Image is cutting off about 1cm from the right-hand side of the screen

[johng1uvy]

I am using GBS-control connected to a radio test set, Motorola R2600. That outputs CGA RGBHV 15KHZ
I am getting a good, clear image, but part of the image on the right had side is missing.
Have tried all resolutions and adjustments for movements and borders, but it still has missing section.
What I have found is the missing part is not as large when the GBS is set at its lowest resolution

Any help please

John