Theory Of Operation (Control Heads); Standard Control Head; Power Supplies; Voltage Regulator Circuit - Motorola MTM700 Detailed Service Manual

THEORY OF OPERATION (CONTROL HEADS)

THEORY OF OPERATION (CONTROL HEADS)
This chapter contains the descriptions of the:
•

Standard Control Head

• Expansion Head
• Remote Mount Head
• Motorcycle Control Head
Standard Control Head
The control head contains the on/off/volume knob, the microphone connector, several buttons to
operate the radio, several indicator Light Emitting Diodes (LED) to inform the user about the radio
status, and a Liquid Crystal Display (LCD) with 21 pre - defined symbols and a 32*96 dot matrix for
graphical or alpha - numerical information e.g. channel number, select code, call address name. To
control the LED's and the LCD, and to communicate with the host radio the controlhead uses the
Motorola 68HC11K4 microprocessor.

Power Supplies

The power supply to the controlhead is taken from the host radio's FLT A+ voltage via connector
J0801 pin 3. The voltage FLT A+ is at battery level and is used for the LED's, the back light, to power
up the radio via on / off / volume knob and to supply the voltage regulator circuitry. The regulator cir-
cuitry provides the stabilized +5 volts which is used for the microprocessor circuitry, the display, the
display driver and the keypad buttons. The regulated +5V taken from the host radio via connector
J0801 pin 7 (line 5V SOURCE) is only used to switch on or off the voltage regulator in the control-
head.

Voltage Regulator Circuit

Voltage regulator U0861provides 5V for the controlhead. The supply voltage FLT A+ for the voltage
regulator is fed via parallel resistors R0861/2 and dual diode D0861 to pin 8 of U0861. The +5 volt
output is switched on and off by the host radios's 5 volt source via line 5V SOURCE and control tran-
sistor Q0866. When the host radio is switched off the voltage on line +5V SOURCE is at ground
level and switches off transistor Q0866. Pull up resistor R0863 pulls input SHUTDOWN (pin 3) of the
voltage regulator U0861 to FLT A+ level and switches off the output of U0861 (pin 1). When the host
radio is switched on the voltage on line 5V SOURCE of about +5 volts switches on transistor Q0866
which in turn pulls input SHUTDOWN (pin 3) to ground and switches on the output of U0861. Input
and output capacitors (C0861 / C0862 and C0864 / C0865) are used to reduce high frequency noise
and provide proper operation during battery transients. Diode D0861 prevents discharge of C0862
by negative spikes on the FLT A+ voltage. This regulator provides a reset output (pin 5) that goes to
0 volts if the regulator output goes out of regulation. This is used to reset the microprocessor
(U0871) and the display driver (J0821-5) to prevent improper operation.
The voltage USW 5V derived from voltage FLT A+ is stabilized using resistor R0855 and diode
VR0855 This voltage is used to buffer the microprocessor's internal RAM. C0856 allows the battery
MTM700 Mobile Radio / Detailed Service Manual 4.3 - 1
CHAPTER 4.3