pac-man screen filled with various colored characters

JimD53

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I've been gradually bringing pac-man back to life. I finally got the board to boot without constant reset. Some bad traces in a couple of the bus circuits and a short in the clock circuit at 3R & 3S. I finally have data on the screen, not the correct data but not a white screen. This screen occurs in test mode or run mode. The Z80 and all eproms, proms, and memory except (2A-2D) have been replaced. Same goes for both daughter cards. Many of the buffer ICs in the various bus circuits have also been replaced.

Somewhere I saw a document with screenshots of various failures but I can't find it now that I need it. Does anyone have such a document? Any suggestions on what I need to check that could be throwing these random characters on the screen?
 

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Search for lawnmower man PacMan repair on Google. I think it's now hosted on Rotheblog.com.

Also there is info with pictures on mikesarcade.com.


Edit to add: fixed a board like this a year ago:

 
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the CPU board caps are starting to go on these more frequently now. if you could measure +5V to the chips to rule out low voltage.
 
the CPU board caps are starting to go on these more frequently now. if you could measure +5V to the chips to rule out low voltage.
My bad. I replaced the caps and verified the voltages are good. The +5 is measuring 5.15.

I also checked all of the clock circuits and all of the V & H clock lines looked good at that time.
 
Got to the shop and swapped the 74LS157s at 4E and 3L. Since I don't have the equipment to debug a bus I decided to swap out all of the chips that interface directly with one of the busses shown in the schematic. I now have a full screen of a white background, no text or any other characters. Same when running in test or run mode.

I double checked and the /reset pin 26 of the Z80 is staying high so the watchdog timer is happy and not popping up to trigger a reset. I'm not sure what it is doing but evidentially it is running code..

Does the white background screen suggest a cause for not displaying either the test screen or normal pac-man start up? If not I'll close out this thread and start a new one if needed on how do tell if the Z80 is actively executing pac-man code.

For some light reading I printed out the pac-man section from the lawnmower man web site.
 
I'm finally back from the white screen problem to a different set of characters on the screen. The white screen seems to be a result of the monitor screen control set way too high and a failure in the horizontal clock circuit at 3R. I finally traced the problem to a solder bridge downstream of a chip being fed by 16H. I now have a new screen with random letters. The video signal appears to be working, now if I can just get the game to get displayed.

Same screen in both run and test. No sound just a loud hum now that I fixed the short which fed the sound chips.
 

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