Partial screen display on Apple IIc monitor

I have an Apple IIc monitor that only displays 3/4 of the screen.
The right 1/3 of the display fades.
This monitor was like this when I acquired it about 5 years.
No amount of adjustment helps apart from turning up the brightness.
Further, I don't have the circuit diagram.
Please see the attached photo of the screen.
Does anyone have any idea of where to look for the fault, or what the
fault might be? I suspect the horizontal deflection circuitry but would
appreciate advice before powering up the soldering iron.