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Troubleshooting Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 240
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 CO
2
MEASURE signal and CO
2
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 25
o
C 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.