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#1
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When you let a CRT down to air the cathode's emissive surface is wrecked. A new cathode is coated with (I think) a carbonate whch is converted to an oxide during pumping using an RF heater. I'm sure somebody will correct me if I'm wrong.
I don't think it would be feasible to transfer a cathode from one CRT to another entirely in vacuo. |
#2
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I believe you are correct, it is activated once all the vacuuming getting and baking is done.
__________________
Yes you can call me "Squirrel boy" |
#3
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Yes, that is correct, but if you are starting with a cathode taken from a new gun intended for a modern tube, then the cathode should still be unactivated, as the gun was never sealed into a tube. The activation of the cathode takes place while the tube is being pumped down, so that the liberated gases can be removed from the tube. The getter is fired once the tube is sealed off from the pump.
It seems highly unlikely that a modern cathode assembly would mechanically fit the older guns, though. The cathode/grid spacing is highly critical for proper cutoff, with tolerances measured in thousandths of an inch. |
#4
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Activating the cathode after pinch off won't poison the tube, I suspect they were activated during bake out to save processing time.
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Evolution... |
#5
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As I understand it, there are 2 distinct processes: "Breakdown" is done on the pump station and is achieved by ramping up the heater voltage to remove the binder used when the Barium Carbonate material is sprayed onto the Nickel cathode surface, as well as Carbon Monoxide from the Carbonate. Both heater voltage *and* RF induction heating are involved in this process. "Activation" , the second process is done after the tube is sealed off and the getter is flashed, and involves ramping up the heater voltage *and* drawing current from the cathode, to create stable emission sites on the very surface of the cathode. After current is drawn for some length of time, the heater voltage is brought down to "normal" voltage, and operated there for a while, a "slump test" may be performed to determine the stability of the cathode emission.
jr |
Audiokarma |
#6
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Not sure about the terminology. There are certainly 2 steps, one during pumping, the other afterwards.
During pumping the cothode is heated with the RF coil (didn't think the heater was needed too but I could be wrong) and the carbonate is converted to oxide. I would imagine this is best done before the tube is sealed off because it liberates some gases. After it's sealed off and cooled down then you draw current from the cathode to increase and stabilise the emission. For the EMI 6/6 that was rebuilt in 1986 I did this for my first and, so far, only time. I had no idea what sort of voltages and currents to use so I erred on the side of caution. The result was that the initial image on the rebuilt CRT was disappointing. After a couple of hours running in the set the picture improved hugely, to the best and brightest that I have ever seen from one of these CRTs. Cathodes also seem to get "tired" from long periods of non-use. Again a few hours running usually improves things |
#7
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Well here is one hard datapoint. A used-but-tests-good 10FP4 just sold on eBay for $227.75.
-Matthew |
#8
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One thing: If RACS has guns for the specific early 80's color CRTs like the M48AAW00X, 19VJTP22, and the 19VLUP22 used in color XY (and a few raster) arcade games could be a potential source of income for the project. I'd think these being "modern" should mean that they're able to be done with a high yield.
I've been searching for new or good M48AAW00X's for a long time -its limited use (high resolution) makes it difficult to replace. The 19VLUP22 is hard to sub because of its 100 degree deflection angle. |
#9
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Hello ColorFixer,
I've queried RACS about their having in stock new ready to mount guns for the 3 CRT references you quote in your message. Will post reply when i get one. I have also queried them about what equipment/glass/parts they may recycle soon - planned date given months ago was July 2013. Have requested detailed itemized list and prices. Best Regards jhalphen Paris/France |
#10
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Thanks Jerome.
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Audiokarma |
#11
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I think colorfixer has a great point - there may be a market for 'newer' CRTs as well as 'vintage' ones. For example, new CRTs for HP 8591 spectrum analyzers (and similar) are selling well at $195 each:
http://www.ebay.com/itm/NEW-CRT-HP-A...item20be07ace8 -Matthew |
#12
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I suspect that in the future the ultimate will be the tubes
for Tektronix 454 and 455 scopes, possibly the higher end 7000 series too. There really would be no way to fix one of these short of replacing just the cathode, the gun is so complex. And there is simply still no digital substitute for some uses for these. Doug McDonald |
#13
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Good point. Also, don't forget the 2445/2465 series. As the 'last and best' analog oscilloscope these things are still used professionally. In our lab alone (at MIT) we must have at least a half dozen.
-Matthew |
#14
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I thought VDC was still doing medical, military, and industrial CRT rebuilding. I thought they only got out of the non-profitable consumer CRT rebuilding.
. |
#15
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I am glad to see some fruitful and informative discussion going on here.
Matt |
Audiokarma |
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