HI Andy--

1.  The heating current is 5mA, the emission sits on top of that.  It is current limited--so the bad batch of filaments that went out a while ago looked like they might have been too thick and carried too much current--so the over all current limit was hit before you got good emission. If the leads are (close to) touching in the source, then the current bleeds thru that way.  Probably not a problem for you  Also I remember hearing about a bad batch of source heaters--they would leave a deposit on the insulators--so a small leak to ground at voltage. 

2.  Our set up has the HV showing in Volts--and it goes from 0 to 3000 mV.  So it should go all the way to 0 mV.  You can even check for problem with voltages at say 1000 mV?  Everything should float on the hV, which, at least according to the readbacks, bounces around a little.  It may also float a little away from your ground--depending on what you're using for ground--but that shouldn't be much.

good luck


On 1/12/2016 10:38 AM, Andrew Schauer wrote:
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Greetings list,

Two questions:

1) Does anyone know what the current range THROUGH the filament is supposed to be? I am not referring to the emission (~ 1.0 mA). We are having a complete loss of signal and right now it looks like the current through the filament is not varying as a function of the potentiometer (physical potentiometer on the face of the instrument). When I vary the potentiometer I see no variation for most of the travel and the a sudden drop to zero volts (across a resistor that escapes me at this moment). 

2) For the Isodat software 2.x on windows xp, if I vary the HV [steps], from 0 to 3000, what should I expect in terms of real high voltage variation? For example, if I measure voltage between (yes using appropriate high voltage probes and caution) ground and "col".

To call out two of you that may be listening (Peter and Christie), we talked at the clumped conference about this loss of signal issue. The source connections are all sound (I vented it and checked each connection which were all very solid). I also measured no connection between any feedthrough and another (apart from the 0.2 ohms for the filament). I also observe expected magnet strength when I hold a metal object near the magnet during hysteresis. 

Thank you.