Midland 70-1341, Service Manual

Page: 43 / 68

CIRCUIT DESCRIPTION
70-1341/1344/1441/1444
5 - 5
Q507. The power control reduces the output power -3
dB whenever PA section temperature becomes
excessive.
LOGIC SECTION

Microcomputer
Radio operation is under control of a microcomputer
system located on the Logic Board. This system is
comprised of microcomputer IC901 and 32K EEPROM
IC902.
All CPU activity is performed step-by-step in time with
a clock. The frequency of the clock is fixed by crystal
X901. Because of the high clock speed,
microcomputer activity seems instantaneous.

Modulator
Voice signals from the hand-microphone are applied to
audio filter of IC401, where frequency response is pre-
emphasized and splatter filtered. Gain is such that
stronger signals bring IC401 into clipping, which limits
modulation. Harmonics above the 3 kHz modulation
pass-band are removed in IC401. Modulation signals
are then adjusted by IC401 so that modulation at
limiting will produce transmitted carrier deviation of
±5
kHz (wide) or ±2.5 kHz (narrow). Output of
processed voice signals at IC402 pin 5 is fed to the
gain control IC407.

Data Control
When the radio is turned on, the contents of EEPROM
IC902 are serially clocked into IC901 so that it can set
up receiver frequency, scan operation, transmit/
receive hold timer, busy-channel lock-out timer, time-
out-timer and reference oscillator frequency control.
When a channel is changed, or when PTT is pressed,
the contents of EEPROM IC902 are sent to IC901.
IC901 then uses this data to send the appropriate
information for the channel selected to IC701,
CTCSS/DCS circuitry, display circuitry, and any
signaling options.

DC Power and Reset
5V DC power to all logic circuitry in the logic section is
supplied from switched 13.6V and is regulated by IC905.
Microcomputer IC901 is powered by the 5V, which is
source by IC904 8V regulator supply.
DISPLAY SECTION

Display and Switches
By pushing A/D switch S301 , a logic low signal is supplied
to pin 35 of IC302. IC302 feeds display data to LCD301.
By pushing AUX switch S302 , a logic low signal is supplied
to pin 36 of IC302. Then IC302 feeds display data to
LCD301.
By pushing SCAN switch S303 , a logic low signal is
supplied to pin 37 of IC302. IC302 feeds a signal to IC901.
Then IC901 makes a radio in the scan mode, and also
feeds display data to IC301 to display SCAN on LCD301.
By pushing MON switch S304 , a logic low signal is
supplied to pin 38 of IC302. IC302 feeds a signal to IC901.
Then IC901 makes a radio in the monitor mode, and also
feeds display data to IC301 to display MON on LCD301.
By pushing switch S305 , a logic low signal is supplied to
Q901. Then Q901 feeds a logic high signal to IC908 to turn
on Q903. Thus power supply is turned on by fB
switch out. By pushing switch
S305 again, power supply is
turned off.
By turning switch S306 , pulses are supplied to pins 33 and
34 of IC302. IC302 performs channel-up or channel-down
with these pulses, then feeds display data to LCD301.
By pushing switch S306 , a logic low signal is supplied to
pin 24 of IC302. IC302 performs switching of the SQ of
menu, group, back-light, and others with this signal, then
feeds display data to LCD301.
During transmit, TXDL is low, TX icon is displayed on
LCD301, turning it on.
When a signal is received, NSQ is turned on, BUSY icon is
displayed on LCD301.