Model 755A
Instruction Manual
a. Photocell Replacement and | a. Case Heater Control Circuit |
Adjustment |
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1.To remove the photocell from the optical bench, perform steps 1 through 5 in Section
2.Install replacement photocell by reversing removal procedure.
3.The photocell must now be adjusted. With zero gas flowing:
a.Place a digital voltmeter on the wiper of the front panel ZERO control (R13) and ground (TP7) ground on the Control Board. Adjust front panel ZERO control for 0.0 VDC.
b.Place the voltmeter from the bottom of R10 and TP7 (see Figure
c.Place the voltmeter on TP8 and TP7, move the photocell to obtain a DC voltage as close to 0mV as possible but not more than ±750mV.
d.Perform steps 8 through 11 in Section
With all internal adjustments now properly set, the instrument may be calibrated in the normal manner.
To ensure against damage from overheating in the event of malfunction, the heating circuits are protected by thermal fuses F2 and F3. If temperature of a heated area exceeds the permissible maximum, the associated fuse melts, opening the circuit.
NOTE
Thermal fuses F2 and F3 are to be plugged in, never soldered. If soldered, the fuse element may melt and open the circuit.
The case heater control circuit receives power via thermal fuse F2 (76°C). This fuse, located on the Case Board, can be checked for continuity.
Case heater element HR3, located on the heater/fan assembly, has a normal resistance of 20 ohms.
To verify heater operation, place a hand beside the
Temperature sensor RT1 has a cold resistance of 22.7K ohms and a normal operating resistance of 20.2K ohms, indicating normal operating temperature of 140°F (60°C). Until thyristor RV1 reaches operating temperature, it bypasses most of the current that would otherwise flow through Triac Q7.
As a further check, disconnect connector plug P8 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.
Set the decade box for 20.2K ohms to simulate RT1 resistance at controlling temperature. The voltmeter should now show pulses of 1 VAC.
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.
Rosemount Analytical Inc. A Division of Emerson Process Management | Service and Maintenance |