dodger741 wrote:
I have an HP 8568B spectrum analyzer which went through shipping
recently. The display unit no longer works and it appears that the
CRT section is the culprit. I checked the HV and signals and on the
surface they appear OK. I did find some glass chips in the case
which leads me to believe the tube is damaged/broken. Taking the CRT
out is a pain. I can measure the filament resistance at about 4 ohms.
If the tube is cracked or broken, wouldn't the filament burn-up with
an infinite resistance??

I would not think so. CRT filaments only need to glow red, unlike light
bulbs that glow white hot. So if it has lost its vacuum, it would still
function as a heater since it does not get hot enough to burn up. You
can try this with a regular light bulb at about 1/5 its rated voltage.
Even with the glass broken (and assuming breaking the glass does not
also break the filament), it will still glow red hot.