[Jeffhs]

The TV may be working just fine. The program you are watching may be in 16:9 format, which will show a picture with black bars at either side of the screen on any set designed for the old 4:3 aspect ratio. In this case, no amount of adjusting the width control will fill the screen. Another possibilty could be the horizontal output tube is so weak that, even with the width set at maximum, the picture will not fill the screen horizontally. The Zenith porthole TVs are late '40s-early fifties vintage, so it wouldn't surprise me if the tube (if original) is very weak, especially if the set's former owner(s) watched the TV a lot. There may be other problems, such as defective capacitors, but I'd check the tube first.