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.

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?

Any info/pointers highly appreciated!

--
tia,
Aniruddh Sarkar
Research Associate
Dept of Electrical Engg
IIT Powai
Mumbai, India

(Aniruddh Sarkar) writes:

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.

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?

While I'm not an expert on SEM design, there is no magic involved.
Replicating the coil as precisely as possible should restore function.
It certainly can't hurt while you search for a new coil.

Roughly how many turns are involved? Is this something you could do
with a manual coil winding machine or by hand?

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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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My first call would be to the manufacturer of the SEM. If that fails,
then I guess you are stuck with rewinding the coil. But, it will be a
long and arduous process. You will have to photograph the old coil and
take many notes to be sure you replicate it correctly. Also, you must
get exactly the same wire and insulation, so that the number of turns
of wire are the same for each layer as in the original coil. But, that
means the wire may be subject to the same shorting problem as you are
now experiencing, so you must find the source of the problem (water
leakage ?) before you install the rewound coil. Altogether, this
sounds like a MAJOR problem. I would exhaust all other possibilities
before starting to rewind the coil.

H. R. Hofmann

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.

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?

Any info/pointers highly appreciated!

--
tia,
Aniruddh Sarkar
Research Associate
Dept of Electrical Engg
IIT Powai
Mumbai, India

"Sam Goldwasser" wrote in message
...
(Aniruddh Sarkar) writes:



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




While I'm not an expert on SEM design, there is no magic involved.
Replicating the coil as precisely as possible should restore function.
It certainly can't hurt while you search for a new coil.


There may well be magic involved. In order to rewind a new coil they
will have to unwind the original to save the steel(?) core to wrap the
new coil on. Big buck$ and no going back.

I bet the original coil had to be tweaked after manufacture because of
impurities in the wire and variations in insulation thickness.
Alignment may require some expensive toys and training.

"Derelict" wrote in message
news:fBOae.3103$Bb.1492@okepread06...

"Sam Goldwasser" wrote in message
...
(Aniruddh Sarkar) writes:



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




While I'm not an expert on SEM design, there is no magic involved.
Replicating the coil as precisely as possible should restore function.
It certainly can't hurt while you search for a new coil.


There may well be magic involved. In order to rewind a new coil they
will have to unwind the original to save the steel(?) core to wrap the
new coil on. Big buck$ and no going back.


Going back? It's already broken, what's he got to lose?

"James Sweet" wrote in message
news:BWQae.3150$yc.3104@trnddc02...

"Derelict" wrote in message
news:fBOae.3103$Bb.1492@okepread06...


There may well be magic involved. In order to rewind a new coil
they
will have to unwind the original to save the steel(?) core to wrap
the
new coil on. Big buck$ and no going back.


Going back? It's already broken, what's he got to lose?


The OP and his multimeter have not done nearly enough troubleshooting to
mess with a device that could easily cost $300K. I bet he didn't pay
for it.

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
-----------------------------------------------