Service manual

3.2 TP0332/2
POWER SUPPLIES AND ON/OFF CONTROL
The main power supply for the radio is the 7,2V battery which is connected directly to the PCB via
the battery contacts on the rear of the radio. The power supply has five separate regulators supplying
power to the following blocks (see Figure 3.2):
VCO PSU +5,6V Frequency generation.
RF PSU +5V Transmitter and Receiver and switched supply to Tx
and Rx.
AF PSU +5,2V Audio and MMI.
Control PSU +5V Supply, reference, reset to control and baseband.
Rx/VCO Bias PSU –12V VCO bias to Frequency generation/Rx Filter Bias.
The power supply distribution system for the radio is illustrated in Figure 3.2 on page 3.3.
Turning On
When the radio is turned on by the rotary control, a positive pulse is applied to the CONT input of the
RF_PSU regulator IC501 via C538. This pulse turns on the RF_PSU regulator output. The RF_PSU rail in
turn enables the CONT_PSU, VCO_PSU and AF_PSU supplies and also powers the on/off latch formed
by TR526/TR527. This transistor latch keeps the RF_PSU supply on by holding PSU_ON high even after
the initial switch-on pulse via C538 has died away.
Once the processor starts running, it issues a periodic watchdog pulse on WDOG#RXD0. This
pumps up the voltage on the gate of TR529 to +5V via the charge pump formed by C541, D505 and C540.
Thus, the continued presence of watchdog pulses keeps TR529 on and, hence, TR528 off. The on/off
transistor latch TR526/TR527 is thus left untouched by the watchdog circuit and stays this way until the
watchdog pulses stop. This is described in the next section “Turning Off”.
Turning Off
Although powering off is initiated by turning the rotary control, the actual power shutdown is
controlled by software. This is to allow time for the software to finish any tasks it may be performing. When
the radio is turned off by the rotary control, the processor is informed of the request to turn the radio off by
means of the OFF signal. The processor then completes any outstanding tasks such as saving current
radio status or saving UMP data before it shuts down the power supplies by halting the generation of the
watchdog pulses on WDOG#RXD0.
In the absence of watchdog pulses, the voltage on the gate of TR529 decays to 0V and turns off
TR529. This turns on TR528 which changes the state of the on/off transistor latch TR526/TR527. This
forces PSU_ON low and turns off the RF_PSU regulator IC501. All the other regulators are then turned off
as they are enabled by RF_PSU.
Reset Circuit
The reset circuit, consisting of IC504_A, IC504_B and associated components, continuously
monitors the control supply voltage CONT_PSU and generates a reset whenever this supply drops below a
minimum acceptable level for the control circuits to continue running. When the supply returns above this
voltage, the reset is released and the processor restarts as if turned on by the rotary control.