Motorola Gm Series Low Band Info Manual

 Professional Radio
GM Series
LB1 (29.6 - 36.0MHz)
LB2 (36.0 - 42.0MHz)
LB3 (42.0 - 50.0MHz)
Service Information
Issue: August 2002 

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Computer Software Copyrights
The Motorola products described in this manual may include copyrighted Motorola computer programs stored 
in semiconductor memories or other media. Laws in the United States and other countries preserve for 
Motorola certain exclusive rights for copyrighted computer programs, including the exclusive right to copy or 
reproduce in any form, the copyrighted computer program. Accordingly, any copyrighted Motorola computer 
programs contained in the Motorola products described...

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Table of Contents
Chapter 1 MODEL CHART AND TECHNICAL SPECIFICATIONS
1.0 GM360 Model Chart............................................................................................. 1-1
2.0 Technical Specifications ...................................................................................... 1-1
Chapter 2 THEORY OF OPERATION
1.0 Introduction .......................................................................................................... 2-1
2.0 Low Band  Receiver...

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Chapter 4 Low Band PCB/SCHEMATICS/PARTS LISTS
1.0 Allocation of Schematics and Circuit Boards ....................................................... 4-1
1.1 Controller Circuits ................................................................................................ 4-1
2.0 LB1 25-60W PCB 8486206B06 / Schematics ..................................................... 4-5
2.1 LB1 25-60W PCB 8486206B06 Parts List ......................................................... 4-15
3.0 LB2 25-60W PCB...

Chapter 1
MODEL CHART AND TECHNICAL SPECIFICATIONS
1.0 GM360 Model Chart
2.0 Technical Specifications
Data is specified for +25°C unless otherwise stated.
GM Series Low Band 29-50 MHz
Model Description
MDM25BKF9AN5_EGM360 LB1, 29.0-36.0 MHz, 25-60W, 255 Ch
MDM25CKF9AN5_E GM360 LB2, 36.0-42.0 MHz, 25-60W, 255 Ch
MDM25DKF9AN5_EGM360 LB3, 42.0-50.0 MHz, 25-60W, 255 Ch
Item Description
XXXGCN6114_ Control Head, GM360
XIMUB6003_SField Replaceable Unit (Main Board) GM360
X IMUB6004_S Field Replaceable Unit...

1-2 MODEL CHART AND TECHNICAL SPECIFICATIONS
*Availability subject to the laws and regulations of individual countries.
Transmitter LB1 LB2 LB3
*Frequencies - Full BandsplitLB1 29.7-36.0 MHzLB2 36.0-42.0 MHzLB2 42.0-50.0 MHz
Channel Spacing12.5/20/25 kHz12.5/20/25 kHz12.5/20/25 kHz
Frequency Stability
(-30°C to +60°C, +25° Ref.)±5.0 ppm±5.0 ppm±5.0 ppm
Power 25-60W 25-60W 25-60W
Modulation Limiting±2.5 @ 12.5 kHz 
±4.0 @ 20 kHz
±5.0 @ 25 kHz±2.5 @ 12.5 kHz 
±4.0 @ 20 kHz
±5.0 @ 25 kHz±2.5 @ 12.5 kHz...

Chapter 2
THEORY OF OPERATION
1.0 Introduction
This Chapter provides a detailed theory of operation for the LowBand circuits in the radio. For details 
of the theory of operation and trouble shooting for the the associated Controller circuits refer to the 
Controller Section of this manual. 

2-2THEORY OF OPERATION
2.0 Low Band Receiver
2.1 Receiver Front-End
The low band receiver is bandsplit into three ranges depending on radio model, covering frequencies 
from 29.7 to 36.0 MHz, 36.0 to 42.0 MHz, or 42.0 to 50.0 MHz. The circuitry of the three models is 
identical except for component value differences. The receiver consists of five major blocks: front-end 
bandpass filters and pre-amplifier, first mixer, high-IF and blanker switches, low-IF and receiver back-
end, and “Extender” (noise...

Low Band Receiver2-3
There are two 2-pole 10.7 MHz crystal filters in the high-IF section and two switched pairs of 
455 kHz ceramic filters in the low-IF section to provide the required adjacent channel selectivity. 
The second IF at 455 kHz is mixed, amplified and demodulated in the IF IC. The processing of the 
demodulated audio signal is performed by an audio processing IC located in the controller section.
2.2 Front-End Band-Pass Filters & Pre-Amplifier 
The received signal from the radio’s antenna...

2-4THEORY OF OPERATION
presented to the high-Q crystal filters FL1102 and FL1103, would cause ringing of the filter 
response, stretching an otherwise narrow impulse into a long and audible output waveform. 
Therefore, source follower stage Q1104 isolates the blanker switches from the crystal filters, 
providing a consistent source impedance via matching network L1106, L1107 and associated 
components. Q1104 has unity voltage gain in this configuration.
Crystal filters FL1102 and FL1103 are 2-pole, 10.7...