yeah diachi covered it. No way to refill the cadmium without a full tube overhaul. The GOOD thing is that these aren't QUITE as picky as argon lasers go (as far I know) when it comes to gas purity, pressure, etc etc.
As long as you're taking apart an existing head and not trying to build your own, I'm sure it can be done. For a quick and dirty fix, which may or may not work, one would probably break/melt off the Cd reservoir, add in more cadmium, melt/epoxy it back on. Find a good place on the tube to melt a fill stem onto, pump the tube down to about the right helium pressure and seal it off.
The one thing I'm not sure of is how to refill the helium reservoir... I think it has a bit more pressure than the rest of the tube. But, there is a section of tube on the helium reservoir which is covered in silicone banding, so there may actually be a little valve in there, so you can have more helium pressure in that part of the tube than anywhere else.
Then just turn the key and see what happens. The good thing, at least with liconix lasers, is they sense tube voltage and add more helium accordingly. If it's too low, it will keep heating the He reservoir until the He pressure is where it should be. Also, there are sensors for the actual Cd temp (versus the set temp) as well, so your Cd partial pressure will be exactly controlled.
So, it would be incredibly difficult, but worth trying; especially if you have a dead tube that is Cd depleted. Trying to build one from scratch, on the other hand, would be a pain in the ass... haha.
Picture of the tube, so you guys can get a better idea of what you'd be working with. Note: this is a liconix tube, which is vastly different from an omnichrome tube. Omni's use a heated cathode, like an argon, and basically have a 'straight' tube design. You can tell from this picture that liconix tubes are definitely not just a straight tube. There are things shooting off everywhere ;D
