Motorola GM338, Service Manual

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2-4 Controlhead Model for GM338
The HOOK input (J0811-3) is used to inform the µP when the microphone´s hang-up switch is
engaged. Dependent on the CPS programming the µP may take actions like turning the audio PA on
or off. While the hang up switch is open, line HOOK is pulled to +5 volts level by R0841. Transistor
Q0841 is switched on and causes a low at µP port PA1. When the HOOK switch is closed, signal
HOOK is pulled to ground level. This switches off R0841and the resulting high level at µP port PA1
informs the µP about the closed hang up switch. The µP will inform the host radio about any status
change on the HOOK line via SBEP bus.
2.8 Speaker
The controlhead contains a speaker for the receiver audio. The receiver audio signal from the
differential audio output of the audio amplifier located on the radio’s controller is fed via connector
J0801-10, 11 to the speaker connector P0801 pin 1 and pin 2. The speaker is connected to the
speaker connector P0801. The controlhead speaker can be disconnected if an external speaker,
connected on the accessory connector, is used.
2.9 Electrostatic Transient Protection
Electrostatic transient protection is provided for the sensitive components in the controlhead by
diodes VR0811 VR00812 VR0816 - VR0817. The diodes limit any transient voltages to tolerable
levels. The associated capacitors provide Radio Frequency Interference (RFI) protection.
3.0 Controlhead Model for GM338
The controlhead contains the internal speaker, 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 14 character Liquid Crystal Display (LCD) for alpha - numerical
information e.g. channel number or call address name. To control the LEDs and the LCD, and to
communicate with the host radio the controlhead uses the Motorola 68HC11E9 microprocessor.
3.1 Power Supplies
The power supply to the controlhead is taken from the host radio’s FLT A+ voltage via connector
J0801 pin 3 and the reg5V via connector J0801 pin 7. The voltage FLT A+ is at battery level
and is used for the LEDs, the back light and to power up the radio via On /Off /Volume knob. The
stab5 volt is used for the microprocessor, the display, the display driver and the keypad
buttons. The voltage USW 5V derived from the FLT A+ voltage and stabilized by the series
combination of R0822, VR0822 is used to buffer the internal RAM of the microprocessor (U0831).
C0822 allows the battery voltage to be disconnected for a couple of seconds without losing RAM
parameters. Dual diode D0822 prevents radio circuitry from discharging this capacitor. When the
supply voltage is applied to the radio, C0822 is charged via R0822 and D0822. To avoid that the µP
enters the wrong mode when the radio is switched on while the voltage across C0822 is still too low,
the regulated 5V charge C0822 via diode D0822.
3.2 Power On / Off
The On/Off/Volume knob when pressed switches the radio’s voltage regulators on by connecting
line ON OFF CONTROL to line UNSW 5V via D0821. Additionally, 5 volts at the base of digital
transistor Q0822 informs the controlhead’s microprocessor about the pressed knob. The
microprocessor asserts pin 62 and line CH REQUEST low to hold line ON OFF CONTROL at 5 volts
via Q0823 and D0821. The high line ON OFF CONTROL also informs the host radio, that the