Hmm! I appreciate the tip. I'll confess it didn't occur to me to try to force more current through the ion pump. The supply I was using was limited to about 300 uA, but I took your advice and tried a beefier one, set to 3 kV. As I increased the current limit from 0 towards 5 mA, the ion-pump current fell back to ~25 uA once I reached 1000 uA.
At the rate I was adjusting the current-limit control on the Glassman supply, this occured about 10 seconds after power-up. It seems stable now at 25 uA, several minutes later. I didn't leave the hot-wire ionizer energized for more than 10-15 seconds, all told. I'll let the ion pump run for a few hours, and then try the hot-wire ionizer again. Should I bother with the other supplies (Cs oven heater, EM, mass spec), or is it reasonable to recondition the tube using only the ion pump and hot-wire ionizer terminals? Keep in mind that I don't have a 5062C mainframe, just the Cs tube and some bench supplies. I'm essentially recreating Tom's experiment from http://www.leapsecond.com/pages/cspeak/ but without benefit of an actual clock mainframe. So I don't have a filament shutdown/recycle controller in the picture. Obviously that would be needed if I were to actually build a clock around the tube, but for now, I just want to see if the tube is functional at all, and determine its figure of merit. -- john, KE5FX > John, > > What you describe with the ion pump curent is normal if the tube has been > powerd off for a long time. > > Th mainframe normally would turn the oven and ionizer filaments off when > the ion pump current pegged. > > After minutes (can be quite a few minutes) the ion pump current will drop > as it pumps the surge of outgassing from the heated filaments. > > The cycle repeats and can take a couple days on a stubborn tube. > > You bypassed that circuit so I'm not sure how much gas you introduced. > > If after a couple days the current is still pegged try connecting an > external high voltage supply of around +3000VDC that can provide at least > 5ma. (turn unit off) > > Let it run overnight and if the current comes down reconnect the internal > power supply and turn the mainframe back on. > > You probably will see it peg again and the see the oven shut down (it > happens fast!), just leave it on and it should start cycling and > eventually the filaments will reach the oven set points and have > outgassed enough so that the protective circuit will not trip. > > Then you can try to see if the rest of the tube has any life. (you may > have to reduce the oven set resistor to 130 ohms as well as reducing its > companion overtemp resistor. This is the value the Navy uses to get the > last bit of life out of the tube, don't do it if you can get the rated > beam current at the original value!) > > If you email me I can give you my evening phone number if you need some > more info. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.