Teledyne 4020 - Hydrocarbon analyzer - Manual (Page 88)

Maintenance Model 4020
Teledyne Analytical Instruments 72
terminal 4 on the temperature control board P/N B30927, heater
wires are black, and check the resistance between that wire and
the remaining undisturbed terminal. If a reading of
approximately 100 Ohms, then the heating element is most likely
OK. If an open circuit is found, check the heater wires and a
possible connector between the heater and temperature control
board. If no problems are found, and the heater circuit is open,
then replace the heater element.
Note: If any of the components located inside the isothermal
chamber has failed, the instrument must be removed for
service. If no problems are found with either the thermistor
or the heater circuits, then replace the temperature control
board.
5.3 Ignition and/or Flame Guard Circuit Checks
If the flame guard circuit will not hold the flame-out lamp off when
the ignition procedure is employed (see section 4.5 Flame Ignition),
perform the following procedure to isolate the problem (consult the
system schematic for details of the circuit):
1. Disconnect the anode-igniter/flame guard thermistor cable from
the socket.
2. Check the flame guard sensing thermistor by measuring the
resistance between pins J4-3&4 of PCB part number B74671,
disconnect the cable plug. The reading should be about 100 KΩ at
room temperature. The actual resistance is not important, since the
thermistor experiences radical changes in resistance as the
temperature changes. No indication in a sufficiently high range on
the ohmmeter indicates an open thermistor. (If the thermistor is hot,
the resistance will be much lower.)
3. Check the anode-igniter coil for continuity by measuring
between pins J4-1&2 of the disconnected cable plug. The
ohmmeter should indicate a short circuit.
4. If either step 2 or 3 does not check as indicated, remove the
electrode assembly of the detection cell and replace it. If the
quartz flame tip is damaged, the top section of the cell may be
removed by disconnecting the vent line, and removing the screws
around its flange. Return the unit complete with attached