Instruction Manual

748213-S April 2002

Model 755R

2.Adjust R29 clockwise and counter- clockwise. The display should follow accordingly and remain steady within the adjustment limits of R29. If this condition is met, refer to Section 6-6a (page 6-7) for Control Board setup. Before replacing the Control Board, test for -15V at the junction of C1/J4- 7. Use the junction of CR1/R2 for

+15V, or any source of ±15V on the board for the respective voltages.

3.If adjustment of R29 is not possible, replace the Control Board.

Alarms

Set RANGE Select to lowest range or use zero and span gases.

Current Output

Set RANGE Select to lowest range or use zero and span gases.

When checkout complete, re-install Detector Isolation Plug. Configure Control Board to original setup.

If the Control Board functions correctly, the problem is either located in the De- tector/Magnet Assembly or related to temperature control.

6-2 HEATING CIRCUITS

To ensure against damage from overheating in the event of malfunction, the heating circuits receive power via thermal fuses F2 and F3. If temperature of a heated area exceeds the permissible maximum, the associated fuse melts, opening the circuit.

NOTE:

The thermal fuses should be plugged in, NOT SOLDERED, as the fuse element might melt and open the circuit.

a.Case Heater Control Circuit

The case heater control circuit receives power via thermal fuse F2 (setpoint

75°C). This fuse, accessible on the Power Supply Board, may be checked for continuity.

Detector compartment heater element HR3, mounted on the heater/fan assem- bly, has a normal resistance of 20 ohms.

To verify heater operation, carefully place a hand on top of detector compartment. Heat should be felt. If not, check the case heating circuit.

Temperature sensor RT1 has a cold resistance of 22.7K ohms and a normal operating resistance of 20.2K ohms, indicating normal operating temperature.

As a further check, disconnect plug P6 on the Control Board, thus disconnecting temperature sensor RT1. Substitute a decade resistor box to simulate the resistance of RT1. Also, connect an AC voltmeter from the hot side of the line to the neutral side of F2, located inside the detector compartment.

Set the decade box for 20.2K ohms to simulate RT1 at controlling temperature. The voltmeter should show pulses of 1 VAC.

CAUTION

OVERHEATING

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 tem- perature. The voltmeter should show pulses of 120 VAC.

6-3 DETECTOR/MAGNET HEATING CIRCUIT

Heater HR1 is attached to the magnet. Heater HR2 is attached to the rear of the de- tector. Combined resistance of these two parallel-connected heaters, as measured at

6-2

Maintenance and Service

Rosemount Analytical Inc. A Division of Emerson Process Management