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#31
02-15-2010, 06:52 PM
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Speaking of 7"crts... A 7JP4 has no ion trap, is not aluminized, was processed under primitive late 40s processing conditions, and yet, I have never seen an ion burn on one.
 I may be very lucky... Or perhaps there is a better explanation for bonus points?...  jr 
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#33
02-15-2010, 07:08 PM
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Remember being a kid and watching the little white dot that would hang around for a few minutes after the 'family' set was turned off? This probably depends on your age. Anyhow, I think many years of that leaves a small burn in the center of the faceplate that is otherwise harmless to the tube. I have seen many examples of ion burned tubes in old repair manuals. It seems like most were a larger circle, but of particular interest, now that I look back, is my memory of an example that was an X burned into the phosphor. I wonder how ions that supposedly can't hit the screen can do the damage I have seen, and read the heavily stressed warnings on.... I see the logic in the 'beam bender' theory, but I think there is a little more to it than that. I guess I should break out the books......
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#34
02-15-2010, 07:32 PM
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I also remember seeing pictures of CRTs with huge ugly "X" burns (rectangular tubes) and have wondered if perhaps these tubes were gassy enough to exhibit glow discharge, in which case the ion trap would do no good because the ions are not generated by the beam striking gas molecules, but are everywhere in the tube?
jr
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#36
02-15-2010, 08:44 PM
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#37
02-15-2010, 10:07 PM
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I have a 10BP4 that came out of my porthole; the screen has dark blotches without any real pattern; I'll have to dig it out someday and take a photo. That tube is entirely used up, with basically zero emission left.
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Bryan
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#38
02-16-2010, 06:04 AM
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The reason there were more ions in early non aluminized tubes was from atmospheric atomic bomb testing. The newer aluminized tubes keep the ions out of the tube the same way aluminum foil wrapped around a technician's head protects his brain from continuing education.
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#39
02-16-2010, 07:22 AM
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The picture is of a 5" pre-war British set that was magnetically deflected. You can clearly see how large the ion burn circle is. This set ran with an anode voltage under 4KV. Darryl Last edited by tubesrule; 02-17-2010 at 07:00 AM.
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#40
02-16-2010, 07:23 AM
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#41
02-16-2010, 11:14 PM
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As far as HV effect... I tried to do a literature search, I found some information but I have to jump through too many hoops to access the articles from home. So, I think that what I am going to say is accurate, but also incomplete as an answer. I suspect that the major ions species are nitrogen and hydrogen since they are not so easy to getter out. There may be some CO2, which the beam would dissociate. I did a damage simulation using SRIM for 5 and 10 KeV for hydrogen ions and nitrogen ions on Zinc Sulfide. The types of damage that they would cause are lattice dislocations, vacancies and displacements. Zinc sulfide would get broken into the metal zinc and elemental sulphur. Oxygen would also make zinc oxide and volatile sulfates. None of these would scintillate. Average grains sizes for ZnS are between 1-10 microns. You'll notice that the damage is from a few hundredths of a micron to a few tenths of a micron in depth. Small compared to the grain size. The emission of light by excited ZnS and the effect of dopants is complicated. ZnS is a semiconductor and much of the light is from recombination of electrons and holes. Impurities, such as iron, not only have an overlapping absorption band, but cause deep wells that can cause a non-radiative recombination of the electrons and holes (no light...) If you look at the damage it seems to be on the surface of the ZnS grains. The metal zinc and elemental sulphur would be randomly distributed in that surface and would easily capture light before it managed to escape the crystal. They, and sulfates from oxygen could also cause non-radiative recombinations. What I'm leading up to is that I don't think there's a big difference between 5KV CRTs and 10KV CRTs. At least not more than a factor of 2 in aging rates. The first two plots are 5 & 10KeV nitrogen on ZnS and the next two are 5 & 10 KeV hydrogen on ZnS. Oxygen would look similar to nitrogen. The plots show the damage per ion per angstrom of travel versus depth. The peak damage along a track is about the same for 5KV and 10KV. The biggest difference is the depth of the damage. There is a much bigger difference between hydrogen and nitrogen. Hydrogen causes damage deeper into the grain but nitrogen causes much more total damage. If the explanations I gave for the quenching or capture of the photon emission is correct, then the difference in depth of the damage isn't going to have a big effect. It's just the total amount of damage that matters and it appears that 5KV might last about twice as long as a 10KV CRT assuming that it has the same amount of residual gas to start with. But, all this hand-waving might be nothing more than hand-waving. John
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#42
02-16-2010, 11:25 PM
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Ion Trap Magnets
Since this thread contain some discussion of ion trap magnets I thought I'd mention this.
The RCA Sept 15, 1949 datasheet for the 12LP4 says that the main pole pieces of the magnet should be longitudinally opposite the internal pole pieces of the gun. Also, the orientation should be such that the north pole is adjacent to pin 6 and the south pole to pin 12. I set the ion trap magnet that way when I installed the 12LP4 that Scotty rebuilt with a rebuilt gun. It required almost no re-adjustment of the ion trap. It was very close to the proper position to begin with. This seems like a way to minimize any damage that might occur by having a badly adjusted magnet when you do an initial adjustment. John
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#43
02-17-2010, 05:50 AM
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I had forgotten about that memory until this post. Must be my earliest recollection:-)
__________________
CW 1950 Zenith Porthole - "Lincoln"
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#44
02-18-2010, 02:36 PM
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Greetings from FixitLand!
The 17LP4 CRTs in the Motorola 17F13 combo set I had some 35 years ago, and in the Zenith K1846R I have now, both have (had) brownish, roundish patches at screen center. Is this not the ion burn we're discussing? Take care, -- J. E. Knox 'The Victor Freak'
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#45
02-18-2010, 08:55 PM
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Linear Mode
