Teledyne gfc 7000ta operation manual Excessive Noise

Troubleshooting Model GFC7000TA Carbon Dioxide Analyzer

10.4.2. Excessive Noise

Noise is continuously monitored in the TEST functions as the STABIL reading and only becomes meaningful after sampling a constant gas concentration for at least 10 minutes. Compare the current STABIL reading with that recorded at the time of manufacture (included in the GFC 7000TA Final Test and Validation Data Sheet-p/n 04271 shipped with the unit from Teledyne Instruments).

1.The most common cause of excessive noise is leaks. Leak check and flow check the instrument described in Section 9.3.

2.Detector failure – caused by failure of the hermetic seal or over-temperature due to poor heat sinking of the detector can to the optical bench. In addition to increased noise due to poor signal- to-noise ratio, another indicator of detector failure is a drop in the signal levels of the CO2 MEASURE signal and CO2 REFERENCE signal.

3.Sync/Demod Board failure. There are many delicate, high impedance parts on this board. Check the CO2 MEAS and CO2 REF Test Functions via the Front Panel Display.

4.The detector cooler control circuit can fail for reasons similar to the detector itself failing. Symptoms would be a change in MR RATIO Test Function when zero air is being sampled.

Also check the SIGNAL I/O parameter PHT DRIVE. After warm-up, and at 25oC ambient, if PHT DRIVE < 4800 mV, the cooler is working properly. If PHT DRIVE is > 4800 mV there is a malfunction.

5.The +5 and ±15 VDC voltages in the GFC 7000TA are provided by switching power supplies. Switch mode supplies create DC outputs by switching the input AC waveform at high frequencies. As the components in the switcher age and degrade, the main problem observed is increased noise on the DC outputs. If a noisy switcher power supply is suspected, attach an oscilloscope to the DC output test points located on the top right hand edge of the Relay board. Look for short period spikes > 100 mV p-p on the DC output.

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