Nautel XR12 Troubleshooting Manual

Also see for XR12: Installation manual Operation and maintenance manual Preinstallation manual

Contents

XR12 Troubleshooting Manual Detailed Circuit DescriptionsIssue 3.0 2009-07-28 Page 2-23High temp detectorComparator U2B in conjunction with two negative temperature coefficient thermistors mounted onthe module's heat sinks, form the high temp detector circuit. The thermistors are connected inparallel and, with R1, form a voltage divider to +15 V. The voltage output of the voltage divider isapplied to the inverting input of U2B. It is compared to a 4.83 V reference, from voltage divider R10/R12, which is applied to U2B's non-inverting input as the high temperature fault threshold.When the temperature sensed by both thermistors is less than a nominal 85°C, the parallel impedanceof the thermistors will be more than 6000 ohms and the dc voltage applied to U2B's inverting inputwill be more positive than +4.83 V. U2B's output will be a current-sink-to-ground, causing U3C'soutput to be an open collector. The high temp detector circuit will have no influence.When the temperature sensed by one or both thermistors is greater than a nominal 85°C, the parallelimpedance of the thermistors will be less than 6000 ohms. The dc voltage applied to U2B's invertinginput will be less positive than +4.83 V and cause U2B's output to be an open collector. +24 V will beapplied through pull-up resistors R16/R17 to U3C's input and cause U3C's output to switch to acurrent-sink-to-ground. This current-sink-to-ground will be applied to:• U2B's inverting input, through CR12, and cause it to latch in the high temp fault state. Thisstate will be maintained until +24 V is removed from R17, by the application of a groundpotential reset command to the junction of R16/R17, by the reset pulse generator (see“Reset pulse generator” on page 2-25).• HIGH TEMP LED DS7, through R103. The TEMP/INHIBIT alarm lamp on the module'sfront panel will turn on.• through CR14, to the inverting input of U2A. The output of U3G, also a current-sink-to-ground, is applied to CR27, CR29, CR41 and CR43, as a logic true Pwr Mod Fail signal whichclamps the PDM (A)/(B)/(C) and (D) outputs to ground. All four low PA volts detectors willbe latched in their no-fault states (see “Low PA volts detector”) and the modulators for allfour power blocks will effectively be turned off.The high temp detector is also used to inhibit operation of the RF power module when it is to beremoved from an on-air transmitter. The module's Inhibit switch applies a ground potential as the RFinhibit command to the RF Inhibit input (J4-23) when it is pressed and released. This ground potentialsimulates an excessive temperature and causes the high temp detector to latch in its fault state asdescribed previously.Low PA volts detectorThere are four identical low PA volts detectors, one for each power block. Since they are all identical,only the circuit for power block A is described.