Filter
Top Products
Theory of Operation
3–82
AM700 Audio Measurement Set Service Manual
LCD Flat Panel Display (A14)
The flat-panel display is a thin-film transistor device. Drive signals to the display
are the 12-bit data (4 bits per color), four timing signals, and the +5 VDC
supply
voltage for the transistor array.
Backlighting for the LCD display is provided by two cold cathode florescent
tubes. The two tubes, one at the top and one at the bottom, are arranged to
provide edge lighting to the LDC display. The lamps require a kickoff voltage of
800 to 900 V
RMS
to turn on. They draw about 4 mA and have a power consump-
tion of approximately 1.6 W. Tube life is approximately 10,000 hours, and the
end of life (time for a replacement) is defined as the point when the maximum
brightness reduces to 50% of the original value or when the kickoff voltage
(voltage needed to turn on the lamps) exceeds a maximum of 900 V
RMS
at 0° C.
Replacement backlight tube sets can be ordered from Tektronix. See the
replaceable electrical parts list section for the part number and ordering
information.
The backlight inverters are small switching supplies that provides the high
voltage level AC required to power the two LCD display back lights. One
version of display has both inverters on a single circuit board; a newer version
uses two separate circuit boards for the two inverters. The maximum voltage
output from an inverter is approximately 800 to 900 V
RMS
at a typical frequency
of 35 kHz. Backlight brightness is controlled by a control circuit on the LCD
driver board. The control circuit uses the brightness control level from the CPU
(derived from the front panel brightness control) to adjust the brightness of the
backlight tubes. The backlight extinguish level is set by an adjustment in the
backlight inverter control circuit (BRT R25) to turn off at about the 8 to 10%
brightness setting.
LCD Backlight
LCD Backlight Inverters