IBM CGA Flickering Pixels Issue

[Great Hierophant]

I have a weird issue with one of my CGA cards. For some graphics, some of the pixels will "shimmer" in an odd way. I took a pair of videos to illustrate :

https://www.youtube.com/watch?v=QKFc1Vnzfbc

https://www.youtube.com/watch?v=FmQsPYQkli4

This is an "old" IBM CGA card, and it is installed in an IBM PC w/640KB. It is more visible with a composite monitor than an RGB monitor, but it appears with both types of display. Anyone have any ideas of what may be causing it and how it can be fixed? It seems to be centered in between dithered patterns, and maybe only to the red/yellow (intense or not) combination. This does not happen with my "new" IBM CGA card.

 

[Hugo Holden]

Hi,
This could be tricky to resolve. If you could get the faulty CGA card on an extender card and set up a plain screen graphic, say just a box of color or white, and the pixel defects were visible on a simple stable display, you could set up a dual channel scope, with the delay timebase function to see the video signal (the composite would do) at the point on one scanning line where the errors were occurring say on channel one. Then with the other scope probe channel 2 you could hunt around on the pcb with the aid of the schematic to try to find a signal transition corresponding to that error signal and it could, possibly, lead you to the faulty gates or IC causing this error. For example it might be due to a signal with multiple rather than a single transition on an IC output. And you could compare that with the other working card. But it might be difficult to find the cause.So it is a good thing you have a spare card.

 

[Great Hierophant]

I tried the card in a different system and could not replicate the flickering pixels issue, even with the same games. Therefore the problem is a system issue, not a card issue.

 

[Trixter]

That's troubling. I wonder if you put the card back into the same system, but a different slot, would you still see the problem...

I have a CGA card that will "blink" the leftmost+topmost pixel of a 40x25 text character if the character to the left of it is actually blinking. It is very subtle, but annoying.

 

[Great Hierophant]

I tried adjusting the trimmer capacitor on my 5150 and was able to influence the shimmering quite a bit. I could get it to the point where it was barely noticeable. Given some "waviness" in vertical lines, I would suggest that this board's filtering is not what it should be and that its tantalum caps need to be replaced.