Uniden SMH 4000, Service Manual

Page: 16 / 72

is rectified by D120 and D121, thus charg­
ing C160. In the absence of a carrier,
C160 will remain charged keeping the out­
put of IC103 1/2 low. When a RF carrier is
present, the "rush" noise is reduced, thus
allowing C160 to discharge through R121,
causing the output of IC103 2/2 to go high.
This high is fed to the microprocessor
IC601 pin 40. The microprocessor sends
mute data to IC101 pin 9 from pin 41 of
IC601 through an inverter Q613.
Receiver Audio
The mute controlled audio signal from
IC101 pin 11 is fed to active high pass fil­
ter IC102. The output of IC102 is fed to the
audio power amplifier IC105 through the
limit control, VR102, and the volume con­
trol, VR751. The audio power amplifier,
IC105, amplifies the recovered audio to a
level sufficient to drive the speaker. In the
squelched mode the output of IC102 is
clamped to ground by 0107.
TX, BUSY, CALL, and DISPLAY
LEDs
The TX LED (D703) is activated by the
microprocessor pin 56 through an inverter,
0615, when the PTI closure is received
on pin 63 of IC601. The anode of D703 is
connected to the emitter of Q701, dimmer
circuit.
The BUSY LED (D705) is activated by the
microprocessor pin 55 through an inverter
0616 when a busy indication is received
on pin 40 of IC601. The anode of D705 is
connected to the emitter of 0701, the dim­
mer circuit.
The
CALL LED (D702)
is activated by
the
microprocessor pin
60
through an inverter
0618 when a correct CTCSS tone or Two­
Tone is decoded by the microprocessor.
The anode of D702 is connected to the
emitter of 0701, dimmer circuit.
The DISPLAY LED (0701) receives data
from microprocessor pin 54 which is
loaded into two shift registers, IC752 and
IC752. The clock signal for the shift
registers is provided by the IC602 pin 52.
During the time that the data is being
loaded into the shift registers the STB line
pin 53 is held low; thus, preventing the
LED from flashing with the data stream.
The anode of D701 is connected to the
emitter of Q701, the dimmer circuit.
LED Dimmer Control
When the ambient light dims, the resis­
tance of the CDS cell (Y701) increases
reducing the current supplied to the base
of
0701;
thus, reducing the emitter voltage
of 0701 causing the LEDs to dim.
Microprocessor
The microprocessor IC601 controls many
of the transmit and receive functions of the
transceiver. These functions include such
items as channel selection, scan, audio
muting, transmit keying, CTCSS and DCS
encode/decode. The microprocessor also
provides programming data for AMX 533
TwoTone option, if installed.
--=--
The microprocessor operates according to
the instructions that are stored in its inter­
nal 4K x 8 byte ROM. This ROM is an in­
tegral part of the microprocessor chip and
is maskprogrammed when the
microprocessor is manufactured. In addi­
tion to the internal ROM, there is an exter­
nal 256 x 8 bit E2PROM (IC603) which
stores the information
unique to the
individual radio system, such as channel
number and frequency, DCS or CTCSS
codes, scan, priority channel, busy chan­
nel lockout, and timeout timer. The SMU
300
is field programmable using the AMX
500C, PROM Writer.
Page 1 4 SMH
4000 December 1 992