Hopping Asteroids

Brick Top

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It's been a long road but I have almost brought an Asteroids machine back from the dead. I have been doing the old "two bad boards to make one good one" trick. First it was a bad 6502, then did some swapping of socketed chips, then I switched out RAMs to get rid of some errors at test, then swapping out the Y 191 counter chips to fix squished graphics.

Now I have a board that passes test and plays games normally...Except when things (rocks, saucers, bullets, ship) scroll up and down, they skip. It is not a smooth scrolling effect. Maybe jumping a half inch at a time, always in the same places on the screen.

Side to side, everything scrolls smooth as butter, it is just when traveling up and down that there is the jumpyness.

My logic leads me to believe that it is something in a circuit that has to do with "Y" but not "X" . Any ideas? I can post a video if my description is not clear.

Thanks guys!
 
I would starts looking at the LS191's - column 'E' I think on the board (depends on the revision).

Thanks, I already replaced those 3 LS191 chips with new ones from digikey. I had wacky garbled graphics before I did that. According to the schematic,There are a few other IC's that work along with them, I may just shotgun everything in that circuit until I can find what it is.
 
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