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Service Note

Replacing the Filament in LEED Optics

Revised 14 December 99

Note: We suggest that you remove the gun assembly as a unit and ship it to us for filament installation. We offer this service without charge to our reverse view LEED customers. Installation by us will ensure precise alignment of the filament tip and the gun aperture. Misalignment of these two parts will degrade gun performance. If you prefer to replace the filament yourself, follow the instructions below.

Overview & Precautions:

All PRI and PHI LEED optics use the same filament assembly, P.N. D71017. The filament is covered with a special burnout-resistant coating which can be easily damaged by contact with anything. The filament is attached to moly posts brazed on to a ceramic button. The posts are fragile and will break if much force is used when tightening the set screws on the filament clamp pieces. Be sure to support the filament clamp pieces when tightening the set screws.


Remove the gun assembly from the optics. For PRI optics, follow the instructions in the manual supplied with the unit. For PHI optics, set the optics face up and remove the shield. Avoid touching the grids with tools or fingers. Remove all wires from the gun assembly. The three outermost screws insulated with ceramics hold the gun assembly to the mounting plate. When they are removed, the gun can be lowered straight down and removed.

Remove the old filament by removing the three tiny screws and the clamp plate. Reassemble in reverse order, being careful not to brush the tip of the new filament against anything. Snug up the three tiny screws lightly (the filament is not yet in its final alignment).

Center the tip of the filament in the first gun aperture. We recommend holding the gun assembly directly in front of the nose of a microscope light and looking into the output snout of the gun using an eye loupe or (preferably) a stereo microscope. Centering may require several iterations since both x and y axis must be adjusted. When the filament assembly is centered, gently tighten the three tiny screws. Reassemble the gun in the optics.

Note: early PRI optics used a ceramic button of slightly different thickness. After removing the old filament you will see a small circular step on which the front of the ceramic button locates. If the new filament will not clamp firmly in place, then the back face of the first anode assembly should be machined down, such that the distance to the step is .055 inches +.000 inches -.002 inches. If you prefer, we can do this modification for you.
Important: the filaments we supply require slightly more current than those formerly supplied by PHI. If you do not get adequate emission from a new filament, the internal current limit on your model 11-020 electronics may need adjustment. See our Service Note for the procedure.

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