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Service Note

Changing the Filament Current Limit
in the Model 11-020 LEED Electronics

Revised 4 Feb 2000


The filaments we supply require slightly more current than those supplied by Physical Electronics, Inc. (PHI) for its LEED optics. The internal current limit in PHI - manufactured versions of the 11-020 electronics needs to be raised to 3.9 to 4.1 amps to accomodate our replacement filaments. Also, some customers have reported better results with our 11-020 units at slightly higher current limits than the 3.9 amps originally set at our lab. Filament emission varies from unit to unit. If you are not getting adequate emission from a new filament, you may benefit from a higher current limit.

CAUTION: Life threatening high voltages are present inside the Model 11-020 whenever AC power is applied, regardless of front panel control settings. Be particularly careful to keep fingers away from the terminals on the two large film capacitors mounted at the center of the unit. 2000 volts DC is present on these terminals at all times when the unit has AC power applied. Only a technician familiar with safety precautions for working on high voltage power supplies should work on this unit.

Measuring the Current Limit (before making changes):

Turn off AC power to the 11-020, disconnect all MHV cables, and put the unit on a bench. Set all front panel controls to the full counter-clockwise position. Connect a DC ammeter (preferably an analog type) with a 0 to 5 amp scale between filament output terminals F1 (J6) (+)and F2 (J2)(-). Connect a one ohm 20 Watt resistor in series with the ammeter (optional but recommended). Turn on AC power. Slowly turn the front panel filament control knob clockwise, and observe the ammeter. The reading should be rising (if not the polarity of the ammeter leads is incorrect). At some level between about 3.4 and 4.1 amps the reading will reach a limit point. This is the level set by trimpot R7. Let the unit stabilize at this current level for about five minutes, re-check the ammeter, and record the reading for reference. Return the front panel filament control to the full CCW position.

Procedures for Changing the Limits:

Leave the ammeter connected as above. Turn off AC power to the unit, and remove the top cover. On the right of the chassis is the filament control PC board (this is the board without the vacuum tube). Make a note of the location of variable resistor R8 (large ceramic, wire-wound) and of trimpot R7 (square blue, top-adjust) on the filament control board. R8 sets the mechanical current limit, triggering a relay that disconnects AC power to most of the unit. This protects against gross overloads. R7 sets the electronic current limit circuit, which limits the front panel filament setting to a predetermined value well below the limit set by R8. In most cases, only R7 will need to be adjusted by the user. For safety purposes, keep in mind that the filament board components are not referenced to ground but instead "float" at the beam voltage across F1 and F2. The beam voltage should be left at zero volts throughout the procedures below.

Electronic Limit Adjustment (R7):

Start with R7 turned full counter-clockwise. Set the front panel filament control to about 4.2 amps. Turn R7 slowly clockwise until the meter reads 4.0 amps. [Turning R7 clockwise lowers the limit, counter-clockwise raises the limit] The limit may be made slightly higher than 4.0 amps if required for a particular filament (emission varies somewhat from unit to unit). After making a change, allow the 11-020 to stabilize again for several minutes. The reading on the ammeter may drift downward as much as .15 amps as the unit heats up. If so, readjust R7 to the desired current.

Mechanical Limit Adjustment (R8):

This is a coarse adjustment that will usually not need to be reset after the 11-020 leaves the factory. However, if the electronic limit set by R7 has been raised substantially, the relay may begin to buzz, or may even trip when the current is at the maximum level. In this case the limit set by R8 needs to be raised slightly:

  1. Turn the front panel filament control full counter-clockwise.
  2. Turn R7 full counter-clockwise to disable the electronic limit.
  3. Loosen the clamp securing the slider of R8, move it one notch toward the front panel and re-tighten. Precise adjustment is not possible here.
  4. Slowly raise the current using the front panel filament control. The relay should trigger between roughly 4.3 amps and 5.5 amps. If not, loosen the clamp and try another setting. If the relay trips during testing, turn AC power off , turn the front panel filament control down, and then turn AC power on to reset.
  5. Readjust R7 as described in the section "Electronic Limit Adjustment" above.
  6. Verify that the front panel filament control can now be turned full clockwise without causing the relay to buzz or trip.
  7. Replace the top cover.

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