Teledyne GFC-7000E - Trace CO2 Analyzer - Manual (Page 208)

Model GFC7000E Instruction Manual TROUBLESHOOTING & REPAIR PROCEDURES
04584 Rev A1 197
11.3.1. Temperature Problems
Individual control loops are used to maintain the set point of the absorption bench, filter wheel,
and IR photo-detector temperatures. If any of these temperatures are out of range or are poorly
controlled, the MGFC7000E will perform poorly.
11.3.1.1. Box or Sample Temperature
Box Temperature
The box temperature sensor is mounted to the motherboard and cannot be disconnected to check
its resistance. Rather check the BOX TEMP signal using the SIGNAL I/O function under the
DIAG Menu (see Section 11.1.3). This parameter will vary with ambient temperature, but at
~30
o C (6-7° above room temperature) the signal should be ~1450 mV.
Sample Temperature
The Sample Temperature should closely track the bench temperature. If it does not, locate the
sensor, which is located at the midpoint of the optical bench in a brass fitting. Unplug the
connector labeled “Sample”, and measure the resistance of the thermistor; at room temperature
(25°C) it should be ~30K Ohms, at operating temperature, 48°C, it should be ~ 12K Ohms
11.3.1.2. Bench Temperature
There are three possible failures that could cause the Bench temperature to be incorrect.
1. The heater mounted to the bottom of the Absorption bench is electrically shorted or open.
Check the resistance of the two heater elements by measuring between pin 2 and 4 (~76
Ohms), and pin 3 and 4 (~330 Ohms), of the white five-pin connector just below the sample
temperature sensor on the Bench (pin 1 is the pointed end).
2. Assuming that the I 2 C buss is working and that there is no other failure with the relay board,
the solid-state relay (K2) on the relay board may have failed. Using the BENCH_HEATER
parameter under the signal I/O function, as described above, turn on and off K2 (D3 on the
relay board should illuminate as the heater is turned on). Check the AC voltage present
between pin 2 and 4, for a 100 or 115 VAC model, and pins 3 and 4, for a 220-240 VAC
model.
WARNING:
HAZARDOUS VOLTAGES ARE PRESENT DURING THIS TEST
3. If the relay has failed there should be no change in the voltage across pins 2 and 4 or 3 and 4.
Note, K2 is in a socket for easy replacement.
4. If K2 checks out OK, the thermistor temperature sensor located on the optical bench near the
front of the instrument could be at fault. Unplug the connector labeled “Bench”, and measure
the resistance of the thermistor. At room temperature it should have approximately 30K
Ohms resistance, near the 48 o C set point it should have ~11K ohms.