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TI99 Video Problem, maybe VRAM - Help Please

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a 99er friend on Facebook has this issue on a TI-99/4A


so i checked screens on ninerpedia https://www.ninerpedia.org/wiki/Troubleshooting

to check if this issue is recognizable  but i can't see a matching...

Maybe it could be more of one chip broken to show this issue? or maybe another thing...

Any Idea and help please? :)






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This one is weird; normally when one of the VRAMs fails, the corresponding bit just gets stuck and garbles the entire screen. It could be the VRAM at U104 seeing the random ' " ' characters, otherwise I'd suspect the VDP.

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11 hours ago, GDMike said:

Looks like bad ram..anyone else chime in

TLDR; still points to what others have concluded, one bad VRAM chip, bit number 6.


This is fascinating.. only three spaces >20 are transformed into quotes >22, each time in column >09. (but not column >19)


Characters A and 1 have a bit set at VDP address >989 and >A09. Other letters in this sequence would be ! and Q, which we don't get to see.


Inside the TI logo chars, >809 is normally set. >849 is also inside the TI logo and NOT set normally or in the picture. >889 and >8c9 are not used.


Then Color bars.. they are all clear and background color, foreground transparent. Characters >60 to >D8 are the color bars. They must have the >02 bit set consistently in VDP memory in at least >B00 - >B07, from >B40 to >B47 and so on upwards through >E47. (pattern descriptor table. .)


VDP RAM above that first 4K is unused by the title screen.


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