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#1
10-31-2009, 04:21 PM
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12LP4 post mortem
I decided to try to find the shorts in a 12LP4 gun I got back from Scotty after a rebuild. I planned on trying to rebuild the cathode for the fun of it, but scrapped that plan for reasons I'll explain.
The photo of the gun as received is in the "Is it possible to know the speed of an electron" thread. The first photo shows the base end after removing the remaining glass. The glass on the left has nickel on it that was evaporated from the cathode. You can see where the leads made shadows. The nickel, even though transparent, has a resistancy of 10Kohm per square. This is one likely source of the shorts. If notice, the glass where the leads pass through have a metallic looking surface that is even darker than the areas where the nickel was deposited. They probably used a reducing flame when attaching the neck cylinder to the feed through. If done to excess this would reduce oxides in the glass leaving some metal and pure silicon. I don't think this was a leakage path, but it makes you wonder. The next photo shows the cathode disassembled. They parts are laid out in order. I can now see the difficulty in rebuilding the gun in the 15GP22. The retainer for the cathode is spot welded in. Even after you remove the welds it is difficult to remove without damage because of the tight fit. You can see some of the nickel evaporated on to the ceramic. The cathode is a very thin nickel cylinder with one end closed. It was probably electroformed. It is supported by a ceramic disk. The closed end has the oxide emitter on its surface. The heater is placed in the open end. The next photo shows the bottom side of the cathode assembly. It has a very heavy evaporated deposit of nickel. This is another likely candidate for shorting. Nickel has a vapor pressure of 10**-8 Torr at 927C. Even at 1072C it's down at 10**-6 Torr. They must have had a brightener on this CRT for some time to cause this much evaporation. The last photo shows the remains of the heater. I wasn't able to remove it without breaking it. This is the reason I'm not doing the test rebuild. It would have been fun. I would have tested it in a vacuum chamber so it would have been nothing more than an experiment. Making an aluminum oxide coated tungsten filament is more than I care to try. There are companies that make them to order, but they probably have a huge minimum order. It looks pretty bizarre too. Like they just stuffed it in rather than make a coil. The exposed end did look like a coil. Maybe if they did use a brightener it overheated it enough to cause it to move around. For the cathode I was going to mix in a ball mill 13% Calcium Carbonate, 31% Strontium Carbonate, and 56% Barium Carbonate along with a temporary binder. Then paint this on to the cleaned end of the cathode tube. Then reassemble and try the activation and stabilization procedure. Oh well, at least I learned something about the shorts. John 
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#2
10-31-2009, 05:13 PM
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Wow! What happened to the heater...all that I have ever seen are a "double helix looking" design. Did it just fall apart when you pulled it out or was it really deformed during processing somehow?
(please pardon the crummy scan) jr
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#3
10-31-2009, 05:19 PM
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John
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#4
10-31-2009, 05:56 PM
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I have seen a few that have developed H-K shorts where the heater has "welded" itself to the inside of the cathode nickle, and have been very difficult to remove without distorting/breaking... did this crt have a H-K short? Most of the heaters that I have seen after years of service look pretty much like the picture that I posted above, although they may "darken" somewhat over the life of the tube.
jr
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#5
10-31-2009, 06:22 PM
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The 10MP4 in my Sylvania is orginal. The striking thing I notice about it is that's its heater is a very dull cherry red. Not nearly the amost white that I see in other CRTs. I wonder if that's contributed to its longevity. It still performs very well. John
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#6
11-01-2009, 08:56 PM
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The gun in my dud 10BP4 in my Bendix tv has an interesting look.
The part of the gun closest to the filament is black. The other part in front is a bluish yellow. Is this from shorts, or, burn out. ? Filament still lights. Bill Cahill
__________________
"Tubes are those little glass things that light up orange unless there is a short.. Then they light up all pretty colors..." Please join my forum. http://www.tuberadioforum.com/
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#7
11-02-2009, 01:32 PM
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Bill...
I have posted a picture of the gun in a 10FP4 that is in a GE 10T4 set that I am working on. The Grid cup and G2 show plenty of yellow to blue color... does your gun resemble this? This tube is not gassy and is a strong performer. I have always thought that the colors that I see here were developed during pumping due to the RF heating of the gun before it was sealed off. I believe that in those days, gun parts were NOT hydrogen fired before processing. But this is only a guess. jr
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#8
11-02-2009, 05:08 PM
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Interesting that 10fp4 is still running strong. I tend to find that when a cathode/g1 assembly looks toasty like that, the tube has high miles and tests weak. I took some pics of a couple of 21" crts guns to illustrate this in this thread:
http://www.videokarma.org/showthread.php?t=246342 I just drug out a pair of 21" roundie crts, and the one with darker cathodes tested weaker than the one with less toasted elements.
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#9
11-02-2009, 08:17 PM
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I've done a lot of evaporations using boats made of various metals in vacuums as bad as 10**-5 Torr and have never noticed anything like this. Maybe when they activate the cathode and convert the carbonates to oxides the excess oxygen that is released isn't pumped away quickly enough. It would require a lot of material to do that though. But now that I see your photo I think my explanation was wrong. It certainly still looks like oxidation, but did they silver solder the leads to the gun assembly rather than spot weld them? Doing that in air would account for the coloration. What do you think? John
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#10
11-02-2009, 09:16 PM
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How about during cathode "breakdown" on the pumps, when the binder is baked out?... I have seen manifold pressure rise almost to 5 scale during that part of the process, and assume that near the cathode , the presssure would be much worse. Quote:
What is the melting point of silver solder, these parts usually become red during RF... 700-750 degrees or so, perhaps. Just for grins, I rechecked the tube with my trusty old B&K 400. It was in the middle of the green "good" scale, after about a minute of warm up... no "hot shot" or "rejuvenate" was used. jr
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#11
11-03-2009, 12:23 AM
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It's that shiny spot that made me think it might be soldered. The melting point of BT alloy, which is a eutectic, is 780C. Other alloys are higher.
There must be a clue in the fact that the end nearest the base is darkest and gradually get lighter as you move away from it. I wouldn't expect that temperature distribution from induction heating or activating the cathode. Maybe another possibility. I forget what you call the glass disk feed-through portion. It's basically a disk until after they mount the gun. Then it's attached to a tube that will later form the neck. Or perhaps done in one step. When they do that joint that end of the gun will get hot. It could oxidize during that step, depending on how they do it. If they join it to the end of a tube then it would see some flame and get hot. If they shrink a tube down around it and then remove the excess, then it would stay fairly cool. Does that make any sense? John
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#12
11-03-2009, 01:58 PM
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I am now starting to wonder if these early guns perhaps did NOT use stainless steel for gun elements. jr
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