21-CT-55 Acquisition

[John Marinello]

Anyone know how a color tube is aluminized? Doesn't the shadow mask get in the way?

[eberts]

The shadow mask is inserted and removed each time one of the color phospers is placed on the glass panel.
After the phospers, a black matrix material is applied, next the glass panel is placed on an aluminizer machine, a table with a rubber gasket, the interior of the panel is brought down to a vacuum in about 90 seconds, then a small pellet of aluminum is vaporized by an electric heating coil.
The superheated aluminum vapor "condenses" on the glass panel.
Vacuum pump is closed, air is let in the table, glass panel is removed, placed on conveyer belt, next.
The same shadow mask has to be kept with the same glass panel, they used to travel down the line together.
In the 70's, many operations of assembly were done by hand in a production line in batches of 7000 each.
Phillips had a very modern almost totally automatic assembly line to make 32" tubes in Europe. Robots and equipment run by computers, they must have spent close to a billion dollars on machinery. The plant closed last year.

[bgadow]

The whole phosphor application process has to take remarkable precision-I have often marveled at it. Would like to know more about how it is done.

[ohohyodafarted]

Interesting stuff from you Eberts. Thanks you!.

In answer to bgadow...

The phosphor aplicaton process is a photo optical process. Each of the color phosphors is applied one color at a time. First one color of phosphor is applied to the glass front of the tube. Then the shadow mask is set in place. The phosphor is then subjected to a beam of light that is focused at the same angle of the corresponding electron gun for that color. Where ever the light penetrates through the holes in the mask, the phosphor, which is photosensitive, addheres to the glass. The mask is removed and the unexposed phosphor is rinsed off, leaving only dots of the color phosphor behind.

Then the next color phosphor is applied over the entire inside of the faceplate of the crt. The mask is again installed. But this time the light source is moved to corespond with the position of the electron gun for this color of phosphor. Because the light is comming from a different angle, it shines through the holes in the mask and strikes the phosphor on the faceplate in a different spot, then it did in the first exposure. The mask is removed again and the unexposed phosphors are washed off. Now there are 2 colors of dots next to each other.

The inside of the faceplate is now coated with the third phosphor color and the mask is installed again. The light source is moved to the position of the angle of the third electron gun and the last color of phosphor is exposed. The mask is removed again and the unexposed phosphor is again washed off and now (if everything was done properly) you have triads of phosphor dots all over the inside of the faceplate.

Then the tube gets aluminized as discribed by Eberts.



AFter the aluminizing the mask is installed for the last time and the front faceplate is mated with the glass funnel and neck assembly using a glass FRIT that when heated, fuses the front and rear into one unit.

AFter that the electron gun assembly is installed and the tube is run throiugh the evacuation oven at between 650 and 700 degrees F.

There may be some technical aspects that Eberts may wish to elaborate on but this is basically how the phosphor dots are applied.

BTW: the process above was patented by CBS Hytron Division, and Sarnoff had to buy the rights to use this process. It must have realy frosted him good that CBS had a better way of doing things. The first tube to use the application of the phosphors directly onto the inside of the faceplate was the 19vp22 built by CBS and used in the set on John Folsoms avitar. Prior to the developement of the photo process above, RCA did the manufacture of the 15gp22 crt using a silk screen process which required the making of several negatives to expose silk screens to silk screen the phosphors onto the flat glass phosphor plate that was suspended inside the glass envelope. It was very inefficient and expensive. It is rumored that RCA lost lots of money on each tube made.

Bob G.

[old_tv_nut]

Like any good technology (internal combustion, rockets, computers), the basics are simple, but there are lots of little things to color CRT design to make them practical and that improve performance and yield.

A few I can think of:

Look at the face of early tubes and you see the phosphor dots touching and covering the entire face. The dots are actually bigger than the holes in the shadow mask, so the beam landing (purity) can wander a bit without the beam falling partly off a dot. This is called positive guard band. This gives some tolerance for purity adjustment, but if the phosphor isn't uniform across the dot width, it's efficiency may change with purity adjustment, resulting in white field purity variations even when the individual primary colors are pure.
Later, black matrix surround was developed, with negative guard band. In this case, the phosphor dots are smaller than the holes in the shadow mask and do not touch each other. Now, the beam landing has some tolerance over which the whole phosphor dot is still evenly illuminated, so the white field variations go away. Plus, of course, the screen reflectance is reduced, for better contrast.
Shadow mask mounting techniques were refined over the years, with temperature-compensated mounts, so when the mask heats up and expands, it is moved closer to the screen. Invar masks were developed, which have less expansion.
The Trinitron introduced tensioned wires for the mask, so that the expansion had no effect unless it became so severe as to relieve the tension and cause sagging. Flat Tension Mask (FTM) tubes had a thin sheet shadow mask tensioned in all directions for a similar effect.

Gradient masks were developed with bigger holes in the center, gradually tapering to smaller near the edges. This allowed higher average brightness and/or more purity tolerance, since the eye is not very sensitive to gradual and smooth variations. However, in some tubes it was rather extreme, with the center nearly twice as bright as the edges, and people could see and complain about it. The spacing of the phosphor triads could also be larger towards the edge for more purity tolerance - this was more likely to be used in in-line-gun tubes, I think.

Some tubes used red pigment on the red phosphor, blue on the blue phosphor, etc., to absorb more stray light and increase the contrast.

In-line gun tubes made purity independent of beam landing in the vertical direction. This allowed the development of self-converging deflection yokes and elimination of all the active convergence circuits. As described in one of the posts above, some tubes were developed with internal magnetizable convergence poles, and they were permanently adjusted at the factory by automatic methods.

The final mating of a tube and yoke and their adjustment was sometimes called "YAMing" for "Yoke and Magnets".

[bgadow]

Fascinating. Thank you, gentlemen!