R
OUTINE
S
ERVICE AND
M
AINTENANCE
7-2
April 2000 Rosemount Analytical 245364-U
Model 755A Oxygen Analyzer
Digital display reads correctly with standard gases but not with sample gas
, the
sample and sample-handling systems are suspect. Check these areas.
Digital display reads correctly with standard gases, but the alarm or output
devices do not,
these devices must be checked individually.
Digital display reads overrange or erratic with standard gases as well as with
sample gas,
the problem is probably in the detector or the electronic circuitry.
•
Overrange
- Turn power to analyzer OFF. Tap detector compartment with
fingers. Wait 30 seconds, reapply power. If the suspension within the detector
assembly is hung up, this procedure may correct the condition. If not, proceed
with tests of the detector and electronics.
•
Erratic
- Problem probably in the detector or the temperature control circuits.
Proceed with tests of the detector and electronics. Verification that all circuits
are operating properly should be performed before replacing detector.
7.2 DETECTOR COMPONENT CHECKS
7.2.1 D
ETECTOR
Before concluding that the detector is defective and must be replaced, verify that all
components and circuits that could cause erratic readings are operating properly.
To isolate the detector as a source of a problem, the source lamp, photocells and
suspension should be checked for proper operation.
7.2.2 S
OURCE
L
AMP
To verify that the source lamp in operating properly:
1. Verify that lamp is lit.
2. Voltage at U7 pin 2 should be 2.2
±
0.2 VDC.
If lamp is not operating properly, replace per instructions in Section 7.3.2.
7.2.3 P
HOTOCELL
To verify that photocell is operating properly, perform the following steps:
1. Keeping power source ON, disconnect the leads of the photocell from connector
J12. See Figures 7-1, 7-2.
2. Note the current measurement between the gray and orange wires (between 300
to 450 mA).
3. Measure between the orange and red wires. The reading should be approximately
the same as step 2.
If photocell readings not correct, replace photocell per Section 7.3.3.
