6-14 888-2408-002
WARNING: Disconnect primary power prior to servicing.
Troubleshooting
6.9 Master Controller Related Faults
The following is a listing of the faults which are handled by or merey related to the
Master Controller. A quick reference for each of these faults is given in Table 6-3 on
page 6-35.
6.9.1 THERMISTOR, Fault
The Thermistor fault is detected if a thermistor temperature reading is 5° C below
the ambient temperature, provided the ambient temperature is above 25° C. Can be
caused by an actual open thermistor but is more likely an open connection between
the thermistor and the controller board monitoring the thermistor.
This fault protects against a thermistor (temperature sensor) circuit being open. The
thermistors are used on all amplifiers, iso loads, and the power supplies to sense
temperatures and use pull up resistors to +V. As the temperature increases, the
resistance of the thermistor goes down, lowering the voltage on a detector input,
which gives an increased temperature reading. If the thermistor circuit becomes
open, the detector input goes high which gives a cold temperature reading. This
leaves the device to which the thermistor is connected, unprotected from an over-
temperature condition. Once this fault is detected it will stay active in the log until
the Fault Log is reset.
NOTE:
If one of the thermistors or connecting cables is shorted, a temperature fault
for the device it is connected to will be registered in the Fault Log, not a
THERMISTOR fault. A shorted thermistor or cable will give a very hot read-
ing which will not change, about 155° C or higher. If the thermistor for one of
the PA ISO resistors has shorted the fault “PA_ISO_OT” is triggered and the
transmitter will be shut off (see PA_ISO_OT under PA Controller Faults).
Troubleshooting:
a. First, locate the problem thermistor by systematically looking at all of the
temperature readings. One of them will show a # sign instead of a tempera-
ture. The temperatures to look for include:
1. IPA Temps - IPA_AB1 and IPA_AB2. The thermistor is located on the
IPA right next to the FETs.
2. PA Temps for A1-A8 and B1-B8. The thermistor is located on the PA
right next to the FETs.
3. ISO Resistor Temps for A1-A8 and Az / B1-B8 and Bz. These are all
located on the Isolation Boards.
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