MORE positive than the Oscillator DC output voltage. When
the comparator’s non-inverting input is more positive, the com-
parator output goes logic HIGH. Because the output of the
Oscillator is inhibited when the transmitter is OFF, U22-11
inhibits a fault indication until the transmitter is ON.
Q.5.8.2
Buffer and Predriver Fault Sensing
The Buffer Amp sample and Predriver RF sample both come
from peak detectors on the Driver Combiner/Motherboard. Both
comparators on the LED Board have a positive reference voltage
at their non-inverting inputs. When RF is present, the inverting
input is positive and the comparator output is LOW; if a fault
causes loss of RF, the inverting input voltage goes below the
reference voltage and the comparator output goes HIGH.
Q.5.8.3
Status Indicator Circuits
The Status Indicator circuits are the same as those used for
“Supply Fault” (Type 4) Status indications. For each status
indicator, a logic LOW to the status indicator circuit illuminates
the GREEN section of the LED and a “Fault-H” illuminates the
RED section of the LED and provides a “Fault” status output to
the External Interface Board.
The only RED RF status indication will be for the first section
where RF is lost, because indicator “Inhibit” logic, including
invertors U7-2 and U7-4, and AND gates U6-3, U6-8, and
U6-11, blocks red “fault” indications for following sections.
Q.5.8.3.1
Oscillator Fault
An “Oscillator Fault-H” logic signal always goes to the Oscil-
lator status indicator circuit and causes a RED fault indication.
If the invertor output is LOW, the outputs of AND gates U6-3
and U6-8 are forced LOW no matter what the other gate inputs
are. The LOW output of gate U6-8 also forces gate U6-11 output
LOW. The “Buffer” and “Predriver” status indicator circuit
inputs remain LOW and both indicators remain green.
Q.5.8.3.2
Buffer Fault
The Oscillator comparator output is LOW and is inverted to hold
pins 2 and 6 of U6. A Buffer Fault causes gate U6-3 output to go
HIGH and causes a RED “Buffer” status indication. The LOW
inverted Buffer Fault signal at pin 9 of U6 forces output pin 8 LOW,
and the LOW input to U6-12 holds its output LOW so the Predriver
indicator remains GREEN.
Q.5.8.3.3
Predriver Fault
Both invertor inputs are LOW (no fault) so that both invertor
outputs are HIGH and U6-8 output is HIGH. A Predriver Fault
causes gate U6-11 output to go HIGH and gives a RED Predriver
“Fault” indication. It also causes invertor U7-6 output to go
LOW. A PA off signal also is generated by U55-15 through the
actions of U39-6, U6, and U40.
Q.5.9
OSC TEST switch S1
The Oscillator is normally held OFF when the PA Power Supply
is not energized. A “Turn-On Request” from the Controller will
enable the Oscillator. If switch S1 is pressed when the PA Power
Supply is OFF, the Oscillator output will be enabled. This is
useful for troubleshooting the RF drive chain with the PA Power
Supply OFF.
Q.6
Maintenance
Q.6.1
Printed Circuit Board Maintenance
Refer to SECTION IV, Maintenance, for general printed circuit
board maintenance procedures.
Q.6.2
Adjustments
Adjustments on the LED Board are identified in the table of
“Controls and Indicators.” All adjustments are described in
SECTION IV, Maintenance.
DX-25U
Q-20
888-2297-002
Rev. R: 11-11-96
WARNING: Disconnect primary power prior to servicing.
