Rosemount 755, Instruction Manual

Page: 85 / 94

S
ERVICE AND
M
AINTENANCE
748183-H
7-11
Rosemount Analytical
December 1997
7.5 HEATING CIRCUITS
To ensure against damage from overheating in 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.
Each thermal fuse should be plugged in, not soldered, as the fuse element might melt
and open the circuit.
7.5.1 C
ASE
H
EATER
C
ONTROL
C
IRCUIT
The case heater control circuit receives power via thermal fuse F2 (setpoint 76
°
C).
This fuse, accessible on the case circuit board, may be checked for continuity.
Case heater element HR3, mounted on the heater/fan assembly, has a normal
resistance of 20 ohms.
To verify heater operation, place a hand beside the right hand side of the detector
housing. Heated air 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 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 plug P8 on the control board assembly, 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.
Do not operate for long periods of time with decade box set for 22.2K
ohms, as overheating of equipment may result.
Set the decade box for 20.2K ohms to simulate RT1 resistance at controlling
temperature. The voltmeter should now show pulses of 1 VAC.
Set the decade box for 22.2K ohms to simulate RT1 resistance at ambient
temperature; the voltmeter should now show pulses of 120 VAC.
7.5.2 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 resistance of HR2 alone.
CAUTION: EQUIPMENT OVERHEATING