MFJ MFJ-8100K User Manual

MFJ-8100 World Band Receiver

Instruction and Kit Assembly Manual

Receiver Features......................................................................................................2

Introduction No. 1: For Radio Beginners.................................................................3

Introduction No. 1: For Experienced Hams, Enthusiasts or Engineers....................6

Schematic Diagram of MFJ-8100 .............................................................................8

Receiver Controls and Connectors............................................................................ 9

Understanding and Using the Regeneration Control................................................. 11

Tuning SSB (Single Sideband) Voice Signals..........................................................12

Some Helpful Terms and Abbreviations................................................................... 13

MFJ-8100 Parts List.................................................................................................. 15

"X-Ray View" of Printed Circuit Board....................................................................17

Before You Start Building !......................................................................................18

STEP-BY-STEP KIT CONSTRUCTION (19-28)

Construction Phase 1 (steps 1-1 through 1-10).........................................................19

Construction Phase 2 (steps 2-1 through 2-20).........................................................20

Construction Phase 3 (steps 3-1 through 3-20).........................................................22

Construction Phase 4 (steps 4-1 through 4-14).........................................................23

Construction Phase 5 (steps 5-1 through 5-11).........................................................25

Testing and Initial Adjustment

Construction Phase 6 (steps 6-1 through 6-18).........................................................27

Final Assembly

Dial Calibration Adjustment of Trimmer Capacitor C5............................................29

Note to Hams or Experimenters................................................................................29

Using and Enjoying Your Receiver...........................................................................30

Setting Up a Useful Shortwave Antenna................................................................... 31

Your Receiver's Audio Circuit..................................................................................31

About the 5 Tuning Ranges of Your Receiver.......................................................... 32

Shortwave Listening in General................................................................................ 33

In Case of Difficulty.................................................................................................. 34

Notes for Radio Hams and Experimenters................................................................ 36

Conclusion................................................................................................................. 37

Circuit Component Index.......................................................................................... 38

MFJ-8100 Receiver Features

• Five separate tuning ranges between 3.5 and 22 MHz.

• Smooth 6:1 vernier-reduction tuning dial

• Sensitive FET RF amp and detector stages

• Dual headphones for sharing the fun

• Use economical "personal stereo" headphones or speakers

• Smooth, well-engineered regeneration circuit

• True choice of AM-CW-SSB reception

++++

• Excellent reception even with a few feet of wire antenna

VERY IMPORTANT PLEASE READ:

Your receiver carries MFJ's respected No Matter What™ guarantee specifically to the extent that you may return the entire kit UNASSEMBLED for credit or refund. If you have never built an electronics kit before, PLEASE study this book carefully before unpacking the small parts. Once you have begun soldering parts, neither MFJ or any dealer can accept the return of the kit for any reason whatsoever.

MFJ-8100 World Band Receiver Instruction and Kit Assembly Manual

Instruction Manual developed and designed by Dan F. Onley, K4ZRA Copyright  1993 by MFJ Enterprises, Inc. All Rights Reserved. No part of this work may be copied or reprinted without the explicit permission of the copright owner.

MFJ Enterprises, Inc. 300 Industrial Park Road Starkville, MS 39759

Printed in the United States of America.

Introduction No. 1:

For Radio Beginners

You're about enjoy a versatile shortwave receiver which employs a circuit concept that is as classic as the 1920's but which uses modern engineering that takes advantage of the advanced capabilities of today's electronic components.

This shortwave radio is designed to let you listen to a great variety of international broadcasts. You can choose from five different frequency "bands'' so that you can count on hearing SOMETHING at any hour of day or night. Also, this receiver lets you hear a generous sampling of ham radio signals (both Morse code "CW'' and voice "SSB'' communications), plus many other government and commercial transmissions.

Even if you have never worked with electronic parts before, you can successfully build this receiver by carefully following all the directions in this book. Step by step, we'll show you how to build it and how to enjoy it. Before you start building however, please read the notice on page 2 so that there is no misunderstanding about your rights as a valued MFJ customer.

