Teledyne 4020 – Hydrocarbon analyzer (special version for automatic background gas switching) - Manual (Page 95)

Total Hydrocarbon Analyzer Maintenance
Teledyne Analytical Instruments 77
standoffs, instead of directly to the printed circuit, to eliminate the
possibility of leakage currents.
To eliminate any possibility of contamination of the insulating
materials employed, the completed PC board is ultrasonically cleaned in
laboratory grade alcohol. Under no circumstances should the parts
described be handled with bare fingers. A freshly-scrubbed finger,
stroked along one of the glass resistors, would deposit enough skin oil to
completely upset the range division of the attenuator circuit.
Resistor R3 is a 1000 MΩ resistor used in the feedback circuit of the
amplifier. R2 has a resistance of 10,000 MW and is used in series with
the zero potentiometer slider. This circuit is used to nullify any offset
signal introduced by the signal electrode. Trimmer P1 is used to nullify
the offset signal generated by the electrometer amplifier.
The output of the circuit is standardized against gases with known
hydrocarbon concentrations by zero and span calibration, so that the
meter and/or recorder indicates the hydrocarbon concentration of the gas
being used.
The positive and negative operating voltage required by the
electrometer amplifier is furnished by a switching power supply
circuit,mounting at the back panel of the analyzer
The stability of the electrometer circuit can be tested as follows:
1. Disconnect the collector cable.
2. Place the analyzer in the auto range.
3. Adjust zero offset value so that the recorder reads at some
point upscale, and record a 24 hour chart.