Galaxian sprite issue

philmurr

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Donor 4 years: 2022-2025
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I've just about admitted defeat on a Galaxian board I'm working on. It has a sprite issue where in the attract mode, the aliens appear on the wrong row (see first photo). In the game the attackers sometimes appear in the right place, sometimes on the wrong row. It's all repeatable showing the same fault every time, and not intermittent.

I wrote some code to display the sprites on screen, starting at the top then working their way downwards - initially 1 on the screen, then 2, 3...up to all 8. It works fine when 1-7 are on there, once 8 is drawn all 8 sprites show constantly at the bottom of the screen (I've tried to show this in the photos the best I can - one shows 3 sprites falling from top to bottom, then 6 sprites. The picture with all 8 is trying to show that they don't move top to bottom but are all pinned to the bottom of the screen regardless of the position they're meant to be at).

Does anyone have any idea what could be causing this, and why it only happens when sprite 8 is brought onto the screen? Attack, object and video RAM are ok, as are graphic ROMs, 74163 counters and all the simple logic feeding the attack RAM. Everything else on screen displays correctly, just the sprites are wrong.
 

Attachments

  • Attract.JPG
    Attract.JPG
    54.5 KB · Views: 34
  • 3 sprites.JPG
    3 sprites.JPG
    42.9 KB · Views: 31
  • 6 sprites.JPG
    6 sprites.JPG
    40.2 KB · Views: 24
  • 8 sprites.JPG
    8 sprites.JPG
    57 KB · Views: 21
So after a bit more studying of the schematics, there's a 377 D-type at 6P that clocks through various data including sprite data from the object RAM to the attack RAM. On a whim I decided to piggy-back another 377 onto it - it shouldn't have had any major impact if the original was working, but it showed corruption at the bottom of the screen suggesting all wasn't well.

Desoldered it and put it into my Chinese chip tester - which showed it to be good...

Soldered a socket onto the main board, then fitted (just to check) a new 377, fired the game up and it works (I know the copyright has changed, I'd swapped out the graphics ROMs as it's an original Midway board!)

I'm not really interested in finding out why the original shows as ok, I'm guessing output level is wrong or it doesn't work when worked under load (it takes in a 6MHz clock so it's worked pretty hard). Main thing is it works.
 

Attachments

  • Working attract.JPG
    Working attract.JPG
    72.8 KB · Views: 18
  • Working demo.JPG
    Working demo.JPG
    71.3 KB · Views: 19
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