RCA "Deluxe" Portable KCS-111D

[Eric H]

These were inexpensive sets back in the day, some overscan was not uncommon and most did not have a way to adjust the horizontal width.

How wide is it? I don't mean to insult, but you are using a 4:3 source for the video?

Low HV will generally make the raster larger in all directions, the vertical can be adjusted to compensate but not the Horizontal.

Do you have a Variac? if so try cranking the line voltage up above normal and see if it improves.