CRT Testing - Low Filament Voltage

[bhegges]

I have a 21FBP22 CRT that I am trying to test with a Sencore CR70. Once connected I cannot get the filament voltage much above 3.5 volts. With the lights out I can see a very very dim glow but that is it. No shorts present. I was temped to let the CRT "cook" a bit but was worried that I might overheat the tester.

The tube is likely bad but out of curiosity what is pulling down the filament voltage? If the CRT lost its vacuum I would assume the filaments would have burnt up. The tester has worked on other tubes so I don't think that is the problem.

Thoughts?