cleaning CRT burn (was: an ADM-3A cam in tod...ay)

AFAIK, there is no way to 'clean' burn off a CRT. The burn you see is discoloration and deterioration of the phosphors themselves, and you're not going to "bring them back" by exposing them to the sunlight. I could imagine that you could theoretically discolor the rest of the tube to look similar though. If there was sun this time of year, I'd set a burnt out tube outside and cover one half with cardboard, see if the sun is capable of discoloring or changing the appearance of the phosphor at all. As far as a dim tube, you can kinda brighten a weak tube by boosting the filament voltage a bit. Don't go too high, or you'll burn out the filament. CRT boosters for TV sets were common in the 50's and 60's. They worked, but not for all that long. The only solution then is a rebuilt or replacement tube. Mold behind the CRT face is pretty common on old TV sets. In particular, the old round 21" color tubes develop "cataracts" caused by the PVA compound located between the safety glass and the tube face deteriorating and molding. To clean this, on these tubes anyway, you remove the faceplate. To do this without cracking it, remove the tube and set it neck down in a container that will hold it (bucket or somesuch). Heat the face of the tube with a heat gun. Heat evenly and try to get bubbles to form in the PVA compound. Once you've gotten bubbles to form, you want to work toward the edge of the tube, and try to free the safety glass from the rest of the tube, lever gently with a popsicle stick or something. This works at least on the TV tubes that have a safety _glass_. I think sometimes the face on terminals is plastic. Might want to stick with low heat. Once the glass is off, you should be able to peel away the PVA coating some chemical cleaner like paint stripper. This works on the TV tubes, I have never seen it done on a terminal tube, but if they were made the same way I'd say it is worth a shot. Maybe start with a hair dryer and see if you can get the PVA to bubble or soften. I know the Zenith TV tubes had a different sort of PVA, and those didn't work with this method. I think people were cutting the faceplates off with nichrome wire. When picture tubes are rebuilt, the guns are cut off and new ones are attached. For color tubes, the phosphors are not replaced, and only tubes without screen burn are rebuilt at all. For black and white tubes, it is possible to replace the phosphors, and this was common back in the 50's. In general though, a big cause of lack of brightness is simply a deteriorated filament/cathode. Early black and white tubes (pre aluminized screen) had a phosphor that discolored much more easily, to the point that the stray electron emission from the gun would eat a mark in the center of the tube. These tubes used ion traps to prevent this, and when rebuilt, new phosphors were the norm. For the vintage computer collector though, replacing picture tubes is generally done with used tubes from something else. A lot of monochrome tubes are pretty much interchangeable. Save junk terminals with decent tubes, they might come in handy. Also, don't overlook TV's - the picture tubes in 12" black and white TV's make a pretty good replacement for many computers, I've used a TV tube in a TRS-80 Model III. Also, one time when I was bored, I transplanted the tube from an IBM 5151 display into a 12" Zenith black and white TV set. It worked fine, although the long persistance phosphor and WordPerfect screen burn made TV unusual. The things to look out for in swapping tubes is the physical mounting, the neck size, and base. Typically, if the base matches up, the yoke fits and you can bolt it in there, it should work. Modifying mountings is OK and necessary sometimes, but NEVER CUT THE RIMBAND OFF A TUBE. That rimband is sometimes part of the mounting ears, and it is there as a critical structural component. It typically looks like a metal pallet strap. If removed, the implosion protection for the tube will be compromized, and the tube may very well implode/explode. I've heard horror stories about this. -Ian