Nautel XR12, Troubleshooting Manual, Page: 91 / 283

XR12 Troubleshooting Manual Detailed Circuit Descriptions
Issue 3.0 2009-07-28 Page 2-23
High temp detector
Comparator U2B in conjunction with two negative temperature coefficient thermistors mounted on
the module's heat sinks, form the high temp detector circuit. The thermistors are connected in
parallel and, with R1, form a voltage divider to +15 V. The voltage output of the voltage divider is
applied 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 impedance
of the thermistors will be more than 6000 ohms and the dc voltage applied to U2B's inverting input
will be more positive than +4.83 V. U2B's output will be a current-sink-to-ground, causing U3C's
output 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 parallel
impedance of the thermistors will be less than 6000 ohms. The dc voltage applied to U2B's inverting
input will be less positive than +4.83 V and cause U2B's output to be an open collector. +24 V will be
applied through pull-up resistors R16/R17 to U3C's input and cause U3C's output to switch to a
current-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. This
state will be maintained until +24 V is removed from R17, by the application of a ground
potential 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's
front 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 which
clamps the PDM (A)/(B)/(C) and (D) outputs to ground. All four low PA volts detectors will
be latched in their no-fault states (see “Low PA volts detector” ) and the modulators for all
four 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 be
removed from an on-air transmitter. The module's
Inhibit
switch applies a ground potential as the RF
inhibit command to the RF Inhibit input (J4-23) when it is pressed and released. This ground potential
simulates an excessive temperature and causes the high temp detector to latch in its fault state as
described previously.
Low PA volts detector
There 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.