Teledyne 9700 Instruction Manual page 42

5 Maintenance & Troubleshooting
The best way to check Q1 is to remove it, then check it, or
substitute a replacement .
The meter drive circuitry, as shown on Dwg. B-l0913, is
straightforward and relatively simple to follow. If any troubles occur
in this circuitry, consult the schematic diagram.
The heater (H1) control is controlled by SW3, R15 and the
Triac (SCR1). During various periods of the heater cycle, both R15
and SW3 are required to carry a heavy load. Consequently, it is
possible for either component to fail.
The Triac can fail as well. If it fails in the "open" condition
there will be no heat from the heater. If it fails in the "closed"
condition, the heater will be on all the time. Heater malfunction will
probably be evident with calibration problems involving the
combustibles sensor, since the heater is installed in a compartment
with this particular sensor.
5.2.3 Combustible Analyzer Section
Refer to drawing C-11751 for the following discussion.
5.2.3.1 Inability to Calibrate
The most common cause of an inability to zero and/or span
the instrument is detector failure. The combustible detector will drift
in both span and zero with time. However, if it fails, it is most likely
to be an abrupt phenomenon, unlike the oxygen cell which "dies"
slowly. Upon failure, the output of the instrument will go up or down
full scale depending upon which bead fails.
The standard combustible analyzer used in the Model 9700
system includes a failure alarm circuit that is described in Section 2.2.
As described, a failure will cause the meter to deflect full scale down-
ward (pegged to the lefthand side of the meter) and the alarm will be
energized. This situation will indicate a sensor failure.
The sensor circuit can be checked either with a new sensor or
a dummy sensor, constructed with two 7.5 Ohm 3 to 5 Watt resistors
installed between pins 14 and 15, and 14 and 13 (see Dwg. C11751).
With the dummy sensor, the zero and span potentiometers will both
function and the and the electronics can be checked. If the span and
zero controls operate satisfactorily with the dummy sensor, then the
5-6 TELEDYNE BROWN ENGINEERING
Analytical Instruments
Model 9700