Service manual

CIRCUIT DESCRIPTION
5-4
April 2001
Part No. 001-9800-203
RC networks R186/C124 and R187/C126 have a
longer time constant than R185/C125 so that the new
logic level applied to pins 1 and 5 is delayed for at
least the duration of the high pulse applied to pins 2
and 4. This prevents oscillation. Q112 provides a
5-volt supply to the latch. C123 stores enough power
to provide supply voltage to the latch for up to a week.
Therefore, if power is removed from the transceiver,
such as by a relay used to switch power, the on-off
mode is the same mode when power is reapplied.
Supply Switching on RF Board
When power is turned on by the front panel
power switch, the base of Q514 on the RF board is
grounded through Q109 on the audio/logic board by
Q114 (low tier) or the power switch (high tier). Q514
then turns on which also turns on the Darlington
amplifier formed by Q511 and Q512. These transistors
are turned on by a 13.6-volt signal applied through
R535. Series-pass transistor Q510 is then turned on
and 13 volts appears on its collector. The diodes in
CR505 and CR506 become forward biased only if the
8-volt supply applied to the collector of Q514 becomes
shorted. This provides current limiting which prevents
damage to the transistors.
Q513 controls the Q511 base current in order to
maintain approximately a 0.8-volt drop across the
emitter and collector of Q510. This provides noise
filtering of the 13.6-volt supply. The emitter of Q513
is biased at about 4.5 volts by R538 and R542 (with a
battery voltage of 13.6 volts). CR504 mirrors the base-
emitter voltage of Q513, and the voltage across R541
is the same as the voltage across R542 when the
voltage applied to R536 is approximately 12.8 volts.
Noise pulses less than 1.6 volt P-P then do not
appear on the output of Q510 because of the emitter
voltage filtering provided by C565. This reduces the
amount of noise applied to circuits powered by the
switched 13.6-volt supply such as audio power ampli-
fier U306. Additional filtering of the switched and
unswitched battery supplies is provided by C548-
C553. Resistor R534 turns Q514 off when power is
turned off.
5.3 AUDIO/LOGIC BOARD DIGITAL CIRCUIT
5.3.1 MICROCONTROLLER (U101)
General
The control logic is based on an MC68HCllF1
eight-bit microcontroller (U101). This device has an
internal 1K-byte static RAM and 512-byte EEPROM
but no internal ROM or EEPROM. Therefore, all
program memory is contained in the external flash
memory device (U108). In addition, the microcon-
troller has several general purpose input and output
pins, an eight-channel A/D converter, and synchronous
(SPI) and asynchronous (SCI) serial ports. The A/D
converter port allows analog signals to be monitored
such as the power amplifier temperature, RSSI signal,
and vehicle battery voltage.
Separate buses are used for data and memory
addressing. The data bus consists of D0-D7, and the
address bus consists of A0-A15. The operating speed
of the microcontroller is set by crystal Y100. The 9.38
MHz frequency of this crystal is divided by an internal
divider to produce a lower internal operating
frequency.
Memory
The operating program and most of the person-
ality information used by the microcontroller is stored
in 128K x 8 Flash EPROM U108. The use of a Flash
memory device allows the program to be conveniently
updated using the standard programming setup and
special Flash programming software. This eliminates
the need to replace the microcontroller or a memory
device such as an EPROM. To reprogram the Flash
device, the microcontroller is placed in a special boot-
strap mode by turning power on with the MODA/
MODB inputs pulled low. This is done by the RPI by
applying 20 volts to the PTT pin of the microphone
jack.
Radio tuning information is stored in the 512-
byte EEPROM in microcontroller U101. External 4K
x 8 EEPROM U102 is used in high tier and data
models to store additional personality information. An
EEPROM can be programmed many times, does not
require a constant power supply, and retains data
indefinitely.
AUDIO/LOGIC DESCRIPTION (ALL MODELS)