Repairing devices

Hi Pat, I'm assuming that your laser is helium-neon laser. If so, then if it's 23 years old it almost certainly has what they refer to as a soft seal between the electrodes and glass tube. The problem with the soft sealed lasers is that the helium atoms are small enough that they slowly leak out of the seal. When they do, the mixture ratio changes and the ignition and operating voltage goes up until the laser will no longer operates. That's what it sounds like is happening with your laser. Your's sounds like it's it border line in that the PSU firing voltage can make it fire but the operating voltage isn't high enough to keep it operating. Further you can't just use a resistor as a dummy load. The laser tube is similar to neon and floresent lights in that it has a negative resistance. That is it has a certain amount of resistance until it fires and then the resistance decreases dramaticly. You have to use a ballast with such devices or else the current will increase drasticly and almost instantly burn out the device. The ignition voltage will be on the order of 20,000 volts but the operating voltage will be roughly 12,000 to 14,000 volts. Be careful using a scope around these things. It probably isn't made to handle nearly the kind of voltages involved in the laser. You need a good high voltage probe for checking these things. You can use one made for TV repair. They're fairly easy to find and are reasonably priced. If you send me the brand and model tube I may be able to tell you if it's a soft sealed tube and other details. Joe At 04:38 PM 10/30/02 -0500, you wrote: