Ericsson Monogram Series LBI-39132, Maintenance Manual

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CIRCUIT ANALYSIS
The VHF and UHF Scanning Handheld series radios are comprised of two main PCB’s (an RF PCB and a
Digital PCB). The RF PCB contains the Transmitter and Receiver circuits. The Digital PCB contain the
Micro controller and associated digital circuits.
DIGITAL BOARD
IC 411
IC 411 is a digitally-controlled analog switch which internally consist of three single pole, double throw
switches. By placing a high (5V) or low (0V) on the control lines which consist of A, B, and C." A"
controls the X ports "B" controls the Y ports and "C" controls the Z ports. Example: A high on control "A"
would connect X to X1. A low on control "A" would connect X to X0.
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NOTE: On Contact Resistance is around 100 Ohms.
CTCSS/DCS Decode Circuits
Discriminator audio from Pin 15 CON 402 is fed into IC406A and associated parts, which are the first 2 poles of
a 6th order 250 Hz Chebeyshev low pass filter. The output from Pin 1 (IC406A) is fed into IC411 (Pin 2) and
output to Pin 15 (IC411). The signal is then fed to Pin 8 (IC407) which is a 6th order low pass Butterworth
switched capacitor filter. The output from the Butterworth filter (Pin 3 IC407) is then fed to the remaining 4
poles of the 6th order Chebeyshev, which consist of IC 406C and one of the two operational amplifiers internal
to IC407 (MF6-100) along with associated components. Both the Chebeyshev and the Butterworth combines for
a 4 db ripple low pass filter when programmed for 250 Hz. The output of IC406C (Pin 8) is fed into the
remaining operational amplifier internal to IC407 (MF6-100), which forms the squaring circuit for the Decode
signal. The signal is output from Pin 2 IC407 (MF6-100) and fed into IC409 (micro) where it is matched with a
preprogrammed frequency. If successful a Decode occurs, which is shown by a green L.E.D. on the top panel of
the VHF and UHF Scanning Handheld and audio is heard. If valid Decode was not seen, the busy
L.E.D.(yellow) would be shown.
CTCSS/DCS Encoder Circuit
During TX encode the tone squelch digital signal is produced as a 3-bit parallel word at Pins 15(A), 16(B)
and 18(C) of the micro controller (IC409). The 3-bit digital signal is converted to an analog signal by
resistors R478, 479 and 480. The analog signal is fed into IC411 Pin 1. The signal is output on Pin 15
(IC411) and fed into Pin 8 of IC407 (6th order Butterworth clock tuned low pass filter). The filtered encode
output from Pin 3 (IC407) is fed into Pin 13 (IC411) and output on Pin 14 (IC411). The filtered encode
signal is fed to IC406B and RV403 (sub-audible gain control), the output of IC406B is then fed to the audio
mixer circuit.
External MIC/PTT Control Circuit
When the external microphone is connected to the six pin Hiroshi connector on the top panel of the VHF
and UHF Scanning Handheld, Pin 4 of the connector is shorted to ground, disabling the internal mic and
internal speaker via pin 12 of Con 402 and IC402A. When external PTT is depressed Pin 5 of the connector
shorts to ground, which drops the voltage on Pin 1 (mic audio) of the connector, to less than 2V. Pin 1 of
the connector is connected to IC405D (Voltage Comparator) via R416, R415, and Pin 11 of CON 4. When
pin 13 of IC405D goes below 2V the output of IC405D (Pin 4) goes high (5V) and turns on Q402. Q402’s
output is in parallel with the internal PTT line, which is connected to IC409 (micro), Pin 24.
Channel Select Circuit
One of 16 channels or 4 channels (depending on VHF and UHF Scanning Handheld model) may be
selected using the channel switch on the top panel. The channel switch encodes the channel number
selected into a 4-Bit binary code. The binary code plus 1 is equal to the channel number. The binary code is
MONOGRAM SERIES HANDHELD
CIRCUIT ANALYSIS
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