Electromagnetic Lens Repair

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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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.

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

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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:

people

coils.

decreasing

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.

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

they

the

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.

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.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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