Ericsson MDX SERIES, Service Manual, Page: 9 / 14

SW A+
Switched A+ (13.6 volts nominal) originates from the
MOSFET switch on the system board. The Ignition Sense lead
and the POWER push-button control the MOSFET switch.
Fuse F901 protects the MOSFET and the radio from high
current failures. SW A+ is supplied through J902 and J903 to
the front cap assembly. It provides power to the 5 volt regula-
tor, 10 watt audio PA, and the front cap display board. J903
provides SW A+ to the PA board which, in turn, passes SW A+
through A4 J151 and A2 J705 to the RF board.
SW A+ enters the RF board on J704 and J705 and supplies
power to three 8-volt regulators and the transmitter power
control circuitry. SW A+ leaves the RF board on A2 J702 to
supply power to 8-volt regulator U805 on the audio/logic
board.
Regulated Voltages
SW A+ is the source of power for all voltage regulators.
Several 5 volt regulators receive power from an 8-volt regula-
tor reducing the power dissipated by the 5-volt regulators.
RF Board
8-volt regulator U502 provides power to the receiver. A
separate 8.3-volt regulator U207 and 5-volt regulator U203
provide power to the synthesizer. 8 volt regulator U102 pro-
vides power to the transmitter. The output of U102 is switched
to the exciter and the power control circuit. U503 powers the
audio/logic board. U102 (TX 8-volt supply) and Q101 (power
control output transistor) are mounted for heat sinking.
Audio/Logic Board
A 5-volt regulator, U801, is used to power the audio/logic
board. The input voltage is derived from the 8-volt regulator
on the RF board. The power-on reset circuitry for the
audio/logic board microprocessor is part of regulator U801.
This reset signal prevents scrambled operation due to low
voltage transients during automobile starting. An 8-volt regu-
lator is used to power the audio circuits.
Front Cap Assembly (Display Board)
A 5-volt regulator on the audio amplifier board powers the
microprocessor logic display, icon LED’s.
Audio Amplifier Board
A +5-volt regulator U727, is used for logic level pullups
and to power the display board. SW A+ is used to power audio
amplifier board U801, and display board backlighting control,
Q850. An 8-volt regulator, U804, is used to power the opera-
tional amplifiers.
AUDIO SIGNAL FLOW
Refer to the Audio Signal Flow Block Diagram to see the
distribution of RX and TX audio signals throughout the radio.
Audio levels at important points are also shown.
Transmitter Audio
Microphone audio (MIC HI) is routed from the mic con-
nector on the display board through the audio amplifier board
to feed the audio/logic board. After processing and summing
the Channel Guard tones, the audio (TX MOD) is fed through
the audio/logic board to the RF board. TX MOD is adjusted
by Deviation Adjust R226 before feeding the modulation input
to the synthesizer VCO U201.
Receiver Audio
Discriminator audio is buffered by Q503 on the RF board
and adjusted by R513. This audio (VOL/SQ HI) is routed to
the audio/logic board for audio processing, tone/code detec-
tion, and volume control.
LOGIC SIGNAL FLOW
Refer to the Logic Signal Flow Block Diagram to see the
distribution of logic signals throughout the radio.
APPENDIX A. GE-MARC TONES
NUMBER FREQUENCY NUMBER FREQUENCY
01 604.2 Hz 22 1556.7 Hz
02 631.5 Hz 23 1628.3 Hz
03 662.3 Hz 24 1717.1 Hz
04 693.0 Hz 25 1795.6 Hz
05 727.1 Hz 26 1877.5 Hz
06 761.3 Hz 27 2051.6 Hz
07 795.4 Hz 28 2143.8 Hz
08 832.9 Hz 29 2239.4 Hz
09 870.5 Hz 30 2341.8 Hz
10 911.5 Hz 31 2447.6 Hz
11 952.4 Hz 32 2556.9 Hz
12 996.8 Hz 33 2672.9 Hz
13 1041.2 Hz 34 2792.4 Hz
14 1089.0 Hz 35 508.6 Hz
15 1140.2 Hz 36 529.1 Hz
16 1191.4 Hz 37 553.0 Hz
17 1243.0 Hz 38 576.9 Hz
18 1304.0 Hz 39 1962.9 Hz(acq)
19 1362.1 Hz 40 2918.7 Hz(alt)
20 1423.5 Hz 41 3051.9 Hz(std)
LBI-38851 LBI-38851
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