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Rosemount 400A Preamplifier Board 620423

Operation (excerpted from 400A manual): The ionization current generated by the burner is measured by an electrometer preamplifier located adjacent to the burner assembly. This small current is amplified and transformed into a signal voltage that is then further amplified by a post amplifier before being converted to a digital display suitable for direct data presentation. To cover the required dynamic range, the amplifier is provided with two gain ranges that differ by a factor of 100. Output voltage from the preamp is a precise function of ionization current.  The most sensitive gain range includes a trim adjustment so that inter-range correlation can be obtained over the entire signal span.

A buffer signal offering unity gain and noise filtration provide a low output impedance to drive the signal cable and post amplifier circuits on the main circuit board. Selection of the low or high range feedback resistors is made by relay K1 on the preamplifier board.  A variable offset current is injected into the summing node of the electrometer amplifier to compensate for background offset current. These currents influence the measurement procedure, and a variable voltage at the front panel allows the user to visually cancel these currents during the calibration procedure. Background current is due to unavoidable traces of carbonaceous material introduced into the burner flame by the fuel gas and air.

Operation comments by RIGAS: K1 is a N.O. relay (shelf state).  When open (de-energized), maximum feedback resistance is applied to U2 (first stage) thus resulting in maximum gain or high sensitivity.  When K1 is closed (energized by +5 VDC when range X100, X250, or X1000 is selected) then R17 is placed in parallel with R18 resulting in less feedback resistance and thus less gain or less sensitivity.

Jumper E1-E2-E3 should be in the E1-E2 position.  E2-E3 is a factory test position but could be used to determine the exact amount of amplifier offset or burner contamination since all Zero Compensation would be removed from the circuit.

Jumper E4-E5 should be in place.  This allows the polarizing voltage to be grounded out during lighting (when switch is set to “ignite”).

Typical Failures:

  • Glass encapsulated, high ohmage, precision resistors get dirty. Dirt conducts so the more dirt, the less ohmage.
  • Glass capacitors. Dirt conducts so more dirt changes capacitance.
  • Coax cable breaks down (signals get noisy)
  • Jumper wires get frayed, brittle, and break
  • Opamps (operational amplifiers) fail (use list of expected voltages here)
  • Purge / ignite switch fails
  • 3 VAC transformer fails
  • Interconnecting ribbon cable gets pinched and fails
  • Burner contact assembly fails (this is a ghost ['looks' like] a preamp board failure)
  • Burner collector ring connection fails (this is a ghost ['looks' like] a preamp board failure)
  • Burner temperature sensor fails (fuel solenoid won’t stay latched after lighting ‘pop’)

Other information:

RIGAS built a special resistor pack to simulate the ion current developed by burning hydrocarbons.  We have six 500 gigaohms resistors in series to mimic the very low ion current (3 x 10-11 amps) in the burner (remember that a 90 VDC polarizing voltage is applied at this end of the circuit). This helps us determine if noise is coming from the burner chamber proper or the preamp board.

Schematic 620424 (with RIGAS embedded notes)

List of expected voltages

Simplified electronic calibration


  • When the Zero pot is up near 10 (full CW) that is GOOD! That means that there is minimal contamination to overcome with a bias signal.
  • Failure to light is usually a fuel/air ratio problem (usually not enough fuel getting to the chamber)
  • Failure to light could be as simple a s blown glow-plug
  • The 400A is a PERCENTAGE readout analyzer and a TOTAL HYDROCARBON analyzer
    • the display reads a percentage of your calibration gas numbers
    • any hydrocarbon will read out on this analyzer.  If you calibrate with 20 ppm methane and inject 5 ppm of butane, you’ll get the same response
    • Click here for the 400A calculator spreadsheet

Normal maintenance:

  • Replace old jumper wires & their connectors
  • Replace coax
  • Clean resistors and capacitors
  • Replace DIP socket with gold plated DIP socket
  • Replace opamps with latest low-noise opamps
  • Test