Today's Mystery Guest (RCA CTC-4)

[dtvmcdonald]

I'm going to make a stab at your problem based of my CT-100 experience:

Bad, very bad, IF alignment and AGC setting. The symptom is that black
magenta bar. Of course, a wrong type diode might do it.
Try viewing 50% or 25% saturated color bars, they won't show as much
problem with the magenta bar.

In any case, I would do really careful, really full IF and RF alignment next.