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400A Parts

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



Rosemount 400A parts list (similar to Beckman 400) (hydrocarbon analyzer)

Part Number Description
25B0001R0 400: Power supply cover
25B0002R0 400: Temperature Controller
400A-2008-0001 400A, 120 VAC on alarm bus
898816 400A: range relay {*}
623983 400A: Range Select Board
9385K61 400A: Foam Insulation
25C0008R0 400A: Display driver
400A ANALYZER 400A: Rosemount Analyzer
25A0001R0 400: Glow plug / igniter
25C0001R0 400A light up procedure
25C0007R0 400A: 4-20 mADC board
400A-ALARM 400A: alarm module
25C0005R0 400A: cable 16 conductor
25C0009R0 400A: Door, Inside plexiglass
25C0004R0 400A: high range capillary
25C0003R0 400A: mixed fuel capillary
25C0002R0 400A: pure fuel capillary
400A-REPAIR 400A: repair of analyzer
620423-REPAIR 400A: Repair of Preamp Board
73659 400: Burner Tip Assembly
630375 400: Cap assy w/glow plug
73658 400: collector assy
73674 400: Electrode kit
617900 400: Glow plug / igniter (*)
834341 400: Lamp, Pilot {*}
643459 400: SS GAUGE, 5psig
6A00324G01 400A: Main Elect Bd Updated
656888 400A: Air restrictor blue dot
620428 400A: BOARD, AMPLIFIER *
621031 400A: CABLE, 16 COND
655178 400A: Door,Inside plexiglas{*}
620446 400A: Flame Out Bd (Obsolete)
656921 400A: high range capillary {*}
860918 400A: igniter toggle switch{*}
630669 400A: Mixed fuel restrict red
620423 400A: Preamp Board
620443 400A: RANGE SWITCH BD
898904 400A: reed relay {*}
904731 400A: reed relay 5V, 1A {*}
898861 400A: relay, 24v {*}
876447 400A: SPAN POT, 30K {*}


25C0007R0 4-20 mADC isolated output board (replaces OEM 620433 & 621023)

Description: This is an isolated signal analog output card used industry wide. The analyzer’s potentiometric output (DC volts) signal drives the signal for the 4-20 mADC card (usually 0VDC input = 4 mADC output & 5 VDC input = 20 mADC output). The term “isolated” refers to the fact that the output signal is not referenced to earth ground and is therefore much less susceptible to interference from other voltage sources. Milliamp signals are very useful when running long signal loops (on the order of tens of feet to thousands of feet long; note that total loop resistance has to be less than 850 ohms).

Where used: 400A,755,755A,755R,870

Static sensitive: This board is not very susceptible to ESD but handle it with some ESD protocols.

Replaces: Rosemount 620433 (in kit p/n: 621023)

Compatibility: RIGAS has released its compatible version of the OEM board. Some nice enhancements were included: [1] hook-type test points, [2] better labeling, [3] heat sinks, [4] zero & span potentiometers can dual configuration (i.e., adjustable from the top or from the front).

Please specify potentiometer type when ordering. Choices: [1] end adjust or [2] top adjust

Typical failure modes (after many years of service): [1] total failure: most likely due to the fullwave bridge rectifier or output opamp. [2] non-linearity: most likely due to the charge pump circuit. [3] inability to calibrate: most likely due to the charge pump circuit. [4] noisy output: output opamp is failing –or– input signal is noisy –or– board mounted isolated power supply(ies) are failing –or– loop resistance is very high –or– it could be that your mADC reader (DCS, DAS, etc) is misbehaving.

Repairable: Yes! Our flat rate fee is $350 and assumes that we won’t have to replace the $250 Burr Brown isolation amplifier (we haven’t had to replace one yet!). Call us at 877-616-0600 for an RMA number.

** (Pricing is valid through October 1, 2013) **