TM8100 Mobile Radio Service Manual Circuit Descriptions 63
May 2004 © Tait Electronics Limited
3 Circuit Descriptions
Scope of Section This section contains descriptions of the circuitry of the control-head
board and each of the modules of the main board. The latter are the
transmitter, receiver, frequency synthesizer and FCL, CODEC and audio
circuitry, power supply, and interface circuitry. Since the digital board is
not serviceable at level-2, no description of the digital circuitry is given.
3.1 Control-head Board
Introduction This subsection describes the circuitry of the control-head boards for the
one- and two-digit-display control heads. The boards differ only in their
layouts; the components are identical. The circuit description given
below is sufficiently general to be applicable to both boards.
Connectors and
UI Devices
The control-head board includes the circuitry for the seven keys, 18
LEDs, and the LCD device of the control head. Also mounted on the
board are the volume-control potentiometer and the following five
connectors:
■ control-head connector
■ microphone connector
■ speaker connector
■ two connectors for optional circuit board
Protection circuitry is provided for the microphone connector. The
optional circuit board is installed when a dynamic microphone is used or
a concealed microphone is fitted.
Basic Circuitry The LCD driver is based on IC2, IC5 and IC7. Electrical contact
between the control head and the LCD itself is via two elastomeric strips
as described in Subsection 2.1 (on page 33). There are pads on the board
for the seven keys — four function keys, two channel-selection keys, and
an
ON/OFF key. The device IC4 reads the status of the function and
channel-selection keys. Of the 18 LEDs, there are red, orange and green
STATUS LEDs, and four green LEDs for the function keys, as well as 11
green LEDs for back-lighting — one for each key and four for the LCD.
The four dual switching transistors Q1 to Q4 control the switching of
the LEDs; the transistors are driven by IC3.
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