On 24 Apr 2005 00:24:28 -0700 (Aniruddh
Sarkar) wrote:

This was traced to an oscillating 24V supply powering, among other
things, its objective lens control board and the objective lens
itself. We found both ends of the objective lens (basically an
elaborate coil of insulated copper wire wound on a conical steel(?)
core) winding were at ~1 ohm to the the instrument chassis while the
coil itself ~2ohm across.

If the windings are indeed wound on a magnetic core, then the shape of
the core will be much more important to the image than the exact
distribution of the windings on it. It is likely that the alignment of
the core to the rest of the SEM is also important.

You might want to try driving the coil with a separate power supply to
see if that will fix the problem, just because it's probably easier to
come up with a suitable power supply than to rewind the coil.

I will also ask if there is any possibility that the polarity of the
original power supply might have gotten reversed at some point, since
it is possible that one end of the coil was grounded by the
manufacturer on purpose. Along the same line, you should check to make
sure that the core was not previously insulated from ground and that
this insulation has failed.

You should be able to rewind the magnet, but you don't want to go thru
all of this only to discover that this was not the problem.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------