Just a Bit of History . . .

The "regenerative receiver'' moved the world of radio reception and broadcasting beyond the limits of crystal sets useful only for hearing a strong local signal. For over a decade, these magical, whistling, squawking, glowing boxes were the norm for home listening as well as for the first generation of radio hams.

Receiver design evolved swiftly. The "superheterodyne'' became the norm during the 1930's. Regenerative receivers, often called "Gennies,'' were left to tinkerers and beginners. Even though these receivers were simple and quite sensitive, they had a number of shortcomings: instability, touchiness, difficulty in separating strong stations, a tendency to generate interference to other receivers, and a general reputation for making odd sounds that resembled everything from pigs to motorboats.

However, the sheer SIMPLENESS of the regenerative circuit remained attractive to experimenters and beginners. In fact, as recently as the 1960's, one company marketed a $14 kit for building a complete transceiver using only one vacuum tube: half of the tube served as a regenerative receiver, and the other half was a low-power crystal-controlled transmitter. In addition, many thousands of engineering careers as well as ham radio licenses were launched with the building of "my first shortwave radio'' from do-it-yourself regenerative receiver kits offered by the major radio companies of several decades ago. (The fondest dream BACK THEN of most of these radio builders was to be able to afford to move up to a "superhet communications receiver.'' Their fondest memory TODAY is that very first receiver kit.)

From the late 1970's through the '80's, as consumer electronics and new ham radio equipment became more sophisticated so very rapidly, interest declined not only in regenerative receivers, but also in kit-building and even in shortwave radio listening. One or two generations of Americans simply missed out on the thrill and satisfaction of building and understanding a simple radio set which could receive signals from anywhere in the world.

Back to Today . . . and the Future!

Your MFJ-8100 is a much better receiver than the "classic'' radio sets which attracted several generations of Americans to the excitement of radio and electronics. In fact, its basic performance is superior to many of the simplest superhet receivers which were considered such a great step beyond one's first regenerative set.

The reason why this receiver works so well is because there is much more precisi on in today's engineering designs and the manufacturing of electronic parts. We looked carefully at the practical problems associated with yesteryear's technology, and we used TODAY'S know-how and components to solve the problems.

A Simplified Explanation of How It Works

When you're ready, please explore the technical explanation of your receiver in "Introduction No. 2.'' In the meantime, you can peek at the schematic diagram and picture the receiver in three basic sections: A. Detector-Oscillator (Q1,Q2) B. RF amplifier (Q3) C. Audio amplifier (IC1)

To put it very simply, a detector converts radio energy from an antenna into audio energy, i.e., a sound which you can hear. A detector can be as simple as a crystal diode, which is the heart of the simple "crystal radio.'' If you've ever heard unwanted radio signals on a stereo, telephone, PA system or intercom, you can assume that some part of those devices has acted as a detector to convert a nearby CB, taxi or broadcast signal into intelligible sound. (This process of detection is also referred to as demodulation.)

In the following explanation, the words regeneration, feedback and oscillation all mean approximately the same thing.

By itself, a detector can interpret or demodulate only very strong signals such as a nearby AM radio station. However, the process of regeneration can make a detector much more sensitive by turning the detector into an "oscillating amplifier.'' The regeneration circuit repeatedly feeds the detected signal back to the input which boosts its strength many hundreds of times. This feedback process must be carefully controlled, which is the function of the regeneration control.

The frequency of oscillation is determined by the choice of inductors (bandswitch) and the setting of the tuning capacitor. If the oscillator is tuned to 10.1 MHz, for example, any radio signal on that frequency will be boosted and detected in the regeneration process. The resulting output from transistor Q2 is a low-level audio signal which is boosted to comfortable listening level by the LM386 integrated circuit amplifier.

The RF amplifier serves two purposes. It boosts the RF signals from the antenna to the detector, and it minimizes the amount of oscillator RF going back out to the antenna.

