Re: PLASMA NOISE IN THE KEYBOARD

This looks like a good place to use ferrites. If the keyboard has a
straight cable then either get some "donut" cores large enough that
the keyboard plug will go through the center and take at least one
turn around the core. There are also some split type cores that are
held together with a spring type clamp. I like the suggestion of
replacing the keyboard. They are cheap and vary greatly as to their
internal workings. I assume you are not using an extension cable.
Some of those are poorly shielded. Lastly I would consider opening
the keyboard up and finding the +5 and ground wires from the
computer. Solder in a 10 to 100MFD 16VDC small electrolytic between
the +5 and a close ground...keep the leads short. Good designs
normally have .1 MFD bypass caps close to each IC between the +5 and
ground.

Sounds like you have chased the noise grimlins around to the last
spot.