Teledyne 4020 – Hydrocarbon analyzer (special version for automatic background gas switching) - Manual (Page 86)

Maintenance Model 4020
Teledyne Analytical Instruments 68
attempt to evaluate performance on sample gas. If analyzer sensitivity is
questionable, use the span gas. For all other evaluations, use the zero gas
and low range for maximum sensitivity. The important consideration is
to control as many variables as possible. Using cylinder-supplied gases
of known hydrocarbon content eliminates the possibility of introducing
an unknown variable.
Do not overlook the seemingly obvious. Check to see that power is
available for the instrument (and of the proper voltage, etc.), and that
connections are correct. Also verify that support/calibration gases are
not depleted.
5.1 Measuring Circuit Electrical Checks
If the analyzer performs erratically on zero gas, the trouble can be
related to either the integral gas control systems, or the electronics. To
isolate the problem, the two systems must be separated. To isolate the
electronics, employ the following procedure:
1. Pull analyzer out of the drawer, remove sample system top cover,
remove and disconnect the collector cable from the sensor
leaving it attached to the electrometer board. (Consult schematic
and assembly drawings for circuitry and location). With this
cable disconnected, the electronic circuitry is completely isolated
from the gas control system and cell.
2. Select the auto measurement range (for maximum sensitivity)
and adjust the zero offset value until the readout device indicates
above midscale. (The span control should already be set)
5.1.1 Loss of Zero Control
If the zero cannot be controlled as directed above, a failure has
occurred in either the power supply or the electrometer-amplifier. Use
the following procedure to isolate the fault:
With a digital voltmeter (DVM) set to read 15 VDC, check the
positive and negative 15 V output of the power supply as follows:
(See schematic D65506 drawing of assembly PCB C65507A for
power supply PC board).
1. Connect the voltmeter leads across tantalum capacitor C1. Note
the positive polarity is marked on the PCB silkscreen. The
measured voltage should be 15±1 volt.