If the filament has blown you will see no emission current, filament voltage, or beam current when the high voltage is on and the filament is turned up to normal settings (500-600). Before replacing a filament , be sure that loss of current isn't due to some other problem, such as the beam being blanked (turn signal selector to BC to check beam current) or the Main Gate valve V1 closing due to vacuum or electrical failure or depletion of the nitrogen gas supply (check the Vacuum Panel LED's). If the emission and Filament Current meters read 0 and the Gun High Voltage light is on , then the filament is burned out.

To replace the filament, proceed as follows:

1. TURN OFF POWER TO THE FILAMENT BY:

• Turning down the Emission Current, turn the black FIL.V knob to zero, on the SEM Console.
  
  
• Turning the Kilovolts gauge, upper left SEM console, to zero by stepping down the Coarse kV buttons, lower right of SEM console, sequentially to zero. i.e. 15, 10, 5, 0.
  
  
• Turning off the Electron Gun Power Supply, just below Emission Current Gauge.
  
  
• WARNING: FAILURE TO DO THIS CAN RESULT IN SERIOUS INJURY TO BOTH YOU AND THE MICROPROBE (15,000 VOLTS WORTH).

2. VENT THE GUN CHAMBER:

Close the electron gun isolation valve V3. This is the manually operated valve with a handle attached to the front of the gun which closes with a 90 degree counterclockwise turn. Pull the lever from the vertical position left to a horizontal position. The V3 light will go out on the vacuum control panel door.

• Check that the vacuum panel switches for V4, V1, and P1 agree with their actual status. That is, make sure that if the light shows V4 or V1 are open (lit up red) then their switches should be in the up position. If a light is off showing the valve or pump is closed or off, then the switches should be in the down position. When you turn off the auto mode (next instruction) the activity of the valves and pump will depend on the switch positions, which are inactive when the probe is in auto mode.
  
  
• If the switches agree with the lights on the Vacuum control panel, turn the key from AUTO to MAN (manual).
  
  
• Vent N₂ gas into the gun chamber by lifting the V7 switch and letting it spring back (it is spring loaded unlike the other switches). N₂ will bleed into the isolated gun chamber until the chamber reaches atmospheric pressure (V7 will stay lit for a few seconds)

3. REMOVE THE OLD FILAMENT:

• When the V7 light goes out, swing the electron gun back until it comes to rest in a stable position. Use a large screwdriver to ground the gun by making contact between the grid cap and the gun housing at the base. Remove the grid cap by turning CCW (cap will be very warm to hot, use the brass heat sink to remove and store the grid cap).
  
  
• Put gloves on. ALWAYS wear gloves when touching anything inside the probe.
  
  
• Grasp the ceramic plate at the base of the filament and rotate 90 degrees (the two posts holding the filament should be vertical). Then pull the filament out.

4. INSTALL THE NEW FILAMENT:

• On the new filament check that the mounting pins below the base are not spread open too far or closed too tight. These pins will be rotated into position so as to contact the brass socket pins in the gun. This holds the filament in place. If the filament pins are spread too wide, they will bend and not make proper contact. If the filament pins are too close, they also will not make proper contact and the filament may pop out. The proper spacing of the pins at the end should be about 1/4 to 5/16 inch.
  
  
• Insert the filament with the pins vertical and rotate the filament 90 degrees. You will hear or feel a slight click as the filament pins snap into place between the gun socket pins ( the filament should now be horizontal). Gently pull on the filament ceramic base to make sure the filament is snug.
  
  
• Remove the grid cap (use gloves) from the brass heat sink and clean the cap off with ethyl alcohol and kimwipes or cotton swabs. Thread the grid cap back into the gun turning clockwise (CW) until the filament tip is visible in the hole (use a flashlight if necessary). Using the four large gun centering screws on the gun, center the filament tip in the hole (diametrically opposing centering screws are used in tandem by turning both hands in the same direction -- one screw will turn CCW, one CW).
  
  
• Turn the grid cap CW until the filament tip is flush with the caps surface. Check by gently scraping a straight edge of paper backwards (i.e. towards you if the gun cap is horizontal) across the hole. When the filament tip emerges through the hole, a clicking noise will be heard as the paper scrapes across the tip (be careful not to break off the tip).
  
  
• Once the filament tip is flush with the grid cap surface, back off the grid cap one full turn (CCW). Re-center the filament with the gun centering screws. Turning the cap one turn will make the filament bias go to the right of center. ARL specs are for 3/4 of a turn which will make the filament bias go to the left of center.

