Multitone RPR 551IS, Technical Manual

Page: 57 / 127

BEC
BEC
RPR 550IS Series
TM1188 Issue 1 Page 3 - 15
POWER SUPPLIES
46. There are four infallibly power limited supplies on the main board. They are:
a) BEPB+, limited by R16 provides power to the beep drive.
b) B+, limited by R61 provides power to the speaker driver.
c) B+, limited by R1A or R1B provides power to the vibrate driver.
d) B+, limited by R22 provides power to all the other circuitry.
3V Supply
47. The pager is turned on when the On/Cancel/Memory Recall button, S1, is pressed and
pins 53 and 55 of IC2 are taken to ground. Pin 17 of IC2 goes low, enabling TR5b,
which in turn supplies B+ to the base of TR7. This subsequently causes TR16 to
conduct. A self-oscillating converter is formed by TR16 and L1. On negative cycles
C37 charges via D2b. On positive cycles the voltage rises on both plates of C37 by
the same amount to maintain the potential difference between them. Diode D2a anode
potential becomes more positive than the cathode and it conducts charging C21 to 3V.
48. Voltage control of the output is by a feedback loop formed from TR14b and TR5a.
TR14a provides a reference voltage. Any changes in V
DD
affect the voltage on the
base of TR14b subsequently causing TR14b collector to alter. This changes the V
BE
of TR5a and therefore the amount of base drive of oscillator TR16. Diodes D17 and
D18 infallibly limit the output voltage of the convertor under fault conditions.
49. The pager is turned off, by pressing S2, the Off button. This applies a reset pulse to
the gate array (IC2) via pin 5. Pin 17 of IC2 goes high, turning TR5b off.
Subsequently, TR7 goes off and TR16 stops generating the 3V supply, switching the
pager off.
50. Pressing S3, the Mute button, grounds pin 59 of IC2 generating an interrupt to IC3
pin 1. If the switch remains depressed for 1.5s the pager changes mode either from
`on' to `mute' or vice versa. In mute mode the alert tones are inhibited by software.
1V Regulator
51. The pager utilises battery economy ( ) mode where it is activated for approximately
30ms of every second in order to search for preamble. The signal from pin 9 of
IC2 enables TR3 via R21. This allows B+ to be present on pin 20 (V
CC
) of IC1, thus
activating the chip. TR28 is also enabled, bringing its collector low and turning on
TR27, the series pass transistor. TR27, in conjunction with IC1, forms the 1V supply.
The voltage is set by R38 and applied to the radio board at PL1 pin 4, via the
decoupling network R3 and C51.