burn-in and bleaching the 7WP4 projection CRT screen
 

bleaching the 7WP4 projection CRT screen

Sorry for the ugly compression -only small files allowed.

Pages from the 7WP4 datasheet about how the tube face darkens, and what to do for removing the darkening (burn-in) from the screen of this projection CRT.

The CRT is placed face-up in a cone shaped implosion shield and four 250W heat lamps are shined on the face for some time. The risk of implosion is due to possible uneven heating of the tube.

I wonder if anyone has tried this on other monochrome CRTs. Aluminized types of course.

Just thinking of the many old green or amber computer CRTs and old picture tubes and R/G/B projection CRTs that have good emission but slight burns.

What is the high voltage value for this tube? Is the bleaching affecting the glass or the phosphor (or both)? In direct view tubes, it is the phosphor that burns, but my understanding of projection CRTs (the old ones at least) was that the glass was browned by X-rays at very high ultor voltages.

About 75KV. Like an xray machine! Also, 2mA, which is quite hot.

Here are a couple pages. Did better this time on the size.

The bleaching was referred to as being the 'repair' or thermal heating process by which the darkening was removed.

What causes darkening in this tube? good question.

Can x-ray or electron beam heating-related darkening of glass be reversed by raising its temperature but not so high as to destroy the metallic backing or phosphor adherence to the tube face? The tube is air cooled by a blower.

Can heating of the P4 phosphor well above its normal operating temperature somehow 'reflow' it or realign the crystalline structure of it to remove a darkened scan pattern?

I sure don't know those answers or what causes the darkening. I guess I wanted to assume 'burn in' as we know it from being caused by stationary patterns on TV sets.

The skiatron, or dark trace tube, is erased by IR and heat. it's not a P4 family of phosphors, but P10, maybe potassium chloride, making a dark trace on white.
One of those is the 4AP10. There is much info about it in volume 22 of the MIT Radiation Laboratory series.

Makes me wonder if some 'regular' phosphors can be renewed by the method described for the 7WP4.

After reading this, my guess is that the glass browning is entirely due to X-rays and that the heating during use is irrelevant. I also doubt that 200 deg C heating of the faceplate during bleaching can do anything to the phosphor.