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HP CRT filament question

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??

Reply to
dodger741
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On the 2 occassions I've come across scopes that have been dropped and the CRT has failed then I've heard bits of glass pillars loose inside the tube if you slowly turn the whole case around every whichway. Any smallbits of external glass is likely to have come away from the socket area which you may get away with.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

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.

Reply to
Caesar Valenti

Two previous posters have it right!

Reply to
hrhofmann

On Tue, 27 May 2008 08:33:21 -0700 (PDT), dodger741 put finger to keyboard and composed:

If air has entered the tube, then you will hear arcing noises and the HV will probably shut down.

- Franc Zabkar

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Reply to
Franc Zabkar

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