Troubleshooting Guide
3
Keysight 8990B Service Guide
67
Display Trouble Isolation
Check the backlight inverter voltages
Access the backlight inverter board by opening the front chassis as shown in
Figure 3-1
Check the backlight inverter voltages
There is an input connector at one side of the board and two output connectors on
the other end of the board, that power the two backlights which are inserted into
the flat panel display.
The output voltage is approximately 300 to 450 Vrms, 40 kHz (measured
differentially between the two wires) when the backlight is illuminated. The
voltage is approximately 1 kV before the backlight tube is illuminated. A red LED
on the backlight inverter board illuminates when the output voltage is present.
When the backlight goes off (when the peak power analyzer’s operating system
switches to screen saver mode) the voltage on pins 1 and 2 (with respect to
ground) slowly decays to 0 V.
The outputs are controlled by the input. The input pin 5 goes low to enable the
output voltage. These pins can be reached at J1 on the display board MP12.
Backlight inverter voltages
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