MASTR II Maintenance Manual: 25-50 MHz, 50-Watt Transmitter
Specifications
| Frequency Range | 25-50 MHz |
| Power Output | 50 Watts (Adjustable from 15 to 50 Watts) |
| Crystal Multiplication Factor | 3 |
| Frequency Stability | ±0.0005% (-40°C to +70°C) ±0.0002% (0°C to +55°C) ±0.0002% (-40°C to +70°C) |
| Spurious and Harmonic Emission | At least 85 dB below full rated power output |
| Modulation | Adjustable from 0 to 15 kHz swing with instantaneous modulation limiting. |
| Modulation Sensitivity | 80 to 120 Millivolts |
| Audio Frequency Characteristics | Within +1 dB to -3 dB of a 6-dB/octave pre-emphasis from 300 to 3000 Hz per EIA standards. Post limiter filter per FCC and EIA. |
| Distortion | Less than 2% (1000 Hz) Less than 3% (300 to 3000 Hz) |
| Deviation Symmetry | 0.5 kHz maximum |
| Maximum Frequency Spread: (2 to 8 channels) | 25-30 MHz: .160 MHz 30-36 MHz: .200 MHz 36-42 MHz: .240 MHz 42-50 MHz: .280 MHz |
| 1 dB Degradation | .320 MHz .400 MHz .270 MHz .540 MHz |
| Duty Cycle | EIA 20% Intermittent |
| RF Output Impedance | 50 Ohms |
These specifications are intended primarily for the use of the serviceman. Refer to the appropriate Specification Sheet for the complete specifications.
Description
The MASTR II transmitters are crystal-controlled, phase modulated units designed for one-through eight-frequency operation in the 25 to 50 megahertz band. The solid-state transmitter utilizes both integrated circuits (ICs) and discrete components, and consists of two main assemblies: the Exciter Board (with audio, modulator, amplifier, and multiplier stages) and the Power Amplifier Assembly (with amplifier, driver, PA, power control, filter, and antenna switch).
Circuit Analysis
Exciter
The exciter employs nine transistors and two integrated circuits to drive the PA assembly. It can be equipped with up to eight Integrated Circuit Oscillator Modules (ICOMs). Each ICOM contains a crystal-controlled Colpitts oscillator. The ICOM crystal frequencies range from approximately 8.33 to 16.67 MHz, and this frequency is multiplied three times (via division by four and multiplication by 12) to achieve the final output frequency. Audio, supply voltages, and control functions are routed from the system board to the exciter board via connector P902. A centralized metering jack (J103) is provided for use with GE Test Sets for measuring modulator, multiplier, and amplifier stages.
ICOMs: Three types of ICOMs are available: 5C-ICOM (oscillator with a 5 PPM compensator IC), EC-ICOM (oscillator only, requires external compensation), and 2C-ICOM (oscillator with a 2 PPM compensator IC).
Figure 1 - Transmitter Block Diagram: This diagram illustrates the signal flow from the audio input through the exciter stages (audio IC, frequency divider, modulators, amplifiers, multipliers) to the power amplifier assembly, showing key components like ICOMs, buffers, and filters.
Oscillator Circuit
Quartz crystals in ICOMs exhibit frequency variations with temperature. Compensation circuits, including cold-end and hot-end compensation, are used to maintain frequency stability. Figure 2 shows typical crystal performance with and without compensation. Figure 3 provides an equivalent circuit of the ICOM compensation mechanism.
Figure 2 - Typical Crystal Characteristics: A graph showing frequency deviation in PPM versus temperature in degrees Centigrade, illustrating the performance of uncompensated crystals versus crystals with compensation capacitors.
Figure 3 - Equivalent ICOM Circuit: A schematic showing the oscillator circuit with cold-end and hot-end compensation components, including varactors and resistors, connected to a compensator IC.
Audio IC
The transmitter's audio circuitry is handled by audio IC U102. Audio from the microphone is processed through pre-emphasis and an operational amplifier-limiter circuit, which includes gain stages and a limiter to prevent overmodulation. A de-emphasis network and a post-limiter filter condition the audio before it is coupled to the phase modulators via MOD ADJUST potentiometer R127.
Figure 4 - Simplified Audio IC: A schematic diagram of the audio IC, showing the operational amplifier-limiter stages, de-emphasis network, and post-limiter filter.
Frequency Divider IC
The output from the ICOM is fed to buffer amplifier Q101, which then drives frequency divider U101. This IC divides the oscillator frequency by four, using two J-K flip-flops configured as a binary counter. The output waveform is a sine wave at one-fourth the oscillator frequency after filtering.
