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- Jan 19, 2024
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I've been working a pacman that was in pretty bad shape and finally got it to display characters on the screen using the abc diagnostic board. First of all I'd like to thank those of you that weighed in on my earlier problems. I've been reading all I can find on fixing pacman boards. Mikes arcade site and the lawnmower man sites didn't have anything that looked like what I see. I'm stuck.
Attached are pictures of what I get when I run the diagnostic program. It changes when I press the 1 player button so I know it is executing code. My main question is what do the three parts of the video circuits do when it comes to displaying text.
1. A path from the Z80 thru 74LS367s at 6R and 6S that feed the 2114 video ram at 4K and 4N that sends (receives?) data thru a 74LS245 at 4H.
2. A path thru the 7489 ram at 3F and 3H or through the 74LS273 at 4D.
3. A path thru the 74LS161s at 1E and 2E to the MB8125 memory at 1A to 1D.
Here is what I currently see on my display. There are top to bottom repeats of text in sections that are easier to see in the first picture. There is another curiosity I spotted . Notice SWITCHES shows up repeated twice on the each of the first two lines of each screen. Also on both screens I see SWITCHES spelled backward but vertically with the column repeated 6 times.
I'm thinking that I have a row/col addressing problem but I can't figure out if the data is going into the 2114 video ram wrong or being mishandled when it goes through path 2 and/or 3 over to the display.
What path does the character data follow to place it a the proper row/col position on the screen buffer?
Has anyone seen this sort of failure?
What do I look for? I have access to a logic probe, oscilloscope, and 8 channel logic analyzer. I have also tested all chips on the board except the Z80 and the MB8125 memory. Replaced any that tested bad.
Attached are pictures of what I get when I run the diagnostic program. It changes when I press the 1 player button so I know it is executing code. My main question is what do the three parts of the video circuits do when it comes to displaying text.
1. A path from the Z80 thru 74LS367s at 6R and 6S that feed the 2114 video ram at 4K and 4N that sends (receives?) data thru a 74LS245 at 4H.
2. A path thru the 7489 ram at 3F and 3H or through the 74LS273 at 4D.
3. A path thru the 74LS161s at 1E and 2E to the MB8125 memory at 1A to 1D.
Here is what I currently see on my display. There are top to bottom repeats of text in sections that are easier to see in the first picture. There is another curiosity I spotted . Notice SWITCHES shows up repeated twice on the each of the first two lines of each screen. Also on both screens I see SWITCHES spelled backward but vertically with the column repeated 6 times.
I'm thinking that I have a row/col addressing problem but I can't figure out if the data is going into the 2114 video ram wrong or being mishandled when it goes through path 2 and/or 3 over to the display.
What path does the character data follow to place it a the proper row/col position on the screen buffer?
Has anyone seen this sort of failure?
What do I look for? I have access to a logic probe, oscilloscope, and 8 channel logic analyzer. I have also tested all chips on the board except the Z80 and the MB8125 memory. Replaced any that tested bad.