Again, we hope you'll also look at the somewhat more technical explanation of how your MFJ-8100 Receiver circuit works. If any terminology used in this book is unfamiliar to you, please check the "Some Helpful Word & Abbreviations" section.

Introduction No. 2:

For Experienced Hams, Enthusiasts or Engineers

Why use a REGENERATIVE circuit for a kit new for the 1990's? A fair question, but the MFJ-8100 is not like any regenerative HF receiver you've ever used before!

Our GOAL determined the design and circuitry of this receiver. We wanted the following features:

• Good reception of both shortwave AM and CW-SSB

• Ease of kit-construction for newcomers

• Reasonable price

• A quality look and feel

• Relatively simple circuit

• No critical alignment requirements

• Low parts count, yet not dependent on specialty IC's

• Purposeful choice of tuning ranges for SWLing anytime.

Satisfactory AM-CW-SSB listening and circuit simplicity were our primary goals. Despite the popularity of NE602-type "direct conversion'' circuits among today's experimenters and some kit vendors, direct conversion is not satisfactory for enjoyable listening to AM shortwave broadcasts. Merely nulling the carrier does not result in true listenability. Similarly, a multi-band superhet with BFO could not fit our goals of simplicity and economy.

To meet our goals, we chose to refi ne the regenerative concept as much as possi ble, usin g contemporary design concepts and component characteristics. Our first goal was to "tame'' the regeneration process itself to minimize the instability and unwanted oscillations so typical of traditional regenerative circuits -- and so that even a beginner can enjoy and understand the use of the Regeneration Control. The result of our re-design is an HF SWL receiver with better performance than many low-end factory-built superhets of yesteryear.

Some highlights of our design efforts:

• Significantly reduced RFI back through antenna, a chronic regenerative receiver

shortcoming, through use of carefully designed RF amplifier stage.

• Effective RF filtering between detector and audio sections of the receiver.

• Simplified L-C tuning; notice that there are 5 band switch positions but no coil taps

or second windings!

• Elimination of antenna trimmer so critical in most regenerative designs. We

replaced the traditional trimmer with an RF gain pot that has little effect on frequency or regeneration.

• Manageable, "tame'' regeneration control circuit. Regeneration begins smoothly with

no pop and has a comfortable adjustment range.

The result, we think, is a receiver design which bridges the classic simplicity of regeneration to the performance demands of the 1990's. Here's how we did it:

In brief, the circuit uses RF regeneration and high levels of DC feedback. Notice that the antenna is coupled directly to the source of RF amplifier FET Q3 rather than through the L-C tuning network. Direct coupling of the drains of Q1 and Q3 isolates the L-C circuit from the antenna input, enhancing stability and greatly minimizing RF oscillator output to the antenna. Such RFI has been a serious problem in traditional regenerative circuits which permitted the oscillating detector to behave as an unstable but potent QRP transmitter.

R4 reduces the Q of L1 (10 µH) for smoother regeneration. The SW1 bandswitch selects a combination of simple inductors. For example, the total inductance for Band A is L1+L2+L3+L4+L5. The inductance for Band E is only L5. And so forth.

Air variable C1 uses its 50 pF range and mechanical vernier reduction to provide smooth "bandspread'' in parallel with C3 and trimmer C5 which perform the traditional "bandset'' function.

Trimmer pot R20 ensures adjustability for smooth regeneration over all tuning ranges, regardless of individual FET characteristics.

C17, C9, C10 and R9 form a low pass filter to block RF from the audio amplifier and provide basic audio filtering.

Volume Control R2 varies OUTPUT rather than low-level input to the LM386 audio amplifier. This approach further isolates the RF stages from variations in the audio section.

The LM386 (IC1) circuitry employs all recommended options for maximum gain and protection from self-oscillation.

To prolong useful battery life, R13 limits current draw by the LED (CR1) to minimum reasonable visibility as an on/off indicator.

Schematic Diagram

Receiver Controls and Connections

Most of the controls are self explanatory. However, it is very important to understand the correct use of the Regeneration Control and the two internal trimmer adjustments of the receiver.

