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??

dodger741 wrote in message
...
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??


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
http://home.graffiti.net/diverse:graffiti.net/

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.

On May 27, 10:45*am, Caesar Valenti wrote:
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.

Two previous posters have it right!

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

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??

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

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.