Multitone FLA 872, Technical Manual

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TLA 852/FLA 872
TM1205 Issue 2 Page 3 - 9
C20 and C21 are part of the internal DC/DC converter of the module’s negative bias
supply.
R11, R12, R13, C17 and C22 through C25 are part of the circuit which controls the
voltage to the LCD driver.
3.3.7 Display Illumination
Two miniature lamps are used to illuminate the LCD display. The lamps are selected
ON or OFF using the buttons on the front panel. The ‘Lamp ON/OFF’ signal, which
comes from pin 8 of the microprocessor, controls transistor TR2 in the lamp circuit
of the TLA852, or TR7/IC2 for the EL display in the FLA 872.
3.3.8 Alerts
a) Audible Alert (Beep)
The transducer, SP401, and the driver circuits TR415 and TR416, are located
on the radio board. The audible alert signal from IC3, pin 2 connects to TR415
through SK1/PL401 pin14. The alert can be selected ON or OFF using the
buttons on the front panel.
b) Vibrate Alert
The vibrate motor is operated by transistors TR3 and TR4 on the decoder
board. The vibrate alert signal from IC3, pin 7 connects to pin 1 of TR3. The
alert can be selected ON or OFF using the buttons on the front panel.
3.4 POWER SUPPLIES
The primary power supply for the unit is a single 1.5 volt, AAA battery (BAT1). The
supply from the battery (B+) connects directly to the display illumination and vibrate
motor circuits, that require relatively high currents. B+ also connects to the radio
board through SK1/PL401, pin 9 and to the voltage converter, IC6, on the decoder
board.
IC6 and its associated components supply the +3.1 volts (VDD) for the unit. TR5, T1,
C9, C11, C26, R10 and D1 form an oscillator giving a frequency between 5kHz and
20kHz. The output is rectified by D1 and smoothed by C12. IC6 samples the voltage
and if it exceeds 3.1 volts, IC6 causes TR6 to switch off and stop the oscillator.
When the pager circuits take current, the voltage drops and the oscillator is turned
on again. This produces a sawtooth voltage on the 3 volt rail which gives less RF
noise than other converter circuits.
In addition to supplying the 3.1 volts, the circuit charges the backup supply (BAT2)
through resistor R6. Double diode D2 provides isolation for the two circuits.