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weirdocollector

Strange video problem

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HI!

 

After a long time, yesterday I took my two TI994a out of the closet and while one of them functioned perfectly, the second showed a strange video behaviour.

 

When first turned on, the image is perfect and both the stripes of colours, the TI logo and the text appear as expected.

 

After a few seconds the image starts to degrade as the logo and the text begin to fade away pixel by pixel, leaving just the two coloured stripes with the wrong colours (see attached photos).

 

If I live the computer off for a minute this behaviour repeats with the image being perfect at the start and corrupting after a while.

 

Turning off and on the machine without waiting brings back the two wrongly coloured stripes.

 

Consulting this troubleshooting guide (https://www.ninerpedia.org/index.php?title=Troubleshooting) is seems that the problem may lay in the third VRAM chip as the corrupted screen depicted for Mask 11101111 corresponds to the one displayed by my TI994a.

 

What really baffles me is the fact that the video corruption does not manifest immediately when turning on the machine but appears slowly pixel by pixel and on the text and TI logo only.

 

Is that a sign that the real culprit may be the VDP and that the VRAM chip is not to be blamed ?

 

Have some of you experimented the same feat ?

 

Thanks a lot for any help! 

20210529_172850.jpg

20210529_172927.jpg

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What happens if you try to go into basic?  Do new characters appear on the screen at all, or does it stay blank?

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Whether the colour bars are affected or not just depends on which particular memory bit is failing. Personally I'd start by replacing the VRAM - looks like the chip is failing as it gets warm. While you've got the console open you can clean up the VDP and apply some fresh heatsink goop.

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3 minutes ago, Hans23 said:

What happens if you try to go into basic?  Do new characters appear on the screen at all, or does it stay blank?

If I press '1' when being in the blank screen with just the two coloured stripes I'm sent to a blank screen.

 

When I do the same while the characters are disappearing from the start screen I can see the text of the BASIC screen fading away in the same way of the one on the start screen leaving me with the same blank screen.

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2 minutes ago, Stuart said:

Whether the colour bars are affected or not just depends on which particular memory bit is failing. Personally I'd start by replacing the VRAM - looks like the chip is failing as it gets warm. While you've got the console open you can clean up the VDP and apply some fresh heatsink goop.

Thanks for the advice.

 

I'll try first to piggyback a 4164 to the one pointed out in the troubleshooting guide to see if the problem persist.

 

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Note: if that bad RAM chip is getting really hot, it may also warm the piggy-back chip to the point that it fails too. . .

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