Service manual

68P81076C25-C July 1, 2002
General Overview: Control Head Assembly 2-3
2.4.3 Display (W9 Model)
The control-head assembly for a W9 model has an 11-character, alphanumeric, vacuum fluorescent
display. It needs three separate voltages to operate; the cathode needs 35 V to accelerate electrons
to the anode; the grid needs 40 V to totally shut off current flow; the filament needs 3.8 Vac at 80mA.
These voltages are obtained from the transformer on the display controller board.
2.4.4 Vacuum Fluorescent Display Driver
This Vacuum Fluorescent (VF) display driver receives ASCII data from the VOCON board, decodes
it into display data, and then scans the display with the data. Once properly loaded into the display,
data is refreshed without any further processor action. The display driver is periodically reset by the
actions of transistors that watch the clock line from the microprocessor to the display driver. When
the clock line is held low for more than 600 ms, the display driver resets and new display data
follows.
2.4.5 Vacuum Fluorescent Voltage Source (W9 Model)
Voltage for the VF display is generated by a fixed frequency, variable-duty cycle driven, flyback
voltage converter. An emitter-coupled stable multi vibrator runs at approximately 150 kHz. The
square wave output from this circuit is integrated to form a triangle that is applied to the non-inverting
input of half an integrated circuit (IC).
During start up, the inverting input is biased at 3.7 V. A transistor is on while the non-inverting input
voltage is below 3.7 V. This allows current to flow in a transformer, building a magnetic field. When
the triangle wave exceeds 3.7 V, the transistor turns off and the magnetic field collapses, inducing
negative current in the transformer.
This current flow charges two capacitors. As the voltage on one of the capacitor increases beyond
35 V, a diode begins to conduct, pulling the integrated circuit’s inverting input below 3.7 V. This
decreases the cycle time to produce the 35 V. The 41 V supply is not regulated, but it tracks the
35 V supply.
Similarly, the ac supply for VF filament is not regulated, but is controlled to within one volt by an
inductor on the display board.
2.4.6 Controls and Indicators
The control-head assembly processes all the keypad (button) inputs and visual indicators through
the microprocessor. Some of the buttons double as function keys for radio options. All buttons are
backlit to allow operation in low-light conditions.
2.4.7 Status LEDs
These LEDs are driven by the display driver as though they were decimal points on the VF display.
Level shifting transistors are required for this since the display driver uses 39 V for control signals.
2.4.8 Backlight LEDs
The microprocessor operates the backlight LEDs. A transistor supplies base current to the individual
LED driver transistors. The driver transistors act as constant current sources to the LEDs. Some
backlight LEDs are connected to a thermistor. This circuit allows more current to flow through these
LEDs at room temperature and reduces current as the temperature rises.