Phase Modulators, Amplifiers and Multipliers
The exciter uses multiple stages of phase modulation, amplification, and frequency multiplication (tripling and doubling) to generate the final transmit frequency. Varactor diodes (CV101, CV102, CV103) are used in the modulator stages, with their bias varied by the audio signal to achieve phase modulation. Class A amplifiers (Q102, Q103, Q104) boost the signal between stages. Diodes CR102, CR103, CR105, and CR106 are used to remove amplitude modulation.
Power Amplifier
The Power Amplifier (PA) assembly uses three RF power transistors and seven transistors for power control to deliver 50 Watts output. The PA is broadband and has only one adjustment: the Power Control potentiometer R216. The Power Control circuit regulates the collector voltage to the first amplifier stage (Q201), thereby controlling the overall power output. Temperature protection is provided by a thermistor (RT201) and associated circuitry to prevent overheating.
RF Amplifiers
The exciter output is coupled to the PA input. The RF input is rectified by CR201 to activate the Power Control circuit and also used for metering the drive level. Class C amplifiers (Q201, Q202, Q203) are used for power amplification, with matching networks and filters between stages. Collector current for the driver (Q202) and PA (Q203) stages is metered via specific resistors. The PA output is matched through a low-pass filter before being sent to the antenna via switch K201.
Power Control Circuit
This circuit regulates the collector voltage supplied to the first PA amplifier (Q201) based on the setting of the Power Adjust potentiometer (R216). It ensures that the power output can be adjusted from 15 to 50 Watts. Temperature protection is also integrated, reducing power output if the heatsink temperature exceeds approximately 100°C.
Carrier Control Timer
An optional feature that shuts off the transmitter after a one-minute transmission cycle, alerting the operator with an alarm tone. The transmitter can be reactivated by releasing and rekeying the push-to-talk switch.
Maintenance
Disassembly
Procedures for servicing the transmitter from the top or bottom are provided. This involves releasing locking handles, removing covers, and unplugging cables. Specific steps are outlined for removing the exciter board and the PA assembly.
Figure 5 - Disassembly Procedure (Top View): Illustrates the top cover removal process, showing the locking handle and pry points.
Figure 6 - Disassembly Procedure (Bottom View): Shows the bottom cover removal and access to internal components, including the exciter board mounting screws.
PA Transistor Replacement
WARNING: The RF Power Transistors contain Beryllium Oxide, a toxic substance. Handle with care if the ceramic encapsulation is damaged.
Detailed steps are provided for unsoldering old transistors, preparing new ones (including lead trimming and identification), and mounting them with correct torque. Silicon grease is used on the mounting surface.
Figure 7 - Lead Identification: Diagrams showing the lead configuration (Collector, Emitter, Base) of the transistors.
Figure 8 - Lead Forming: Illustrates the correct method for forming transistor leads to ensure proper soldering to the printed circuit board.
Alignment Procedure
This section details the steps for aligning the transmitter, including modulation level adjustment and ICOM frequency adjustment. It specifies test equipment required and provides step-by-step instructions for tuning various components (L101, L102, L103, T101-T108) to achieve maximum meter readings at specific stages. Power output adjustment using R216 is also covered, with guidance on setting it according to battery or collector voltage.
Figure 9 - Frequency Characteristics Vs. Temperature: A graph showing the relationship between frequency deviation (PPM) and ambient temperature, used for ICOM frequency offset adjustments.
Figure 10 - Power Output Setting Chart: A chart that helps determine the correct setting for the Power Adjust control (R216) based on battery voltage and desired output power.
Test Procedures
This section outlines routine tests to verify transmitter operation, including Power Measurement, Voice Deviation, Symmetry, and Audio Sensitivity, and Tone Deviation with Channel Guard. It lists required test equipment and provides detailed procedures for connecting equipment and interpreting readings. Cautions are given regarding input voltage limits to protect transistors.
Outline Diagram
This section lists the main assemblies and sub-assemblies of the transmitter, such as the Exciter Board and Power Amplifier Assembly, along with their respective part numbers and revisions.
Schematic Diagrams
Detailed schematic diagrams are provided for the Exciter Board and the Power Amplifier Assembly. These diagrams show the interconnections of all components, including transistors, ICs, resistors, capacitors, inductors, and connectors, along with typical voltage readings at various test points.
Parts List and Production Changes
This extensive list details all components used in the Exciter and Power Amplifier assemblies, including their GE part numbers, descriptions, and specifications. It also notes components added or changed by production revisions (Rev. A, B, C, etc.).
