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Electromagnetic Lens Repair
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 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! -- Return address is VALID but some sites block emails with links. Delete this sig when replying. .. Wanted, PCMCIA SCSI Card for HP m820 CDRW. FS 500MHz Tek DSOscilloscope TDS540 Make Offer Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121 Wanted 12" LCD for Compaq Armada 7770MT. Bunch of stuff For Sale and Wanted at the link below. MAKE THE OBVIOUS CHANGES TO THE LINK htremovethistp://www.geocities.com/SiliconValley/Monitor/4710/ |
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 ----------------------------------------------- |
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