Motorola ASTRO W5, Руководство по обслуживанию

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September 26, 2003 6881096C77-O
3-8 Theory of Operation: W9 Control Head
U1-6 controls the entire voltage converter. During startup, U1-6 pulls to ground, biasing U3-2 to
about 1.85 volts via R51 and R52. U3-1 and R61 then begin to switch Q50 on and off at a 210 kHz
rate. This allows current to flow through T60, building a magnetic field, then collapsing it. When the
field collapses, the back EMF (flyback) forward biases CR51 and charges C52 (after a few cycles) to
about 37 volts, causing zener diode VR50 to conduct. The high voltage is regulated by the effect that
VR50 has on the bias voltage at U3-2. If the high-voltage supply increases, VR50 pulls U3-2 higher
in voltage resulting in a decreased duty-cycle for Q50 (the switching frequency remains constant;
only the duty-cycle of on-time versus off-time decreases). Less power is up-converted to C52 when
the duty cycle decreases; hence, regulation is maintained.
The voltage converter also produces an a.c. filament voltage for the VF display across T60 pins 4
and 6. R101 drops this voltage to approximately 2.4 Vrms. This a.c. voltage rides on a positive d.c.
offset due to R50, depending on brightness. The positive offset allows a VF display grid to be
reverse-biased with respect to the filament when that particular grid is supposed to be shut off. This
prevents “character ghosting.” Display dimming is exclusively controlled by multiplexing information
sent from U1 to the VF driver IC, U101. Display dimming has no effect on the voltage converter
circuitry except in the OFF mode. In this mode, U1-6 goes high, resulting in U3-2 going to +5 volts.
This keeps U3-1 low and Q50 turned off. The high-voltage supply drops to about 12 volts and the a.c.
filament voltage drops to zero, shutting off the display.
3.2.13 Vacuum Fluorescent Display Driver IC
The VF driver IC, U101, receives data directly from the microcomputer for multiplexing the VF
display. All of the grid and anode driver lines from U101 are either at, or about, 34 volts (ON) or
ground (OFF).
3.2.14 Vacuum Fluorescent Display
The VF display is an 8-character, 14-segment device. Anodes are the actual phosphorescent
segments, and grids are the wire-mesh structures located over each character. Multiplexing allows
character generation with a minimum of interconnecting lines between display and driver. Anodes
and grids operate at, or about, 34 volts when ON and are at zero when OFF. The filament operates
on ac at about 2.4 Vrms, with current flow at 100 mA through the filament. The d.c. current flowing
from anodes and grids to the filament and through R50 to ground increases filament voltage by up to
5 Vdc.
3.2.15 Backlight LEDs
Most buttons on the control head are backlit by ten LEDs for easy night-time viewing. In medium and
low brightness level settings, U1-19 goes high, turning on Q131 and causing a fixed current to flow
through the LEDs.
In the OFF mode and the maximum brightness mode, U1-19 goes low, turning off Q131 and the
LEDs. Microprocessor U1 does not duty-cycle change the brightness level of the backlighting LEDs.
3.3 W9 Control Head
The W9 control head has solid-state microprocessor circuitry that operates the standard and optional
features built into the system. The control-head design allows installation in even the smallest of
down-sized vehicles.
3.3.1 Display
The control head has an 11-character alphanumeric VF display for indicating the following:
• Zone/Mode names
• Squelch level