[Coco] Yellow tinge

[Carlos Camacho]

I have two CoCo 2 for sale on eBay and a customer pointed out one of them
has a yellow tinge running vertically on the left side. You can't see it on
the BASIC screen. You can see it with a white background screen like Canyon
Climber. I recall some video composite boards have this issue, but haven't
heard of it with regular RF out. It's not the monitor as my other CoCo for
sale doesn't have it.

So what is the yellow bar and how to remove it?

Thanks,

Carlos