I am running a JEOL 733 and have run into some difficulties. Last tuesday
I did a sample exchange before closing the office up for the night. A
couple of hours later I realized that there was no current on the cup, so I
figured it was time for a filament change. I inspected the old filament
and realized it had not blown at all. I installed a new filament and
pumped the probe down. The probe pumped down, 15KV was achieved.
Normally when I have run the probe, the coarse gain knob on the condenser lens
has been on step 3 or 4, but now no cup current or absorbed current are
achieved at this setting. I adjusted the aperature with no significant
change. I cycled through the coarse gain knob of the condenser lens (on
the Electron Optical System (EOS)) from 1-7 and at step 8 (one step above 7 on
the knob but not labelled), I obtained picoamps of current on the
cup. At one point, i did achieve 12 nano-amps at the step 7 setting, but
that disappeared. Since I was not able to get any current on any of
the other steps while cycling through the condenser knob, I figured there was a
problem with the condenser lens. I swapped the EOS module, the ML cooling
Box at the bottom of the console, the actual condenser lens, and the
accelerating voltage module (thank goodness for spare parts), however, no change
in the current, and again, I could only get picoamps of current on step 7 or
8. We have checked voltages down at the ML cooling box, and at various
other locations and they seem to coincide with the JEOL circuit diagrams.
We then decided to investigate the brass/copper tube liners which run the center
of the column, cleaned them and re-installed them (are these column tube liners
supposed to be grounded?). Again no change in the current on the cup nor
on the sample and only picoamps achieved at step 8 on the condenser lens.
I am running out of things to try. Some other possibilities I am
considering is the potential of charging somewhere in the column; perhaps the
leaded OM glass has become charged? Any thoughts, suggestions, or things
to try would be much appreciated.