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Maintenance
6–32
THS710A, THS720A, THS730A & THS720P Service Manual
Follow these steps to troubleshoot a nonfunctional backlight.
1. Remove the front cover using the procedure Front Cover on page 6–11.
2. Turn the instrument on, remove the switch mat, and then fold the switch
flex-circuit assembly toward you to access the inverter board.
WARNING. To avoid electrical shock, do not touch the two-wire output connector
of the inverter board. The output connector is the one closest to the display
module.
3. Using a DMM, measure the DC voltage across pins 1 and 2 of the input
connector on the inverter board. Follow the instructions in the table below.
Voltage Measurement at Inverter
Input Connector
Instructions
Approximately 0 V
DC
Check that the inverter input cable is properly
connected at both ends.
Fold the switch flex-circuit assembly back over the
instrument. Use a
1
@
8
inch flat-bladed screwdriver to
lift the lower edge of the chassis assembly out of
the back cover until you access the edge of the
main board.
Remove the inverter input cable from the main
board. Measure the DC voltage on pins 1 and 2 of
the connector on the main board.
If the measured voltage is approximately 12 V
DC
,
the inverter input cable is defective. Replace it.
If the measured voltage is approximately 0 V
DC
, the
main board is probably defective. Replace it.
Approximately 12 V
DC
Proceed with step 4.
4. Using an oscilloscope and 10X probe, attach the probe ground lead to the
ground terminal on the right side of the instrument. Set the oscilloscope time
base to 1 ms/div and vertical sensitivity to 1 V/div.
NOTE. The inverter output voltage is approximately a 250 V
RMS
, 40 kHz sine
wave. Capacitive coupling to the oscilloscope probe tip is sufficient to determine
if the inverter output voltage is present.
Backlight