adding one additional ROM in an unpopulated socket at J14. After burning the ROM and populating it, it showed that RAM 17C, all the between 11A and 14A, and the ROM at 19E was bad.

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This is a bit suspicious, because that implies that nothing is working at all with the sprite engine, you would think something would be working. Looking at the schematic, all the data gets sent from the main Z80 CPU to a Konami 501 custom IC, then through a buffer to the 17C RAM and beyond. Here's the problem, I don't have an easy way to verify the 501 is working other than checking it's signals. Everything looked fine as far as I can tell with the outputs of that, so I shifted focus to buffer at 13E (a 74ls245). Looking at the signals on my oscilloscope, I could see that it was getting all the signals but a few of the outputs were stuck low. That was awfully suspicious as it really should be toggling.

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After socketing and replacing 13E, the moving sprites were all back!  Great to know that it was the issue tying everything up. I've never played Gyruss before, it was really fun and I wouldn't mind owning one of these cabinets in the future.

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