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400A Hydrocarbon Analyzer

Beckman 400, Beckman 400A, Rosemount 400A Total Hydrocarbon Analyzer

This particular analyzer is one of RIGAS’ favorites to work on (repair).

Here is a list of some of our checks that we perform before we release your analyzer for shipment:

  1. Noise on range X1 should be minimal (±1 count)
  2. All ranges should be correlated (especially those with an optional range trim board)
  3. Response to certified calibration gas should be equal to or better than factory specifications
  4. When TP5 = 5 VDC then [a] the display reads 100.00, [2] analog output is either 100mV or 1V or 5V (depending on jumper configuration, [3] isolated current output = 20 mADC
  5. Analyzer is clean
  6. Insulation is re-glued
  7. Frayed wires are fixed
  8. Plexiglass is grounded
  9. A/C power switch leads are coated with “liquid electrical tape” insulation
  10. Ribbon cables are pristine
  11. Rear terminal strip has a sticker that shows what functions are on what terminals
  12. Zero pot (potentiometer) is greater than 50% (the higher the number here, the less contamination in the analyzer tubing)
  13. It needs to look good again (paint job if necessary)
  14. Flame safety circuits MUST work properly
  15. Backpressure regulator works as designed (and controls a very reliable value)
  16. There’s a bunch more, but we don’t want to bore you with the entire list!


Give us a try by calling us at 877-616-0600.

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 Hydrocarbon Analyzer

What a great little machine this is! The Rosemount 400A is a workhorse! Its only downfall is the same as any other THC analyzer … contamination. Call us for parts or service.

Typical failures are:

  • Clogged restrictors (makes it hard to light and/or easy to flame-out)
  • Clogged sample capillary (range switch has to be set lower to compensate for less sample being burned)
  • Burner contact assembly is burned off (noisy display and no response to cal gas)
  • Contamination (zero potentiometer will be very LOW; normal zero pot is > 6.0 turns)
  • Preamp board failed (many symptoms; won’t respond to cal gas or won’t change from high gain to low gain)
  • Thermistor failed (fuel solenoid won’t latch open when ‘purge’ switch is released)

Not so typical failures include:

  • 90 VDC (polarizing voltage) failure (no response to cal gas)
  • ± 12 VDC failed (display won’t be lit)

Here is a Microsoft Excel spreadsheet that will help you calculate the analyzer’s response to various hydrocarbons.   Please call if you need help getting started with this.   Click here to download the calculator:  RIGAS 400A calculations

Pictures: 400A_front_view

Parts (recommended): RIGAS 400A parts list

Manual: Rosemount 400A manual rev L

Rosemount 400A components get hot on Main Electronics Board

Affected components:

  • 620428 Rosemount 400A Main Electronics Board (schematic 620429)
  • 620433 Rosemount 400A Isolated 4-20 maDC board (schematic 620434) … (item is now obsolete by OEM … replaced by RIGAS 25C0007R0)

Situation: The analyzer appears to operate normally (mostly anyway). U13 seems to get very hot (so does U4 [voltage output buffer amplifier]) and there is a 620433 (V/I option board) installed. Also, when attempting to light the analyzer, the analog display will overrange and remain overranged until the power is cycled on the analyzer (TP-5 will be saturated at about 13 vdc).

Problem: someone has employed the E1-E2 and E3-E4 jumpers and this is causing a nasty feedback loop that U13 is trying to compensate for. When the analyzer goes upscale (it always spikes during startup) this causes the analog signal to spike which, in turn, causes the 4-20 maDC card to spike. The 4-20 signal being fed back to U13 has now locked it railed high.

Resolution: remove the 4-20 maDC board or remove the E1-E2 and E3-E4 jumpers.

Other info: U12 & U13 & U4 are µA714 opamps (a.k.a., uA714); they can be replaced with OP07 or OP77 or OP177.