5. EVACUATE THE GUN CHAMBER:

• Check that the o-ring and rim of the gun chamber housing are clean. Close the gun chamber by pulling the housing forward
  
  
• On the vacuum panel, with the key in "MAN" position, turn on Pump 1 ("AUX") and then open valve V5. This opens the auxiliary pumping line to the gun chamber for evacuation. Turn the meter readout switch on the left to G3 (GUN) and monitor the gun chamber pressure (on the vacuum control panel). The pressure should drop fairly rapidly to below 100 microns (in a minute or two).
  
  
• If the pressure does not drop below 50 microns after 5 minutes then the gun o-ring and housing surface should be cleaned. To do this close V5, turn the AUX P1 off, and toggle V7 to vent the gun chamber (as discussed at the beginning of this section). After the V7 venting stops, open the gun housing and clean the flat surface that the o-ring seals against using a kimwipe with alcohol (this will be the upper surface on the cap). Clean the o-ring by gently rubbing with a very clean piece of the special lint free cloth. Reestablish the vacuum.
  
  
• When the pressure drops below 50 microns, close V5, turn OFF Pump P1, and open the gun isolation valve V3 by rotating the handle on the gun 90 CW slowly towards the vertical position. While opening V3 watch the high vacuum gauge on the right of the vacuum control panel. As the low vacuum air enters the main chamber you will see the gauge jump up. Close V3 again for a few seconds and then reopen it fully (handle in the vertical position). This allows the diffusion pump to remove the "air" from the gun chamber with less contamination. Allowing air in direct contact with the diffusion pump will burn the oil and can damage the pump. The oil costs $500 per pint.
  
  

6. DO NOT MIX UP THE SEQUENCE OR DAMAGE TO THE MICROPROBE CAN RESULT. CLOSE V5, TURN OFF P1, OPEN V3.

7. RETURN THE VACUUM KEY TO THE AUTO POSITION IMPORTANT!!

CENTER AND SATURATE THE NEW FILAMENT (see StartUp instructions for detail)

• Make sure the probe is on the Brass Saturation Spot.
  
  
• Turn the gun power on and turn the voltage up with the push buttons to 15 kV (incrementally stepping through the buttons)
  
  
• Be sure that the beam is not blanked on either the SEM console or by the computer (Blank button out on the SEM console).
  
  
• Turn the signal selector knob to BC and select the 1x10^-6 range for the Picoammeter
  
  
• Turn the Filament Voltage knob up until the Emission Current starts to increase. Raise the emission current to about 30 - 50. Let the filament stabilize for a few minutes (the emission current will increase slightly as it sits).
  
  
• As you turn the Filament Voltage higher, a Beam Current signal should appear on the Picoammeter. Make sure that you pass the false peak (if present, it may not show up until the filament is centered a bit better). Be careful not to exceed 1.8^-2 amps on the Filament Current Gauge (directly above the Filament Voltage knob).
  
  
• Generally once the BC reads about 5 - 7 on the 1x10^-6 scale, you can begin centering the filament using the large screws on top of the gun chamber. Grab the two opposing screws and turn them in the same direction, as long as the Beam Current is increasing. As soon as it decreases, back the screws up until the maximum is reached. Repeat the process with the other pair of screws. Now drop the Filament Current down and saturate the filament (you may still not have a false peak at this point, and again make sure you don't exceed 1.8^-2 amps on the Filament Current Gauge). Continue reiterating this process until you have maximized the Beam Current on the Picoammeter and have a good false peak at about 2 - 3 on the Picoammeter
  
  
• The Filament Current Gauge should read no higher than 1.8^-2 amps and the filament voltage should read between 600 - 650.
  
  
• After saturating the filament, adjust the emission current to 100 microamps using the gun BIAS knob.
  
  
• Then switch to the .03 range on the Picoammeter and change from BC to SC (Sample Current). Using the C2 Condenser Lens, set the Sample Current to 1.5 on the .03 x 10^-6 scale. This is 15 nanoamps, which is the current used to analyze samples.
  
  
• Recheck the centering and resaturate every half hour for the next 3 - 5 hours until the filament stabilizes.

*** Note that the USGS Reston Microprobe used 150 microamp for the emission current setting.

Need to add figures from Reston Book to this document.