Nautel XR12, Troubleshooting Manual, Page: 93 / 283

XR12 Troubleshooting Manual Detailed Circuit Descriptions
Issue 3.0 2009-07-28 Page 2-25
U7A's output will switch to a current-sink-to-ground. A ground potential will be applied to the
junction of DS3/CR35 through CR34. The 15 V current source will be removed from J3-11. The PA
input/output PWB's RF amplifier fault detector circuit will be reset if it was activated. Its fault
detection capability will be inhibited as long as the junction of DS3/CR35 is a ground potential (see
“RF amplifier failure detector” on page 2-28 ).
Modulator short circuit detector
There are four identical modulator short circuit detectors, one for each power block. Since they are all
identical, only the circuit for power block A is described.
Comparator U10A and its associated components form the modulator short circuit detector. A
+15 V reference voltage is applied to U10A's non-inverting input. The
PA Volts (A) input (J1-5) is a
dc voltage that is representative of the PDM1 signal. A portion of the PA Volts (A) input is applied to
U10A's inverting input by dual voltage divider R51/R56/R57.
During normal operation, the PA voltage sample applied to U10A's inverting input will be less
positive than the reference voltage applied to its non-inverting input. U10A's output will be open
collector.
When a modulator malfunction occurs, which results in the PA voltage being the same as the B+
voltage (i.e., a shorted FET in the modulator), the PA voltage sample applied to U10A's inverting
input will be more positive than the reference voltage applied to its non-inverting input. U10A's
output will switch to a current-sink-to-ground, causing the
MOD SHORT
LED (DS9) to turn on.
Reset pulse generator
Transistor Q1 and inverter U3A are connected as a one shot pulse generator. When a momentary
ground potential is applied to J4-10 as a logic true
Reset PM command, Q1 will turn on. 5 V will be
applied to the input of U3A and cause its output to switch to a current-sink-to-ground while Q1 is
on. This current-sink-to-ground will:
• Remove the 24 V pull-up voltage from the junction of R16/R17 and effectively clamp U3C's
input to ground potential. U3C's output will be clamped to its open collector state for the
reset pulse period. If U2B was latched in its high temp fault state, U3C's open collector state
will unlatch U2B and reset the high temp detector.
• Remove the 24 V pull-up voltage from the junction of R16/R15 and effectively clamp U2D's
output to ground potential for the reset pulse period. If U2D was latched in its low RF drive
fault state, the removal of the 24 V pull-up voltage will unlatch it. The latching low RF drive
detector will be reset.