[jeyurkon]

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