Change Filament

Step 1. Release vacuum and open x-ray tube
1.1 Open vacuum release nut located below the x-ray tube front cover. Simply loosen the nut and you'll hear air leaking noise. Wait until the noise is gone, which means that the x-ray tube is under atmosphere pressure and you can open the system now.

1.2 Check the o-ring on the nut for defects and dirts. If necessary, pull it out and rubber a tiny little vacuum grease on it uniformly. Then put the nut back in and tighten it. Don't wait until the last minute to do this because you can easily forget after the many steps below.

1.3 Loosen the four screws on the front cove of the x-ray tube and remove the cover. Be aware that there is an o-ring in a groove on the cover's back side. Again, check the o-ring. Also, make sure you do not touch any inner part of the x-ray tube with your bare hands. Wear gloves!

Step 2. Slide down the cover for filament holder terminals
2.1 Here is an inside view of the tube. Loosen up the two screws that hold the lower part, the terminal cover, of the filament holder. Do not try to remove the two screws completely because you may well drop them into the tube and that's going to be a big trouble.

2.2 Once the screws are loosened enough, the terminal cover can be pushed down with a minimum force. However, the screw on the left on both of our sytems were partially damaged already* while the one on the right is still okay. You may not be able to loosen it up to the degree you want and you'll have to try to slide the cover down with some effort. Do so gently with a wooden stick inserting in the gap between the cover and the filament holder.
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*The screw on SYS I was already damaged before Andrew Lai came, while the one on SYS II was damaged once when we were in a hurry to change the filament before the system was cooled down. Because of variation in thermal responses of different metals, this happened. It is still workable with the screw - just that you won't be able to undo completely or fasten very tightly. However, with the other one still good, there has been no problem as far as the tube's funtions go.

Step 3. Take out the filament holder
3.1 This is a view after you've slide down the terminal cover. You need to first undo the two screws in the middle that hold the terminal (a pair of lead wire in orange color. Again, be careful and make sure the screws do not drop into the x-ray tube.

3.2 Then undo the two outer screws that hold the filament holder in place. Note that you should not try to remove these screws - just loosen them and take out the holder together with screws. Again, don't drop the screws into the x-ray tube.

Step 4. Disassemble filament holder
Now, take the filament holder to somewhere, such as a work bench in the lab, away from the x-ray tube* and undo the two screws on top of the holder. This allows you to access and change the filament in the box.
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* You'll now see a window inside the x-ray tube and the anode drum. Inspect the drum, which is made of copper. However, usually you should not expect to a bright copper color, instead, a brown or even dark brown color. If the color of the drum has turned black, or it has too many stains on it, it's time to polish the anode. Also, you'll notice that in the middle part of the drum is very light groove. This is caused by the electrons emitted from the filament.

Step 5. Take out burned filament
5.1 Now you see the inside of the filament holder. The filament assembly is mounted to the front cover the holder. Simply dismount it by removing the two screws that hold the filament (see next picture for a break-down view).

5.2 Do not through away the burned filament. Save it in the original filament box. Burned filaments can be rebuilt.

Step 6. Clean filament holder
6.1 You see now a "bare" filament holder. There is an opening (Wehnelt opening) on the front cover. You need to clean both front and rear covers, especially the area near the opening. Use a fine house cleaning pad (you may need to cut it into small pieces) to polish the surfaces of the covers. Special care should be given to a) both surfaces of the front cover and b)inner surface of the rear cover. You must wear gloves throughout this step.

6.2 Soak the polished parts in acetone for a few minutes. Then take them out and use acetone (in a sqweeze-bottle) to wash again; use duster also if necessary. The filament holder must be absolutely clean and free of metal particles; otherwise you'll have a lot of trouble later when you pump vacuum and/or power up the system.

Step 7. Mount new filament and finish up
7.1 Take a new filament from a filament box (in one of the drawers in the lab). Inspect visually for obvious defects, if any.

7.2 Mount the new filament on the holder. The up-down positioning of the filament is determined by pressing the filament seat plate against the holder. The left-right positioning of the filament, however, must be visually determined in such a manner that the filament is at the center of the Welnelt opening.

7.3 Put on back cover and mount the holder back on cathode.

7.4 Connect the terminals firmly and make sure the two lead wires are parallel to each other and straight down!

7.5 Cover the x-ray tube and start pumping vacuum.