View Full Version : 7jp4 crt and phosphors


timmy
11-21-2015, 07:13 PM
With all the 7jp4 tubes out there with good emissions but a dark picture due to bad phosphors but no way to get inside to fix such a simple problem, It's like so close but yet so far. So if we use a 7jp1 and the pic is green and we take clear red cellophane and now the pic is black & white, because I've done it already but it becomes dark, but is b&w. So if the green can be cancelled by adding a color that neutralizes green then why can't we figure out some kind of phosphorus clear type cellophane like the red to somehow restore some of the picture. Maybe some kind of cellophane that reacts to hi voltage and or radiation, of course on a small level at least what the tube would emminate for it to react to improve the brightness. So many of these tubes I'm told have good emissions but they are dark and unusable. So if this was at all feasible then maybe the beams that work on the inside surface of the tube would extend radiantly to the phosphor cellophane on the outside of the tube and brighten up the picture. Just a crazy thought that we can put our minds together like the people did many years earlier and came up with the idea of a crt and what it does, think about it, it's amazing. Where would one get the phosphor that makes a b&w picture ? :smoke:

Tom Albrecht
11-21-2015, 11:28 PM
Although it is easy to make white light green with a filter, it is not at all straightforward to make green light white with a filter. White light contains lots of wavelengths in a mixture, and the filter removes all but the green. But to fill in the missing wavelengths to make white out of green? Not straightforward at all.

I would not expect a red filter to make a good black and white picture out of light emitted from green phosphor. It must be really dark indeed.

The real solution for now is to simply recognize that there are far more TVs out there looking for a home than there are collectors to take them. If you're willing to buy a distressed set with a 7JP4 in it to harvest the picture tube out of it (and scrapping the set), you can get one for probably $50 - $100. The odds are that set will not be saved in the long run, so go ahead and get your picture tube out of it. You'll also get a bunch of useful tubes and some other good parts (such as extra HV coils). There are quite a few Motorola VT-71s out there looking for a home, as well as many of its cousins made by other brands.

In my experience, most 7JP4s are reasonably good, with only an occasional one that's simply too dark to be useable. The really bright ones are fairly rare, but that's life.

3KP4s are a bit more problematic, but even there if you're willing to keep your eyes open and buy one or two distressed TV-37s, you'll likely end up with a good tube. I lucked out when I bought a nice one a number of years ago. Original 3KP4 is nice and bright.

EdKozk2
11-22-2015, 12:06 AM
So I must be lucky I kept the 7JP4 I bought in 1974 for 50.00 dollars brand new at Richardson Electronics near Chicago. I've only used for two or three hours on a homebrew set I built in the early eighties.
Ed

cbenham
11-22-2015, 04:57 PM
With all the 7jp4 tubes out there with good emissions but a dark picture due to bad phosphors but no way to get inside to fix such a simple problem, It's like so close but yet so far. So if we use a 7jp1 and the pic is green and we take clear red cellophane and now the pic is black & white, because I've done it already but it becomes dark, but is b&w.

The filter color you need is purple. I have done what you are doing and made a 7JP1 look B&W but it was only viewable in a dimly lit room. Lighting gel for stage use is what you need to find, usually about $6 per 20X24 inch sheet. I used a
piece of Roscolux #359: Medium Violet. Hope this is if some help to you.