Firstly, clean the screen if you've not done so already. You can laugh, but it caught me once (after spending a morning with a 'scope looking at a VT100's video amplifier...) If you've got a low emission, CRT there are a few tricks that might help. Firstly, if there's a 'g2' or 'screen' control on the PCB, turn it up a bit. It won;'t do any damage, and it might get a bit more brightness out of the CRT. Increasing the CRT heater voltage can help too, although you do run the risk of buring out the header and ruining the CRT. If there's a series inductor on the CRT base PCB, short it out (it's often enough). If there's a series resistor, drop the value a bit (shorting it out is probably going too far. A fw turns of wire round the flyback transformer core, connected in series with the heater, can provide a bit of a voltage increse too, but, again, be careful (and you are increasing the load on the horizontal output stage and flyback if you try this, which might lead to ehm failing). There used to be things called CRT boosters. Typivally, they worked by over-running the header (about 8-10V for a 6.3V heater), and then applying a high voltage between the cathode and first grid. The strong electric field at the surface of the cathode rips said surface off and exposes fresh emissibe material. It worked reasonably well on older CRTs, but then the manufacturers put less emissive material in in the first place so you couldn't really expose a new layer. It may work, it may totally ruin the CRT, In fact, what abvout replacing the CRT? Is there anything special about it? -tony

--- David Griffith <dgriffi at cs.csubak.edu> wrote: There are numerous ways of brightening old CRT's, all having to do with the filament. Some people "zap" the filament with a sudden jolt to "revive" it, others raise the operating filament voltage, and another trick is to run the tube's filament at a higher than normal voltage for several hours, to "cook" it. Basically, you're trying to get the filament to have a higher electron emission by stripping off the coating of old material. Cooking the tube overnight seems to work pretty well - just hook up a small AC transformer or DC suply of a volt or two more than the tube's filament (for a 6.3 volt filament, try 7 or 8 volts). You might need to build something to supply that voltage. Unhook the tube's neck connector, and connect the wires directly to the filament pins of the tube (aligator clips and tape or something), and run the filament at the higher voltage for a while (this is where a variable DC bench supply comes in handy). Be careful not to burn out the filament. Try it for a few hours, then see how it works. I've heard of people cooking tubes for a couple days (really weak ones) and had them come back well. Another thing I have found is that sometimes if a tube sits for a long time, it will be dim at first, then begin to "wake up" after just letting it run. You might try this first, just turn on the terminal, leave the contrast all the way down so it doesn't burn, and let it run all day. That might even be enough to improve emissions. This is typically something I see on really old TV's though, don't know if a fifteen year old terminal is old enough to have sat long enough to do that. But before you go to the tube brightening stage, I'd look at the terminal. There might be a pot you can adjust inside, or possibly even a bad component causing this - It seems as if I have seen a couple Wyse terminals with this type of failure - might not be the tube. Good luck. -Ian