If you are using Y-R,B you must be using the Component Output from your source into the 1800’s component inputs.
In addition, to get REFERENCE SYNC to the 1800…
Hook the Composite Video OUTPUT from your source,
into the REFERENCE Input on the 1800. It will sync to that just fine.
Make sure to turn the 75 ohm (Reference) termination ON for this setup.
If your source does not have a Composite Video output, LOOP the Y cable from the component output into the REFERENCE input jack on the 1800 and then send the looping (bridging) output (from the REFERENCE) into the Y input, as well.
Make sure to turn the 75 ohm (Reference) termination OFF for this setup.