Found a Capehart

[Tubejunke]

My 1958 Sylvania Electronic Products Technical Manual lists the 16RP4 as REQUIRING a single field ion trap. Here is the answer to your problem as long as you don't have a 16RP4A. "A" stands for aluminized which CAN mean no ion trap is required. I think the gun type plays a role with the aluminized tube. The ion trap is sometimes referred to as a "beam bender" because if you notice most sets with a trap have a kind of crooked gun. Hence the need for a "beam bender". What could have happened is the tube was rebuilt off of the aluminized bell with the older type gun maybe accidentally installed. I know one thing for sure a CRT that needs a trap will show NO raster at all.

You could put an ion trap in proper position and rotate it 360 deg. and see if you get a raster. If you do then make sure you adjust for maximum brightness.

The funny thing is that my manual makes note of both aluminized and non-aluminized tubes, while saying nothing about one of them NOT requiring an ion trap. Hmmm???

The only other things that come to mind are a bad crt socket connection (is the heater lit?) or a bad brightness potentiometer. Helpful hints I hope......