Tait TM8000 Series Service Manual

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Tait TM8000 Series Service Manual Manual pdf 63 page image

TM8100 Mobile Radio Service Manual Circuit Descriptions 63May 2004 © Tait Electronics Limited3 Circuit DescriptionsScope of Section This section contains descriptions of the circuitry of the control-headboard and each of the modules of the main board. The latter are thetransmitter, receiver, frequency synthesizer and FCL, CODEC and audiocircuitry, power supply, and interface circuitry. Since the digital board isnot serviceable at level-2, no description of the digital circuitry is given.3.1 Control-head BoardIntroduction This subsection describes the circuitry of the control-head boards for theone- and two-digit-display control heads. The boards differ only in theirlayouts; the components are identical. The circuit description givenbelow is sufficiently general to be applicable to both boards.Connectors andUI Devices The control-head board includes the circuitry for the seven keys, 18LEDs, and the LCD device of the control head. Also mounted on theboard are the volume-control potentiometer and the following fiveconnectors:■ control-head connector■ microphone connector■ speaker connector■ two connectors for optional circuit boardProtection circuitry is provided for the microphone connector. Theoptional circuit board is installed when a dynamic microphone is used ora concealed microphone is fitted.Basic Circuitry The LCD driver is based on IC2, IC5 and IC7. Electrical contactbetween the control head and the LCD itself is via two elastomeric stripsas described in Subsection 2.1 (on page 33). There are pads on the boardfor the seven keys — four function keys, two channel-selection keys, andan ON / OFF key. The device IC4 reads the status of the function andchannel-selection keys. Of the 18 LEDs, there are red, orange and greenSTATUS LEDs, and four green LEDs for the function keys, as well as 11green LEDs for back-lighting — one for each key and four for the LCD.The four dual switching transistors Q1 to Q4 control the switching ofthe LEDs; the transistors are driven by IC3.