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!