Filament Change
Procedure for
changing burnt x-ray filament and replacing it with a new filament*
Introduction
During the regular use of the x-ray generator (Rigaku generator is only covered in this write-up) the filament that produces the electrons burns out and needs to be replaced. The useful life of a typical x-ray filament is approximately one thousand hours of operation (assuming continuous generator operation this means filament needs to be changed once in forty days). The symptoms of a burnt out filament are one or combination of the following:
§ Alarm at the x-ray generator with a lit red LED next to FC
§ Needle at the filament current meter shows no measurable value
§ Hour meter does not change value even after several minutes of generator operation
§ No diffraction pattern after eight to ten minutes of exposure
§ Absence of water ring and/or absence of beam stop shadow in diffraction pattern.
This notes describes how to remove the burnt filament and replace it with a new one. Due to the fact, we have to break and regenerate high vacuum, the whole procedure can take eight to ten hours.
Removal of burnt filament
1.
Switch the x-ray generator off, close the
chilled water supply to the Haskris water chiller[1],
however, maintain the water circulation to the generator and wait for ½ hour.
2.
Shut the rotating anode (RA) off, if it is
not already off, and shut down the vacuum pumps. Switch off the main powers at the gray breaker box on the wall,
switch off the Haskris water chiller, and wait for another ½ hour.
3.
Using the special tool (flat steel plate with
a wedge[2]),
open the vacuum release valve[3]
(see Figure 1) underneath the front covers to the anode
chamber and let in air slowly. Once the
anode chamber is equilibrated with outside pressure, tighten the vacuum release
valve back to the original position (hand tight).
Figure 1 Vacuum release knob located beneath the rotating anode chamber
4.
Using a Phillips screwdriver carefully remove
the four screws (see Figure 2) found on the front vacuum
cover assembly of the x-ray generator head, ensuring that the removal of the
screws proceed uniformly.
Figure 2 Front vacuum cover assembly with screws highlighted
5.
Remove the front vacuum cover and its
'o'-ring making sure NOT to touch the interiors of the cover or the
chamber. Place the cover and the
'o'-ring aside. Now you can see the
interior of the anode chamber (see Figure 3).
Figure 3 Cathode assembly & anode chamber upon removal of front vacuum cover plate
6.
Now place a long lint-free Kimwipes â EX-L[4]
inside the space made available with the removal of front cover. The idea behind the Kimwipes is to protect
any screws, shims, and such falling into the vacuum chamber located directly
underneath the cathode assembly while one is removing the burnt filament (see Figure 4). So, stretch
the wipe underneath the cathode assembly and let part of it hang outside the
chamber. Essentially all operations are
done above the wipe and anything falls will be caught by the wipe.
Figure 4 Cathode assembly with Kim Wipes placed beneath the assembly
7.
Visible in front will be the back of the
cathode assembly (it may be VP3 cathode, i.e., 0.3 x 3.0 mm2 fine
focus cathode). The terminal
cover-plate[5] protecting
the filament terminals is located directly beneath the cathode assembly. Carefully undo few turns of the two Phillips
screws holding the plate, with out completely removing the screws.
8.
The terminal cover plate should automatically
slip down revealing behind it the two terminal posts for the cathode (see Figure 5). Now
tighten the Phillips screws back with the terminal cover plate down.
Figure 5 High voltage wires visible after terminal cover plate has been moved down
9.
Using a metric hex wrench undo the two hex
heads holding the high voltage wires to the bottom legs of the cathode
assembly. These are captured-screws and
can’t be removed. Remove the wires away
from the legs of the cathode.
10.
Using one gloved hand, hold the cathode
assembly with an electronics-lab glove [6](made
of white nylon). Now with the other
hand, unscrew the hex heads at the bottom corners of the cathode assembly. Once the screws are completely undone, you
can remove the whole cathode assembly away from the chamber retaining the two
hex screws (captured-screws[7]).
This should reveal the rotating anode, filament terminal and cathode holding screws.
11.
Place the cathode assembly on a clean
Kimwipes sheet with the Wehnelt window facing up. Careful examination will reveal that the filament is broken. Most of the time filament breaks at the
middle. This breakage is accompanied by
bend in the filament as well. (See Figure 6 for the frontal & backside view of the VP3
cathode assembly).
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12.
Now turn the cathode such as way that the hex
heads are facing up on a clean Kimwipes sheet.
Now undo the two Phillips screws found on the top corners of the cathode
assembly completely.
13.
The back cover of the cathode should now come
off completely, revealing inside the burnt filament (see Figure
7).
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14.
The filament itself is held to the Wehnelt
window (i.e., the front of the cathode assembly) with two more hex screws[8]. Removing the two hex screws will release the
filament from the cathode assembly. Now carefully store the hex screws. The removed filament will look similar to
one of those shown in Figure 8 below except for the
missing filament.
15.
Using very fine Emery polishing paper clean the
inside and Wehnelt window[9]
of the cathode assembly taking care not to create big scratches.
Figure 8 Replacing a new x-ray filament in side the cathode assembly
This completes the removal of the burnt filament. Now we will proceed to installation of a new filament in the next section.
