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TI99 Video Problem, maybe VRAM - Help Please

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Hello, 

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? :)

 

 

66179716_2652236874839574_2241636319040110592_n.thumb.jpg.782bd04327cc7ec49d8b1a9c554aeffd.jpg66395733_2652236854839576_4151296933392023552_n.thumb.jpg.9b036641a24624bf94788ec8770f8118.jpg

 

 

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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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