Simpson 430 color bar generator

[old_tv_nut]

Nice picture - is that video in or RF?

One way the distortion may go: The modulator may not be able to go to 100% modulation and actual clipping, and may have residual carrier at the signal peaks. This could be worse at higher channels. Depends on the layout and stray coupling. Another thing, the unit may just be undermodulated so that it is just not trying to make over 100%. A modern wideband scope is able to display the modulated waveform at low VHF channels and could tell you how well behaved it is.