Teledyne 4020 – Hydrocarbon analyzer (special version for automatic background gas switching) - Manual (Page 89)

Total Hydrocarbon Analyzer Maintenance
Teledyne Analytical Instruments 71
intermittent open circuit. If there is, replace the cable. Do not attempt to
repair the cable, as special tooling is required to disassemble and
reassemble the cable plugs.
5.2 Temperature Control Electronic Check
If the heating circuit fails, the output of the analyzer will tend to drift
with changes in ambient temperature. Such a failure will be more
evident in the low range. If the temperature environment surrounding the
analyzer is closely regulated, failure in this circuit might go unnoticed
after the initial failure. If the environment follows day and night
temperature changes, the analyzer will show a diurnal, bi-directional
drift when operated on zero gas. The magnitude of the drift will be a
function of the temperature differential experienced by the analyzer. To
check the circuit, employ the following procedure:
Consult the 4020 schematic and assembly drawings, as well as the
temperature control PC board schematic and assembly drawings at the
rear of the manual for circuit details and component placement.
An indicator light on the control panel cycles on and off with the
heating element; the light is on when the heater is on, and vice versa.
Failure of the light to come on at all when the cell compartment is cold
indicates a problem in the temperature sensing or control circuitry or the
wiring that interconnects the thermistor to the circuit. If the light stays
on constantly, but the compartment does not heat up, then a problem
with the heating element or connecting wiring is indicated.
1. Check the sensing thermistor by measuring the resistance
between its connecting wires. Disconnect one of the thermistor
wires from terminal strip on the temperature controller board,
P/N B30927, the wires out of the thermistor are yellow, and
measure resistance between that wire and the remaining
undisturbed terminal. Resistance of the thermistor varies with its
surrounding temperature. A reading of between 10KOhms and
16Kohms at around 25 °C may be measured. (Under very cold
conditions, the resistance could be as high as 50KOhms; under
hot conditions, just a few thousand ohms.) If the thermistor
measures anywhere in this range, it is most likely OK.
Otherwise, if the circuit is short or open, check the wires leading
to the thermistor,
2. Check the heating element by measuring its resistance.
Disconnect one of the heater wires from either terminal 2 or