Gday mate'

The cable should be fine, you just might have to re-pin the chassis end. I would try this as all your saying is pointing to the Vid amp..

Can you measure the voltage on the H sync line, measure on the input to the amp and the output to the chassis, Pin 13 of the vga connector is H-Sync.

Also if you have any old video games ( 15K games) or a 60 in one board or similar set to 15K, you could connect them up to the chassis to double check your leads and confirm your chassis is good..