When a VSWR self-test is initiated, U50-11, U26-2, and U8-8
generate a 0.5 millisecond logic LOW pulse at U8-8 output as
follows:
a. Initially, the output of U50-11 is LOW, the capacitor at
U26-1 is discharged, and the output of inverting Schmitt
Trigger U26-2 is HIGH. NAND gate U8-8 has one LOW
and one HIGH input, and its output is HIGH.
b. When the self-test pulse causes U50-11 to go HIGH, the
0.5 millisecond delay capacitor begins charging. U26-2
remains HIGH so that pins 9 and 10 of U8 are HIGH and
U8-8 goes LOW. The 10 msec self-test pulse then starts
the 0.5 msec logic LOW pulse at U8-8.
c. After 0.5 milliseconds, the charging capacitor triggers
inverting Schmitt trigger U26 and pin 2 goes LOW. Pin
10 of U8 is now LOW and the output(U8-8) goes HIGH,
ending the 0.5 msec logic LOW pulse.
Q.5.4.6.4
“VSWR Self-Test” Latch U42A
Refer to the simplified diagram, Figure Q-6, or to Sheet 2 of the
LED Board schematic. Latch U42A detects the self-test result
and “latches” the self-test status indication. Signals to the latch
include a DATA input from the VSWR logic, and CLEAR and
CLOCK inputs derived from the pulse generator. The Q and
NOT-Q outputs drive status indicator circuits.
When the VSWR self-test pulse goes to the Output Monitor, the
latch is also CLEARED. About 0.5 millisecond (500 microsec-
onds) later, the latch CLEAR-L is released. After another 100
nanoseconds the latch is clocked and the data at the latch D input
will be stored in the latch.
The Data input to the self-test latch is the output from NOR gate
U3-10 in the VSWR logic circuit, through invertor U39-12.
When a VSWR “hit” is detected, the Data input will be HIGH,
and when no VSWR fault is detected, the Data input will be
LOW.
A logic LOW at the “CLEAR” input clears the “self-test” latch,
and a LOW-to-HIGH transition at the CLOCK input “Clocks”
the latch. The 0.5 millisecond logic LOW pulse from U8-8 clears
the latch. When the pulse goes HIGH again the latch can be
clocked. A 100 nanosecond delay line, DL1, ensures that the
“Clear” input is high before the LOW-to-HIGH transition occurs
at the clock input.
When the CLEAR input goes LOW, the Q output goes LOW
and the NOT-Q output goes HIGH. This turns the green LED
“OFF” and turns the RED LED “ON” for 0.5 milliseconds, until
the latch is clocked. The VSWR Sensor “Status” LED will thus
flash RED each time a self-test is done.
A RED “VSWR SENSOR” status indication can only be reset
by performing a successful VSWR self-test, that is, by correct-
ing the VSWR logic fault and depressing the Manual Test
pushbutton again. Until then, the VSWR Sensor Status LED will
remain RED.
Q.5.4.6.5
VSWR Self-Test Passes (VSWR Logic Is Functioning)
If the simulated VSWR fault generated by the self-test pulse to
the Output Monitor is detected, the D input (pin 2) of U42A will
be HIGH when the latch is clocked, and outputs and indication
circuit conditions will be as follows:
a. Q Output: HIGH. Invertor U26-8 output: LOW. RED
LED Section: OFF.
b. NOT-Q Output: LOW. Invertor U39-10 output: HIGH.
GREEN LED section: ON.
Q.5.4.6.6
VSWR Self-Test Fails (VSWR Logic Has Failed)
If the simulated VSWR fault generated by the self-test pulse to
the Output Monitor is NOT detected, the D input (pin 2) of
U42A will be LOW when the latch is clocked and outputs and
indication circuit conditions will be as follows:
a. Q Output: LOW. Invertor U26-8 output: HIGH. RED
LED Section: ON.
b. NOT-Q Output: HIGH. Invertor U39-10 output: LOW.
GREEN LED section: OFF.
Q.5.5
Type 4 Fault Circuits
Type 4 Faults generate a PA OFF command which turns the
Power Amplifier stage OFF. The PA Power Supply remains
energized. Regulated voltage faults on the Analog Input Board
and Analog to Digital Converter are Type 4 Faults.
The Type 4 Fault sensing circuit includes a voltage regulator
“Fault Alert” output from the Analog Input board or Analog to
Digital Converter and voltage comparators on the LED Board.
The five Type 4 Fault circuits are described in the following
paragraphs. Figure Q-7 includes simplified diagrams of the supply
fault sensing circuit configurations used for Type 4 faults.
Q.5.5.1
“Supply Fault” Comparators On The LED Board
The Type 4 Fault comparators are shown on Sheet 5 of the LED
Board schematic, 839-7855-184, in the Drawing Package. The
comparators operate from a 5 VDC supply. When a
“fault” is sensed, the comparator output goes to +5 VDC through
a pull-up resistor.
The non-inverting input of each comparator is a fixed positive
reference voltage, derived from the Cont5 VDC regu-
lated supply. All “Type 4” comparator reference inputs are tied
together and come from a common voltage divider. If a supply
fault occurs the inverting input voltage goes below the reference
voltage and the comparator output goes HIGH.
Q.5.5.2
Regulator Fault Alert Outputs
For a description of regulator IC operation and faults which
cause a “fault alert”, refer to SECTION M, “DC Regulator.”
When there is no fault, the “Fault Alert” output is an open circuit.
When a voltage regulator senses a supply “fault,” the internal
“Fault Alert” transistor conducts, effectively connecting the
Fault Alert output to the regulator internal ground. For POSI-
TIVE supplies, this is the transmitter ground. For NEGATIVE
supplies, the internal regulator “ground” goes to the unregulated
negative input voltage through a small resistance.
Q.5.5.3
Analog Input Board: +15V Supply
The Analog Input Board +15 VDC supply “Fault Alert” output
circuit includes a capacitor and a pullup resistor. If the “Fault
Alert” goes LOW, the capacitor will discharge through the fault
DX-25U
Q-16
888-2297-002
Rev. R: 11-11-96
WARNING: Disconnect primary power prior to servicing.
