Q*Bert finally wore me down

Cmndr Brain

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This game finally wore me down to ask for help. The game plays fine but, as you can see from the pictures, the graphics are corrupt. Both, foreground and background. I've changed all the sockets. No words are legible. The foreground graphics "seem" to be whole, but its like there are invisible bands on the screen that when any of the foreground graphics pass, the image disapears. Thought it was a ram problem so all the ram is socketed and checked (no change). It's not a eprom issue or any of the socketed chips, I've verified the info and they all work in a known working board. It's not the jumper caps, or so I believe... they have all been replaced and I've verified the shorts across them. I thought it was the 74ls166's in row L? (there are 4 of them next to the foreground eproms). Nope, not them either, although they seem to be the only chips that give me different o-scope readings when compared to a known working board. Now, I'm lost and need suggestions.
 

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You Haven't mentioned the filter board, does your machine still have it? If so take it out and check for cold solder joint on he solder side, some funky shit can happen with these filter boards.
 
I'm using both a test rig and the original setup. The filter board is still in the original setup, but the connectors have been replaced. I have a known working board that works just fine in the original setup and the test rig. Great suggestion, but I don't think that's the problem.

Oh, and yes the monitor needs a cap kit, but that's a different issue.
 
So the video problems have not changed at all since you started working on this board?

It is definitely a PCB problem because you swapped out all socketed parts, CPU, RAM (3 or 4 types), EPROMs, what else?

And you did verify all the jumpers by ohming them out on the back side of the board?

Please measure the +5V at the 4 corners and the middle of the board.
 
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This game finally wore me down to ask for help. The game plays fine but, as you can see from the pictures, the graphics are corrupt. Both, foreground and background. I've changed all the sockets. No words are legible. The foreground graphics "seem" to be whole, but its like there are invisible bands on the screen that when any of the foreground graphics pass, the image disapears. Thought it was a ram problem so all the ram is socketed and checked (no change). It's not a eprom issue or any of the socketed chips, I've verified the info and they all work in a known working board. It's not the jumper caps, or so I believe... they have all been replaced and I've verified the shorts across them. I thought it was the 74ls166's in row L? (there are 4 of them next to the foreground eproms). Nope, not them either, although they seem to be the only chips that give me different o-scope readings when compared to a known working board. Now, I'm lost and need suggestions.

This has different readings than the working one so that is where I would look the 74ls166's, before them and after them.
 
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