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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
Posted to sci.electronics.repair
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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?? |
#2
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HP CRT filament question
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/ |
#3
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HP CRT filament question
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. |
#4
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HP CRT filament question
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! |
#5
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HP CRT filament question
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. |
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