I need help understanding the operation of the video processing circuits on a pacman upright arcade board

JimD53

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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.
 

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The VRA and SBC daughtercards were apparently made when NAMCO couldn't supply enough NVC284 and NVC285 custom chips for all the pacman boards that were being sold. They had the same crappy silver legs that midways masked roms of that ERA did, so they're renowned for losing corroded legs.

You can replace the paralyzed chip with any of the aftermarket replacement chips/boards.
 
Start by replacing the VRA socket -- it looks like a lot of the v counts are shorted / stuck / floating.
Already did. I went a different direction on replacing the socket based on my experience with arduino or raspberry pi computers. They use this sort of 0.100 pin header. I had to replace the pins on the daughter cards with longer ones. The female socket is the tallest I could find so that I had enough clearance above the nearby ICs. A word of caution, assemble all connectors before soldering. This helps make sure that they will be aligned properly.
 

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What is all that white junk on the IC pins? Hopefully not corrosion.
 
The VRA and SBC daughtercards were apparently made when NAMCO couldn't supply enough NVC284 and NVC285 custom chips for all the pacman boards that were being sold. They had the same crappy silver legs that midways masked roms of that ERA did, so they're renowned for losing corroded legs.

You can replace the paralyzed chip with any of the aftermarket replacement chips/boards.
Thanks for the help. You got me to the right area but I'll confess to an unforced rookie error. I had replaced the chips on the VRA with tested chips in good double wipe sockets and had plugged the 74LS257 in the socket incorrectly. I've been involved in digital electronics since the Z80s first came out so I should have known better.

At any rate, here is what I see now with the ABC diagnostic program. The screen changed to random characters with each press of 1P which starts the program. After three or four presses I see this screen of the video ram test.. Sometime I see all bits fail and sometime I see some pass as in this screen shot. What path does the data take from the Z80 to the 2114s and what is the return path when it reads the data back? Given I'm getting a report of video ram errors for 4K and 4N what is the best way to find my error in the address or data buses?

What I've done so far.

Tested all 2114 and 7489 ram.
Tested all chips on the board except the ram chips on row 1
using my multimeter verified continuity on all connection (about a week worth of fun)
checked all clock circuits
checked for activity on address and data buses

All that I probably missed something in my checking or broke something trying to fix the scrambled screens.
 

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Thanks for the help. You got me to the right area but I'll confess to an unforced rookie error. I had replaced the chips on the VRA with tested chips in good double wipe sockets and had plugged the 74LS257 in the socket incorrectly. I've been involved in digital electronics since the Z80s first came out so I should have known better.

At any rate, here is what I see now with the ABC diagnostic program. The screen changed to random characters with each press of 1P which starts the program. After three or four presses I see this screen of the video ram test.. Sometime I see all bits fail and sometime I see some pass as in this screen shot. What path does the data take from the Z80 to the 2114s and what is the return path when it reads the data back? Given I'm getting a report of video ram errors for 4K and 4N what is the best way to find my error in the address or data buses?

What I've done so far.

Tested all 2114 and 7489 ram.
Tested all chips on the board except the ram chips on row 1
using my multimeter verified continuity on all connection (about a week worth of fun)
checked all clock circuits
checked for activity on address and data buses

All that I probably missed something in my checking or broke something trying to fix the scrambled screens.
I made it out to the shop and with several tries I got this screen. I still see the video memory error but I also see in the text at the top of the screen that one or more switches are closed. Could a stuck switch goof up the video?
 

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What is all that white junk on the IC pins? Hopefully not corrosion.
No, all new from the board up. The white residue is left over flux after spraying the board with flux cleaner. I need to take a toothbrush to finish the cleaning.
 
No, all new from the board up. The white residue is left over flux after spraying the board with flux cleaner. I need to take a toothbrush to finish the cleaning.

Use Kester 245 no-clean solder, then clean any remaining flux off with a Qtip and acetone.

Or better yet, use Kester organic flux (aka water wash) solder when replacing that many sockets, then just rinse the board under hot water to remove the flux, then stick in front of a fan to dry. Saves tons of work and time.
 
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