Bryan is suggesting that you check the CRT filament for continuity. That's pins 1 & 14. They're on either side of the key. Set your multimeter or VOM for resistance and check those pins.
If you don't have a meter, you can hook up 6 volts AC or DC to those pins and check for a glow. Be very careful that you don't go much above 6 volts or you could blow the filament!
You can download a datasheet here:
http://www.tubedata.tubes.se/sheets/127/8/8BP4.pdf