Controller Board Circuit Operation - Motorola Handie-Talkie MTX Series Theory/Troubleshooting Manual

dual NPN transistor IC, U704. When either of the
outputs (U705 pin 66, PC1 or U705 pin 65, PC0) is
at a logic high, the corresponding output of U704
(pin 3 for the green LED, pin 6 for the red ) is at
approximately 4.3volts. Note that it is possible to
have both LED outputs on simultaneously, in which
case the LED emits an orange/yellow light.
B. Controller Board Circuit Operation
The circuits considered here are those circuits that
involve:
• the transmit audio path between the microphone
and the transmit RF section,
• the transmit data path between the microcomputer
and the transmit RF section,
• the receive audio path between the receive RF
section and the speaker,
• the receive data path between the receive RF
section and the microcomputer, and
• the alert tone path between the microcomputer and
the speaker.
The transmit and receive audio paths are disabled
in the standby mode and selectively enabled by the
microcomputer when the radio transmits or receives a
signal. Also, there are minor differences in the function-
ing of both paths depending on whether an internal or
external (accessory) speaker/microphone is being used.
1. Transmit Audio Circuits
There are three major circuits in the transmit audio
path. Some require enable lines and some are
active devices that are always operating. When the
PTT is depressed, the radio will monitor the channel
for traffic (smart PTT). If the channel is not busy, the
microcomputer will enable the path between the
microphone and the RF section.
The microphone in the front cover (internal mic )
and remote microphone (external mic) are of the
FET electric type. They require a dc biasing voltage,
provided by resistors R701 and R756, respectively.
The INT MIC audio is routed to module U701 pin 2.
The EXT MIC audio is routed to module U701 pin
54. Logic inside the ASF IC selects one of the sig-
nals for amplification and processing.
a. Internal Microphone Path
The internal microphone (MK401) is located on
the front cover of the radio and is connected to
the controller board via connector plug P701
pin7. On the controller, from connector jack
J701 pin 7, the audio signal is routed to resis-
tors R701 and R703. Resistor R701 performs
dc biasing and resistor R703 provides input pro-
tection for the CMOS amplifier input. Filter
capacitor C703 provides low-pass filtering to
eliminate frequency components above 3KHz,
and capacitors C706 and C779 serve as dc
blocking components. The high-pass filter
formed by capacitor C779 and resistor R704
attenuates objectionable low-frequency audio
components of speech. The audio signal is
passed on to the ASF IC, U701 pin 2.
b. External Microphone Path
The external microphone signal enters the radio
via universal connector P403 pin 3, and is con-
nected to the controller board through
connectors P701/J701 pin 14. The external
audio signal is routed through a filtering circuit
composed of L701, C721, R702, and C702,
through dc blocking capacitor C705, and
passed to the ASF IC, U701 pin 54. Resistor
R756 provides dc bias for the stage.
c. PTT Sensing and Transmit Audio Processing
Depressing the internal PTT switch (S406) pro-
vides a ground path for the microcomputer via
the controls flex to controller connector
P703/J703 pin 1 and an internal pull-up resistor
at the input of U705 pin 61, PF2. Depressing an
external PTT switch provides a ground path for
both input lines (OPT SEL 1 and OPT SEL 2,
via the universal connector (P403 pins 1 and 5
respectively). The ground is read by the micro-
computer at U705 pin 38 (PG5) and U705 pin
37, (PG6). When either PTT is sensed (internal
or external), the microcomputer configures the
ASF IC for the proper audio path. Inside the
ASF IC, the audio input signal is amplified, fil-
tered to eliminate components outside the
300-3000Hz voice band, pre-emphasized, and
limited. The limited microphone audio is routed
through a summer circuit, which adds PL or
DPL sub-audio band modulation, and then rout-
ed to a splatter filter to eliminate high frequency
spectral components generated by the limiter.
After the splatter filter, the audio is routed to two
modulation attenuators, which are tuned for the
proper amount of FM deviation. The transmit
audio signal emerges from the ASF IC at U701
pin 13, and is dc coupled to the synthesizer
(U204 pin 5) on the transceiver board through
connector jack J704 pin3.
2. Transmit Data Circuits
There are three major types of transmit data: sub-
audible data (PL/DPL), DTMF data for telephone
communication, and MDC data for use in Motorola
proprietary MDC systems. The deviation levels of
the latter two types are tuned by a 5-bit digital atten-
uator inside the ASF IC. For each data type and
each bandsplit, there is a distinct set of tuning val-
ues programmed into the ASF IC before the data
can be generated and transmitted.
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