Apple Monitor IIc horizontal sync

[option8 <option8@gmail.com>]

I'm in the midst of putting together another of my Frankintoshes -
this time wedging a G4 Mini into the body of an Apple //c. One of the
finishing touches I'd like to add is video out to an original Apple
monitor. The model I have is a 9'' green monochrome like this one:
http://upload.wikimedia.org/wikipedia/commons/4/48/Apple_IIc_with_monitor.jpg

Using schematics from this site - http://www.geocities.ws/podernixie/htpc/cables-en.html#vgavideo
- I built an adapter which seems to work at 640x480@60Hz, with one
glaring problem.

As you can see from this photo - http://www.flickr.com/photos/charles_mangin/6926854065
- i'm having issues with the horizontal sync. The picture seems to be
clear, but shifted horizontally.

Does anyone here have a suggestion for how to adjust either the
monitor or the adapter I've built to connect the Mini's output to
monochrome RCA? The monitor has Vsync and Vcenter knobs but nothing
for horizontal. Maybe there's something internal i could look for?