Nautel XR12, Руководство по устранению неисправностей, Страница: 92 / 283

XR12 Troubleshooting Manual Detailed Circuit Descriptions
Page 2-24 Issue 3.0 2009-07-28
Comparator U6A, inverter U7A and their associated components form the low PA volts detector 'A'
circuit.
The PDM 1 signal applied to J2-2 is applied through R52 to an average detector formed by R58/C48.
The average detector's output, which will be a positive dc voltage, is applied to U6A's non-inverting
input. Since the
PDM 1 input is 0-15 V, varying pulse width, the magnitude of this voltage is
dependent on the on/off ratio of the pulses. The carrier level component of the PDM information,
will produce a mean voltage level between 1.5 V (minimum) and 6.0 V (maximum). Modulation
components are filtered out by capacitors C46 and C48.
The PA Volts (A) input applied to J1-5 is a dc voltage which is a derivative of the same PDM
information as the PDM 1 signal, but it is referenced to the B+ (330 V) voltage. The carrier level
component of the PDM information will produce a mean voltage level of 33 V (minimum) to 130 V
(maximum). A portion of the
PA Volts (A) input is applied to U6A's inverting input by dual voltage
divider R51/R56/R57. The voltage divider to +15 V formed by R56/R57, ensures a minimum of
3.0 V is applied to U6A's inverting input during initial turn on while the B+ power supplies are
ramping up to full power.
During normal operation, the PA volts sample applied to U6A's inverting input is more positive than
the reference voltage applied to its non-inverting input. U6A's output is a current-sink-to-ground and
cause U7A's output to be an open collector.
• 15 V is applied to J3-11, through pull-up resistor R68,
BLOCK A
LED DS3 and diode CR35,
as the current source for the thyristor in the PA input/output PWB's RF amplifier failure
circuit. The logic level of the
RF Amp Fail (A) signal is dependent on the status of the RF
amplifier failure detector circuit (see “RF amplifier failure detector” on page 2-28 ).
• 15 V is applied to the pwr module fail detector circuit, from pull-up resistor R68, as a logic
false block fail (A) output to the power module failure detector circuit; provided a ground
potential is not being applied to J3-11 as a logic true
RF Amp Fail (A) input.
When a modulator malfunction occurs that results in the PA volts being less than the expected level,
the PA volts sample applied to U6A's inverting input will be less positive than the reference voltage
applied to its non-inverting input. U6A's output will switch to an open collector. 24 V will be applied
through pull-up resistor R64/CR32 to U6A's non-inverting input and through R65 to U7A's input.
U6A will latch in its fault (open collector) state and remain there until it is reset by the application of
a ground potential reset command through CR26.