d. Troubleshooting the Temperature Compen-
sator Circuits. There are three temperature com-
pensators in the tester. One is on the calibrator
board, one is on the temperature board, and one is
on the probe control and function switch board.
With power turned off, check the resistance of the
precision 34K dropping resistor; R5 (19, fig. 4-36)
on the calibrator board, R64 (28, fig. 4-34) on the
temperature board, or R2 (43, fig. 4-37) on the probe
control and function switch board. Do this by lifting
one end of the resistor and measuring it with a digi-
tal multimeter. It shell measure 34K ± 34 ohms at
25° C (77°F). With the end of the resistor still up,
measure the resistance of the compensator between
To prevent electrical shock or dam-
age to test equipment, use an isolation
transformer to power the oscilloscope
or tester when troubleshooting the
secondary winding of T1 on the probe
control and function switch board or
the SCR circuits.
Suspend the probe away from flam-
terminals B and E. The compensator shall measure
33.2 ± 0.4 ohms at 20°C (68°F).
Turn the TEMPERATURE switch to HEATER
PROBE TEMP SET, and adjust PROBE CONTROL
e. Troubleshooting Heater Probe Control Cir-
to an elevated temperature (not to exceed 600º C).
cuit. When troubleshooting the heater probe con-
Use an oscilloscope to check the waveforms of table
trol circuit, open the probe controller assembly per
4-14. Test points are identified in figure 4-5 and on
paragraph 4-7a (2) (a). Connect the BH405 heater
the probe controller schematic (fig. FO-2).
cable to the tester and connect a known good heater
probe to the heater cable receptacle.