Section 4
OM6636-B-00
12 February 2015
4-2
has exceeded 34 °C 3 C, either from a high ambient temperature or failure of the
control circuit. Proceed to paragraph 4.3.3. Check the internal temperature with the
imbedded PRT to compare the calculated temperature with the value given in the Report
of Calibration.
4.3.2.
Temperature Control
If the temperature is low and the HEATER indicator is lit, check the continuity and
resistance of the heater circuit.
If the temperature is low and the HEATER indicator is lit, check the operation of the
control circuit as follows:
1)
Locate Rl connected from pin C to pin D of P2.
2)
Short out Rl and the HEATER indicator should light. If not, then check for a fault in
the control circuit. If the indicator lights, then check the resistance of Rl and the
thermistor pair RT1/RT2. Rl should be 3 kΩ to 5 kΩ. RT1/RT2 is equal to Rl at
30 °C, and is larger than R1 if the temperature is less than 30 °C. It has a temperature
coefficient of approximately -4 % per °C. If necessary, replace Rl and RT1/RT2 as a
matched pair.
Control circuit board testing - Measure the voltage at the pins of connector P1 as
indicated in Table 4-1.
Between PIN
And PIN
Should Read
Components To Check
20
16
14.0 V 60 Hz
T1
21
16
14.0 V 60 Hz
T1
7
16
-12 VDC
CR106, CR107
9
7
-3.2 V
see NOTE 1
10
7
-3.2 V
see NOTE 1
12
7
-3.2 V
see NOTE 1
14
7
-0.3 V
-0.7 V
when heater off
when heater on: see NOTE 2
Table 4-1: Control Circuit Board Testing
NOTE 1: If this reading is incorrect, then check the voltage across C101, 470 F capacitor. It
should read 8.2 V 5 %. If not, then check R107, CR101, CR102 and C101. If
correct, then check R102, R105, C103 and Q101.
