R
OUTINE
S
ERVICE AND
M
AINTENANCE
245364-U Rosemount Analytical April 2000
7-9
Model 755A Oxygen Analyzer
CAUTION: OVERHEATING HAZARD
Avoid prolonged operation with the decade box set at 22.2K ohms. Overheating
may result.
Set the decade box for 22.2K ohms to simulate RT1 resistance at ambient
temperature. The voltmeter should now show pulses of 120 VAC.
7.4.2 D
ETECTOR
/M
AGNET
H
EATING
C
IRCUIT
Heater HR1 is attached to the magnet. Heater HR2 is attached to the rear of the
detector. Combined resistance of these two parallel connected heaters, as measured
at pins 15 and 16 of detector connector J12, should be approximately 17 ohms. If not,
remove pin/leads 14 and 15 from the connector to measure the resistance of HR2
alone. This resistance should be approximately 89 ohms.
If resistance the resistance of HR2 is correct, and yet the combined resistance is
incorrect, heater HR1 may be open.
To reach the leads of HR1, remove the printed circuit board on the heater assembly.
Resistance of HR1 should be approximately 21 ohms.
To check operation of the heater circuit, connect a voltmeter across R61 on the Case
Board. Normally, the voltage will be 4 VDC when cold and will drop to approximately
0.4 VDC at control temperature. Temperature sensor RT1 is mounted in the detector,
with leads accessible at pins 10 and 11 of detector connector J12. The sensor
resistance, as measured at these pins, should be 1M ohm at 25
°
C and approximately
149K ohms at operating temperature of 65
°
C.
