Aniruddh Sarkar wrote:
Hi,

I am not very sure whether this is the best suited forum for my query
but still I am sure this is not totally off-topic. So here goes..

Our 1990-purchased JEOL 6400 Scanning Electron Microscope recently
developed an intermittent problem of black bands randomly
appearing/sliding on its display.

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.

What's the specification for the resistance of the coil? Are you sure
that's not the correct reading?

If the coil were shorted, wouldn't the image distortion be a lot worse
than the bands caused by the oscillation??? In fact, are you sure it's
oscillation and not ripple? Black bands "sliding" on a video display
sounds like ripple. I'd go find a person with an oscilloscope
and try to figure out what's happening.

For something that critical and expensive, I'd check with the vendor
before I tried fixing something that ain't broke. Once you take apart
the coil, it's all over. Your homemade replacement is likely to be
less good than a "real" one. It's hard enough to get even spacing by
hand on a flat winding.
I'd be more inclined to work on fixing the power supply oscillation.
Even if the coil is shorted to the frame, it's probably easier to
build a floating supply to drive it than to try to get the shaped field
correct in a rewound coil.


Disconnecting the lens has stopped the supply oscillation etc but
ofcourse now there is no image!

We are guessing that either the condensation due to the running
cooling water or the heating due to lack of it has shorted the
innermost layers of winding to the core.

While we are still hunting for a spare, I would like to enquire people
here for pointers on design & rewinding of electromagnetic lens coils.
The winding looks specially designed - with number of turns decreasing
in each layer in a particular fashion as one goes up the core.

Will just replicating the winding - wire gauge & the turn pattern -
bring it back to function?

If you match the wire and the winding exactly, it "has" to work.
But if you fixed something that wasn't broke, it won't fix the system.
mike



Any info/pointers highly appreciated!




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