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Hi Probe folks;

It's the holiday season, that means it's time for everyone's machines
to break down, right?   At least, that's what always happens to me.

I've got a strange beam drift that started up a week ago without
provocation.  It might be hard to describe so I have a little movie of
it to watch if you're so inclined.

This is a (brown) JEOL 733, tungsten filament.  We developed this
issue where if you put the beam on a spot, there's drift.  In fact
there's a lot of drift.  The drift is co-incident with the gun shift-x
(but isn't in the gun shift-x, since the beam stays bright from
end-to-end, whereas the gun shift of course it gets dark as the gun
misaligns).  It's 45 degrees to the "fine shift x y" on the mag box
controls (like both were being adjusted at once).  It's not stage
drift, it's beam drift.

Very conveniently we put the beam onto a fluorescing mineral so you
can watch the beam drift on the optical microscope, that's what the
movie is of.

Here's the part that I find weird:  The beam steadily drifts in one
direction until it reaches it's end, then it instantly 'snaps back' to
the start of the path it was following and repeats.  I've likened it
to using a manual typewriter and hitting the return for the carriage
at the end of the line.

Weirder still:  If I put the PCD in, it appears to 'not' drift while
the cup is inserted.  That is, you put the cup in at a point, wait a
little while, and when you withdraw the cup again the beam is at the
same spot, and then continues on it's merry drift path.

Probe does this regardless if imaging mode: spot, picture, reduced
picture.   The area scanned is just fine, it just progressively drifts
up and to the right.  This, however, makes it pretty much useless for
work if it drifts off your spot during analysis.

I've gone as far as to manipulate the mag box while this drift is
going on.  I can pull the mag box completely out of the frame (thus
becoming unpowered) while the drift is going on and it has no effect.

This 733 has one of those "JEOL Scan Clear Boxes" in it, I'm going to
try and remove that from the system and see if that affects things.
We've had weirdness before associated with the scan-clear-box's ground
line.

To me this looks like a progressive drift in the fine-shift alignment
coils down in the objective lens.   In the freaky department, I took
the side panel off the 733 (the right side where you can get at the
electronics rack) and it went away.  Eventually it did come back
though.   Could this be a Deflector Amp issue?

If you'd like to watch a little video showing what I'm talking about,
you can check http://gallery.me.com/cogswell/100000 , it shows the
image from the optical microscope as the beam (the bright fluorescing
spot) moves along.  At one point I put the PCD in and out, and you can
see the beam disappear but come back to the same spot.

I figured I'd go out to the experts here on the list and ask "Have you
seen this before, any hints?", I'm not ambitious to tear down the
deflector amp system looking for a needle in the haystack.

and hey, Happy Holidays to everyone.


Best regards,

Steven Cogswell
UNB Microscopy and Microanalysis