[time-nuts] Fwd: HP5061B Ion Current
Donald E. Pauly
trojancowboy at gmail.com
Tue Mar 21 22:49:15 EDT 2017
You bring up an interesting point. As the cesium is used up over the
years, it forms a cesium frost which deposits on everything inside the
tube. If it gets thick enough it conducts or can arc between frost
grains. A charged capacitor can blow some of this away. Cesium melts
at 83° F. If you heated the beam tube up in a hot car or out in the
hot sun, you could turn this frost into dew. The dew might run down
hill into some cesium lakes where it was harmless. Those lakes would
freeze when back at room temperature and might be even more harmless.
I have seen two TV picture tubes that developed leakage between focus
and screen electrodes. Adjusting focus changed the brightness and
adjusting the screen changed the focus. I applied the high voltage
between the two and the arc burnt out the high resistance leak. It
saved both picture tubes.
If you cannot get the ion current below 50ua or so after a week at 5kV
then you are out of luck. Most likely you have resistive deposits on
the ion pump insulator. If you can get the tube to give a decent SN at
those levels then you can
run with the alarm circuit bypassed.
I have run tubes a couple years this way (at <50ua) but if you let them
sit cold you will have to manually cycle the ovens at turn on until the
excess gas load is pumped. If you have a bad ion pump supply when you
open it up to repair it I would suggest adding a 66K resistor across
the 75K resistor. This will increase the alarm trip up to about
40-45ua. If it's the older one the oil capacitors will need changing
as well as the 200Meg resistor.
If the later one (in the 5061B) then the 47uf radial cap is open. both
these units are interchangeable. One other trick is to use a spark
discharge tester (miniature hand held tesla coil used in the neon sign
industry) to ZAP the ion pump lead. This can blow out any whiskers
that the ion pump has developed. You can
also (carefully) with the HV on, remove and replace the ion pump
connector a few times. Sometimes you will get a sharp discharge that
blows out the whisker. I have used both methods with good success.
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