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Time for another bump.
Anybody tell me the theory of what is happening? I have the schematics. The tools I have are my DMM and a logic probe.
Wow. I usually look for regular patterns in these types of video problems in order to narrow down the problem ICs. But I am not familiar with this game and it is not drawing a lot of variety on the screen.
It looks like there is a problem every 8th vertical line (so the fourth bit, or 4V in some schematics).
YEah so do I. In fact I usually fill the video memory with a constant value (0x00 or 0xFF) and then trace the output through the sprite ROMs. Or find a NULL character in the character ROM and have the mem point to that. Then trace the output of the sprite ROM through the palette logic to figure out what line is pulled high/low.
Every 8th would mean 8V I think?
MSB to LSB is:
...... 8V, 4V, 2V, 1V
1V toggles every line, 2V every other, 4V every 4 and 8V every 8.
Interesting. Do you set the values by programming the ROMs? How do you "point the mem"?
It apperently depends on the game! Q*Bert schematics use H0-7 and V0-7. Galaga uses the binary counting method.
The HO-H4, V0-V7 outputs are driven by 74LS86 chips. I have checked these with my logic probe and I think they are all working properly. The LS86's are driven off the outputs of 4 74LS161 chips. I am have trouble understanding what the output's should be from these. The logic probe is pulsing at different frequency's on some but not on others. Or the difference in frequency on some is so small I can't tell the difference. The fequency difference's are highest and easily notable on the V4-V7 outputs.
Any help on how I can use the logic proble and/or DMM to troubleshoot the 161's would be great.
Murph
OK. Sounds like my tools are limiting me here. Based on the schematics, what do you think are the high likelihood areas? I could go shotgun and change a couple of chips. Likely candidates of the H0-H4 and V0-V7 chips to start with?
What would be a good tool to invest in for the future to help with situations like this?
Mike