RCA XL100 portable with a dark screen

[Electronic M]

Get the chassis number (a lot of totally different chassis wore the XL-100 badge) it should begin in CTC- , use it to get the sam's photofact, then measure the voltages on the CRT and compare those to the listed voltages in sam's and check back.

There are a lot of things it could be. knowing for sure which voltages are wrong will point you down the right path and reveal any other issues.

Given the tube brightens with the room there is probably an auto-brightness sensor that senses room light and compensates for it. To confirm put the set in a dark room and shine a bright flashlight all over the case (but not the CRT it's self), if the screen gets bright then there is a sensor. The sensor or it's circuit could be bad, the CRT could be bad, the HV could be weak, etc. Hard data will reveal the truth.