Rejuvenation of the CRT in a 15" GE B&W TV, using a B&K 470 CRT tester/rejuvenator

I was recently given a late '70's 15" GE B&W TV that worked; but, it took a long time for the CRT to warm up enough to produce an acceptable picture. When a CRT starts to weaken, the cathode becomes contaminated; thus, causing reduced electron emission from the cathode. Over time, the condition will become worse until the CRT becomes too weak to produce an acceptable picture. In fact, I've seen some CRT's that were so weak that they'd barely light up in a dark room.
The only permanent fix for this problem is to replace the tired CRT with a good one. It may not always be practical to spend the high cost for a new CRT and, in some cases, a good tube may be impossible to find. That's where rejuvenation comes in.
During the early days of TV, companies started to make CRT testers that would not only test a tube for emission and shorts; but, would also rejuvenate a weak tube. The way this works is by applying a high voltage between the cathode and G1 electrodes of the CRT. The resulting arc between the two elements will blow off the contamination on the cathode; thus, exposing more "good" surface, which will increase electron emission. Unfortunately, these older rejuvenators didn't have any type of current limiting circuitry in rejuvenation mode and the result was often a cathode that was not only stripped of it's contamination; but, it's usable material. Often, the CRT would be worse after rejuvenation than before. My first CRT tester was a mid '60's era B&K model 465. It was a good tester for testing older color and B&W tubes; but, it's rejuvenation process was risky. I had luck with rejuvenating quite a few B&W tubes with it; but, I also blew up a lot of tubes with it (no matter how careful I was).
By the mid-to-late '70's, manufacturers started to offer more advanced CRT rejuvenators that offered current limiting circuitry on the rejuvenate function that resulted in less danger of ruining a CRT from rejuvenation. The Sencore CR70 and the B&K models 467, 470, 480, and 490 fall into this category. The above mentioned testers will also easily test more recent CRT's, as well as older tubes.
I've been using the newer B&K testers since the mid '90's and I've rejuvenated a boatload of newer and older CRT's, both color and B&W, with it. And, I've had very few tubes that were ruined by these newer testers.
Just remember that rejuvenation is a temporary fix and it's hard to determine how long the tube will hold up. I've rejuvenated tubes that died a day later and I've had tubes that lasted for several years. Some tubes will be totally worn out and no amount of rejuvenation will help them, no matter what tester is used.
If you plan to get into the hobby of restoring vintage TV's, I'd highly recommend getting a good CRT tester. When I get a vintage TV, the first thing I do is check the CRT to make sure it's good. There's no need to overhaul a chassis just to find out that the CRT is a dud and can't be saved.
When these testers were new, they cost several hundred dollars and some even crossed the thousand dollar line. Since the advent of flat panel HDTV's, the demand for mainstream CRT TV reapir has dropped off, and, as a result, used CRT testers can often be bought for well under $100. Check ham and antique radio swapmeets, local TV shops, and ebay.
Again, get a later model one from the late '70's or newer because these will more readily check the later CRT's and the rejuvenate function is more reliable. The older models are limited to what tubes they can easily check and the "brute force" method of rejuvenation can often cause more harm than good.