Beaten by VT100 PSU.

[tony duell]

> When I used a variable load on the 12V both the amplitude and the period
> varied. More load less amplitude.

More load _reduces_ the ripple? That's very unusual.

> Added the AVO option. Then I have perfect picture. So the extra loading
> makes a difference.

But that mostly loads the +5V line.

I am wondering what on earth can generate a waveform of around 390Hz in that
supply. 

Have you checked that the +12V Startup supply is nice and smooth. I doubt that's
the problem, but it is worth eliminating it.

-tony