Have you tried
it without the KVM switchj, just temporarily, in case
that's messing about with the signal?
It's a mechanical KVM switch. Not much going on there.
Obviosuly I've not seen your setup, and you are probably right, but I
rememebr a rule I learnt the hard way many years ago : 'That which is
obviosuly correct, beyond all need of checking, is the problem'.
I am almost sure, thoguh that your problem is down to poor design of the
monitor. What it needs to do is sample the video signal in the back porch
[1] (the time between the end of the sync pulse and the start of the
actual video information) and take that to be the black level. Unless you
have schematics for the monitor and enjoy designing video circuits,
there's no real way to solve this.
[1] I am told that the schematic for one of the Tektronix video test
monitors has a cartoon of a back porch on the diagram for the
sample-control part of this circuit. And a $\phi$ in a prison cell for
the 'phase correction' part of the horizontal sync circuit.
-tony
