pac-man scrambled text

JimD53

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I've been using the twobit diagnostic tool successfully to get the colors straightened out and at that time the text was correct, but I've regressed a little. Now, the text on the screens are mostly scrambled. Some letters appear to be correct but most are wacked out. I still had problems with sprites so I spent some time resoldering questionable pins and finishing my inspection of chips on rows 1-4. I had already done the rest earlier where I found some very tiny solder bridges that caused a constant reset and a white screen. Immediately prior to this problem I had removed the chips at 3F & 3H to verify the traces back to their source using my multimeter. I did find a metal leg from an old IC socket I replaced possibly shorting the leads on one of them it was loose but came away when I pulled the chips/ I also replaced the 74ls158s at 3A and 3E.

The first line should read TWO-BIT SCORE. The second line reads ABC DIAGNOSTICS. Notice the first part of DIAGNOSTICS is almost readable.

All eproms and proms are new as are the sockets for them and the Z80. Same goes for the 284 & 285 daughter cards and their sockets.. Has anyone run across this sort of failure? Any ideas on what area of the schematic I need to investigate would be greatly appreciated. I've taken this pac-man from a dead monitor and board. I'd like to finally push this project across the finish line.
. IMG_5240.jpg
 
Pin 23 on 5E is stuck low...

Could just be a bad connection in the socket or an issue with 4D.
yep, pin 23 of 5E is low, now to figure out why. How did you figure that out based on the information I had in my post? I want to add that to my meager knowledge of pac-man.
 
That was it. I used some braid to wick the solder away from pins 10 (gnd) and pin 9 of 4D and the short to ground went away. I didn't observe it by eye but that was the only way pin 9 (CA8) could short to ground.
 
Still on a scrambled text problem. The above fix got the screen text correct. I developed a more severe version, now the dots in the individual letters are all over the screen. Nothing is ledgible but I can see changes when I operate thee abc diagnostic program. Horizontal and vertical sync signals are correct. What is the path that text follows to get displayed on the screen? I'm guessing I have another solder bridge or another IC that has gone bad. An idea where to look would be greatly appreciated.


IMG_5258.JPG
 
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