Re: 15GP22 Rebuild Attempt in France - Soon!

[ohohyodafarted]

Hi Eric,

The entire design of this tube and in particular the Ultor Ring Assembly, is predicated on a number of design constraints.

I will attempt to take the issues one at a time.

First RCA needed a method of making the envelope in two pieces. A front half and a rear half, so as to facilitate the instalation of a third assembly inside the tube. That third assembly is the phosphor dot plate/shadow mask assembly.

The technique of fusing glass to metal was well understood. So it was decided to make a glass front face assembly fused to a metal ring, and a rear glass funnel and neck assembly fused to a metal ring. Then the phosphor dot plate/shadow mask assembly would be mounted to the flange on the rear assembly, and the front half and rear half would be brought together and the two ultor ring halves would be heilarced together to form a complete crt assembly.

The phosphor dot plate/shadow mask assembly mounts to the light guage rear half of the ultor ring inner flange assembly with 3 bolts.

The reason for the two light guage metal rings with the bulge on the outer circumference, is that when the tube is evacuated, great amounts of mechanical force are exerted which squeeze the front and rear halves of the tube together exerting great amounts of pressure on the two light guage metal rings. The rings need to flex under the pressure. The pressure needs to be distributed evenly around the entire surface where the two halves of the ultor ring meet.

The corrugated metal washer that is sandwiched between the front and rear rings, is designed to act as a cushion to evenly distribute the tons of force squeezing the two rings together. If the corrugated washer was not there, the front and rear metal rings would squeeze together, and in the places where the rings would touch all of the force would be exerted in just those spots. That would cause huge stresses and probable breakage of the glass to metal bond on the front and rear rings. Even if the glass to metal joints survived, there would be pressure points that would make the tube vulnerable to implosion if bumped or jared.

The reason for the heavy metal rings which are the bonding surface for the glass is thought to be as follows. The light guage metal would likely melt under the high temperatures needed to fuse glass to metal. To solve this problem, heavy guage metal rings were bonded to the light guage sheet metal rings. Probably a rolling fusion weld under great force and very high current. This is the joint which probably leaks.

I had long suspected that the leaks were occuring in the metal to metal fusion jonts between the heaavy and light guage metal rings. So when John was in Milwaukee a couple months ago we decided to cut a cross section of an ultor ring that we had been using to experiment on. We used a 3 inch high speed carborundum disc to slice a section of the ultor ring away. We then ground and polished the cross section to a high finish so we could closely examine the integrity of metal to metal joint.

AS you can see from the cross section photo of the ultor ring in John's post, the heavy guage ring is not bonded uniformly to the light guage sheet metal ring. There is a very evident partial gap between the two components. This was only a random cross section and we found deficiencies in the metal to metal joint. I strongly suspect that some areas are joined better and some are joined much more poorly.

It is this metal to metal fusion weld between of the heavy guage ring and the sheet metal ring that will need to be sealed. We can't be sure, but we do not think the glass to metal bond is where the leaks occur.

Glass to metal bonds were well perfected and being used in metal funnel crt's for many years before the 15GP22 was put into production.

There has been much discussion betewen John and myself about the usability of frit glass to try sealing the metal to metal fusion joint. And John and I have differing opinions on the use of frit glass to seal 15GP22 leaks.

I had a very long and interesting discussion with one of the elderly engineers who worked at Rauland picture tube. I also spoke to a fellow that works for one of the companies that make frit glass. These fellows both told me that frit glass will not work for the metal to metal leak we are trying to fix. The problem is that the coefficient of expansion of the frit glass has to be matched to the metal very precisely. And even if we were able to match the expansion coefficients properly, due to the fact that the sheet metal ultor ring flexes when the the tube is evacuated, the frit glass would crack due to the flexing.

Frit glass is very useful to bond two glass surfaces of a crt together because the glass all moves at the same rate. When you get dissimilar rates of expansion or physical stresses such as the ultor ring flexing, then frit glass will fail. It will either crack or debond from the surface. Even in glass to glass joints, if the frit glass in not perfectly matched to the rate of expansion of the glass parts, the joint will fail.

As to your query on leak detection, as was discussed in another post the spark method is not going to work on the metal ring as it is electrically conductive throughout. As to pressurizing, you are correct the leak is too small to find this way and may explode the tube because it is not designed to be pressureized, it is designed to be under vacuum.

The only practical method of leak detection would be with a helium leak detector. IN this method a strong vacuum is pulled on the crt by a machine that is designed to count Helium atoms in the vacuum stream. Helium is used because the atom is so small it can squeeze through the smallest of leaks. So while the tube is under vacuum via the HE leak detector, the operator introduces small amounts of HE from a tiny supply tube into the areas where the leak is suspected. Almost instantily the HE leak detector will detect the HE atoms in the vacuum stream and a meter on the machine will register a large number of atoms. As soon as the meter registers, you know you have found the leak.

All this leak detecting is in my opinion not of much use. Even if were to find the exact pont of the leak, and seal it, additional leaks could crop up in other spots. The solution is to seal the entire metal to metal joint all around the tube to make sure that the leak is fixed and to prevent any possibility of future leaks occuring.

I hope this has shed some light on the subject of why the tube was built the way it was and where we suspect the leaks are.

Bob