Need help with Golden Axe PCB weird text problem

action76

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Got a very clean Golden Axe board that I have de-suicided. Boots and plays fine but has some scrambled background issues and some weird problem with text that I have not seen anywhere on anything I have looked up. I cannot even go into test mode to troubleshoot because I cannot read anything. Anybody have any idea where to look or what to do to try and clear up the garbled text so I can troubleshoot further?

Thanks
 

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Your E (hex 45) has become a D (hex 44), your U (hex 55) has become a T (hex 54),etc. So bit 0 for the tilemap layer is likely stuck low or disconnected and floating low.

A very quick look at schematics suggests you should check D0 on ram chips H15/H16 as well as the LS244 latches that feed them (H11/G11). Check for continuity first, then logic probe to see if anything stuck low on that path.
 
Thank you very much for your help and detailed description of what to do. Unfortunately, it is a little too high level for me to fully understand. Also I'm not the best schematic reader and the one I found online is not the best quality. So when you say check continuity, you mean between the ram chips and the LS244. If yes, which pins on the chips are supposed to be connected. I cannot tell from looking at this schematic. Sorry for the ignorance, but I'm trying to learn.
 
The schematic is a bit hard to read..

D0 is pin 9 on both H15 and H16. You can see it then goes to an LS244 - bit hard to read what pin - I think it's 9 there (then 12 then 7 then 14). It also goes to the chip on the right - again hard to read I think pin 5.

But if you look on the board hopefully you will see the traces and can see exactly what goes were! If you have a logic probe you would touch it to this line to see if it is pulsing or stuck low.

img009-X2.jpg
 
Thanks for the pic and explanation! Well it looks like pin 9 on H15 is pulsing but H16 is stuck low. What next? Replace the chip or would something else be causing it not to pulse?
 
Conceptually the CPU puts the text data in the RAM and the graphics chip reads it out. So the fact it is stuck low could equally mean it never got there. If you look up the LS244 datasheet you'll see data goes in one side and out the other. You've already verified the 'out' side is stuck low, so use the probe on the 'in' side to see if D0 pulses there. If it does then you know the problem is definitely 1 of the LS244, the RAM or the graphics custom. If it's stuck low on the input, then the problem must be upstream and probably not related to the RAM at all.

Before that though - verify we're looking at the right RAM chip :) Use your logic probe to short D1 to D0 on the RAM - does the text on screen change? Short D1 to D2 as well for a test. If text changes you're know we're in the right place.

Edit - looking at the schematics again D0 (VR0) is shared with a few other chips as well, so use the probe to see what pins 12 & 13 are doing at H12.
 
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