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

No, in general the phosphors were fine, it was the cathode emission that failed (or in rare cases the heater burnt out). So replacing the gun would bring an old tube back to life. There were a couple of tricks that were tried to extend the life of failing CRTs. One was to over-run the heater somewhat. In a series-string heater TV, the CRT was almost always at the shassis end of the string, with one terminal connected to the chassis. A suitable resistor from a 'live' point on the mains dropping resistor to the other terminal of the CRT heater would do that. If there was a shunt resistor across the heater, removing it (or increasing the value) helped. If the CRT heater was fed froma low-voltage sourve (secondary of a power transformer or the flyback transoformer), there was often a series resistor you could short out. The other trick was to over-run the CRT filament on the bench (give it as much voltage as you dared -- 10V for a 6.3V heater was not uncommon) and apply a few hundred volts DC between the cathode and the first (control) grid. The indea was to have a strong electric field at the surface of the cathode which would tend to 'strip off' the top layer of the cathode, exposing fresh emissive material. Both tricks were a 'last resort' THer was the risk of burning out the heater, and in the case of the second trick of stripping the cathode totally (so no emission at all). And they tend to work better on older CRTs (modern ones have a much thinner emissive layer, or so I am told). None-the-less, I managed to use the second trick to get a bit more life out of a Mac+ CRT... -tony