https://www.febo.com/pipermail/time-nuts/2017-April/thread.html
I spoke to Corby on the phone a few days ago about our first HP5061B that locks fine but has ion current on the order of 76 μA. It has not gone down significantly in two months of pump operation. He mentioned that the electrodes in the pump are made from titanium and that sharp whiskers can form on the electrodes from metal migration. They apparently cause corona and keep the ion current high indefinitely. According to Corby, the vacuum in the tube may be fine and the leakage current make it appear otherwise. We performed an experiment using our second HP5061B that we suspect of being out of cesium. When we first got it, we had about 10 μA of ion current and within a day it went down to nearly zero. Today we jumpered R12 on the cesium oven board to raise the oven temperature. We previously had checked all waveforms for normal operation on the board. This includes measuring cold resistance of the cesium heater at 2.6 Ω and hot wire ionizer of 0.1 Ω. Power to each was close to the nominal 2.6 Watts and 4 Watts respectively. We bumped oven heater voltage up to 11 Volts with the short on R12. This could have put up to 48 Watts into the oven heater unless its resistance went up significantly. After a couple of minutes the oven 150° C overtemp circuit shut down the switching regulator. We saw no increase of beam current even though normal oven temperature is 85° according to the tube data plate. We let the tube cool down and repeated the experiment several times. We had turned the beam current adjust all the way up to -2,880 V. On our good instrument we can get 20 μA beam current with only -1,700 V or so out of the -2,500 V supply. We therefore concluded that the beam tube was hopeless and decided on the risky experiment. We removed the +3,500 V supply on the suspected bad tube from the ion pump and connected the -2,500 V supply to the pump. We left the -2,500 supply on the electron multiplier as well. We saw no drop in its -2,880 Voltage. We would easily have seen 200 M Ω worth of leakage on the ion pump. Therefore the ion pump will work with either polarity of voltage. We have decided to take the risk of reversing the diodes on the +3,500 V supply on our good instrument and watch the ion current. We hope that the reverse polarity will burn out the whiskers or other leakage caused by long application of positive voltage. We have devised a test that will show up to 1,000 M Ω of any resistive leakage on the tube before we apply reverse voltage to it. πθ°μΩω±√·Γλ WB0KVV _______________________________________________ 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.