I noticed a pressure "bump" at Y-end recently and have now confirmed that the ion pump mounted at BT port Y2-8 has been off for the past few days.  Today's troubleshooting including re-enabling the High Voltage output.  This resulted in power limited (700V @ 0.5 amp) energization (no ion pumping while inadequately energized).  Next, the HV cable was moved to he other HV channel with a similar result.  The controller-end of the HV cable was disconnected and a VOM used to measure the cable+pump resistance (2.5 Mohm, insufficient output voltage of the meter makes this test "almost" useless).  The HV cable was reconnected to the controller and the pump-end disconnected. In this configuration, the controller wouldn't output HV as the cable connector shield wasn't grounded and, it would seem, the cable isn't shorted.  

I noticed that the pump-end of the HV cable was noticeably "warm" after applying 700V at 0.5 amps.  This observation combined with the interpretation that the cable or pump-end connector aren't shorted indicates that the low resistance is internal to the pump itself.  Ahhh!  The fabled "whisker" theory.  

Monday, we will adapt a custom HV cable and use the "Big Gun" 10,000V 600 watt power supply to, hopefully, vaporize any troublesome whiskers that have formed between the electrodes.