I've seen this before too; it appears to be a problem within the VFD glass and may well affect all units. The shadow is so faint it's not really worth bothering about though, it's almost invisible through most front panels (but much more visible with no panel when indoors).

It's *not* the wrong data being clocked to the display, I believe the phantom illumination might be a design issue in the grid drivers of the actual VFD glass. It's definitely not a software issue though, the pixels would be fully illuminated in that case and not a fraction of the brightness of otherwise identical columns.

The thing to remember is this: each pair of columns in the VF is connected in parallel, but only emits light if the grid above the phosphor is powered. The display is scanned by scanning adjacent grid pairs and clocking in the pattern you want to see on the 64 pixels below the grids. You draw column 0 and 127 at the same time, then 1 & 2, 3 & 4 and so on.

It almost seems like one of the right-hand grids is getting partially powered when a left-hand grid pair are powered - this would appear to be a slight flaw in the chips embedded in the display glass.

Hugo