Service manual

CIRCUIT DESCRIPTION
5-2
April 2001
Part No. 001-9800-203
Parameters which change from transceiver to
transceiver such as programmed system and groups
and option key programming are also stored in the
Flash EPROM and also EEPROM U102 which is used
only in high tier and data models. This information is
programmed when data is downloaded to the trans-
ceiver by the programming software described in
Section 4.
With high tier models, a second microcontroller
(U2) on the display board controls the front panel
display and tri-color indicator and also detects the
option switch, Select switch, and microphone on/off
hook state. Control information is exchanged between
U101 and this microcontroller via a serial bus. The use
of a second microcontroller minimizes the number of
interconnections that are required between the audio/
logic and display boards. The operating program for
this microcontroller is permanently stored on the chip
and cannot be changed.
5.1.5 RECEIVER
The receiver is a double conversion type with the
following intermediate frequencies:
UHF Models - 45 MHz and 450 kHz
800 MHz Models - 52.950 MHz and 450 kHz
900 MHz Models - 45 MHz and 450 kHz
Two bandpass filters in the front end attenuate
the image, half IF, injection, and other frequencies
outside the selected receive band. Receiver selectivity
is enhanced by a four-pole crystal filter and two 450
kHz ceramic filters.
5.1.6 TRANSMITTER
The transmitter amplifies the synthesizer signal
to produce a power output of up to either 25 or 40
watts (UHF) or 15 or 30 watts (800/900 MHz) at the
antenna jack. Frequency modulation of the transmit
signal is performed by modulating the synthesizer
TCXO and VCO frequencies. A control circuit senses
forward power to maintain constant power output. It
also senses final amplifier current and cuts back power
if it becomes excessive. The use of a digital potenti-
ometer allows the power output to be set from the
front panel when the test mode is selected and also
allows two different power levels to be programmed
for each system. The microcontroller also monitors
power amplifier ambient temperature and voltage and
cuts back power or disables the transmitter if either
are excessive.
5.2 POWER DISTRIBUTION AND SWITCHING
5.2.1 POWER SWITCHING CONTROL
A diagram of the power distribution and
switching circuits is shown in Figure 5-1. The main
power switching is performed on the RF board by
Q510 and other transistors. This switch is controlled
by the front panel power switch, the ignition sense
input, and the microcontroller as shown in Figure 5-2.
For the front panel on-off switch to be detected, Q109
must be turned on by a high signal applied through
R170 or from the ignition switch. R170 is installed if
the ignition switch is not used to control power. Q107
and Q108 provide power switch on and ignition on
signals to the microcontroller so that it can sense those
conditions.
Q110 allows the microcontroller to hold power
on for a time after it has been turned off by the power
or ignition switch. For example, when the microcon-
troller senses that power was switched off by the front
panel switch, it holds power on for a short time so that
switch settings can be saved to memory. It also holds
power on when a power-off delay is used. Once power
turns off, power is also removed from the microcon-
troller. Therefore, power can be turned on only by the
power and ignition switches.
NOTE: The front panel power switch of low tier
models is a momentary push-button type, and the
power switch of high tier models is a push on, push off
type (pressing it toggles between open and closed). It
is not a momentary switch like low tier models.
5.2.2 SUPPLY SWITCHING
Low Tier On-Off Toggle
Since low tier models have a momentary-type
power switch, a latch is required to hold power on
after the switch is released. When the power switch is
pressed, the base of Q113 on the audio/logic board is
POWER DISTRIBUTION (ALL MODELS)