BANDSWITCH (SW1) This quality rotary switch selects any one of the 5 tuning ranges from A to E indicated on the tuning scale.

TUNING (C1) The Tuning knob controls an air-variable capacitor (C1) which also has a built-in 6:1 vernier reduction drive to which the dial pointer is attached. This reduction permits very smooth tuning. The frequency markings on the dial scale must be understood to be approximate due to the 10% tolerance ratings of the fixed inductors (L1 through L5).

PUSH SWITCH (SW2) AND LED. INDICATOR (CR1) While the purpose of the on/off switch and LED is obvious, remember to turn your receiver OFF when not in use. A weakened battery degrades receiver performance.

REGENERATION (R1) Because understanding and controlling regeneration is at the heart of your receiver's performance, we've provided a separate section on its use. In brief, it controls receiver sensitivity and adjusts between AM broadcasts and CW-SSB.

VOLUME (R2) This potentiometer performs the normal function of any volume control. Of interest to the technically-minded, it controls the output of the LM386 audio IC, rather than the input, which enhances the stability of the regenerative detector.

RF GAIN This trimmer potentiometer is adjustable with a small screwdriver. Maximum gain is clockwise when viewing the rear panel. A good normal setting is 3/4 of its full rotation. If you are using a marginal antenna (5 to 10 feet of wire indoors), keep R19 at its maximum setting. If you are using a very good antenna (a long, high outdoor wire or ham antenna), keep R19 at about 2/3 or so of its range. If your listening interests require frequent RF gain adjustments, install an external 10K control in series with your antenna.

REGENERATION RANGE TRIMMER Ordinarily, this trimmer is adjusted only after kit construction or in the unlikely event that any of the FET transistors are replaced. This adjustment assures smooth regeneration over all five of the tuning ranges. See Construction Phase 5.

(R19)

(R20)

DIAL CALIBRATION TRIMMER (C5) This one-time internal adjustment is made with a miniature screwdriver in order to assure that the frequency markings on the front panel are as accurate as reasonably possible.

EARPHONE JACKS These two jacks accept 1/8'' (3.5 mm.) stereo plugs as used in "Walkman'' type headphones or mini-speaker systems. The audio output is monaural; the two jacks are wired in parallel to permit the use of two headphones.

Note

If a mono 1/8'' plug is used for any reason, it must NOT be pushed all the way in,

or it will short out the audio.

ANTENNA CONNECTOR (J1) This binding post permits easy hookup of any wire, or a banana plug may be inserted in its end. 10 to 20 feet of ordinary hookup wire (also called "bell wire'') provides good basic reception, even when installed indoors. See the section on Antennas in this book for more information.

GROUND CONNECTION For casual operation, a ground connection is optional. However, a wire from this connector to a ground rod or cold water pipe will reduce unwanted noise and interference from nearby electrical devices or AC wiring and may boost receiver sensitivity. Attach the wire between the two washers, then tighten the wing nut.

(J2,J3)

Understanding and Using the Regeneration Control

In theory, your receiver's Regeneration Control adjusts the level of feedback or selfoscillation of the FET detector section (Q1 and Q2). In practice, this control is like a "joystick'' for managing and optimizing receiver performance. Your ability to handle this "joystick'' saves you many dollars over today's cost of receivers which perform similar functions "automatically.'' In fact, you might even get more control over receiver performance in varying situations than may be possible with more elaborate receivers.

With the control turned fully to the left (counter clockwise), the receiver is virtually silent. "Regeneration'' begins at a certain point as you turn the control clockwise. The exact point varies not only from band to band but even as you tune within a given band. Regeneration begins as an audible increase in background noise followed by a soft hiss. The hiss, or any signals that may be on frequency, increases as you continue to turn clockwise. If you go too far, the signal becomes distorted, or the receiver begins to squeal (oscillate).

Always use the LEAST amount of regeneration necessary for good reception of a given signal.

As a rule, the best reception of AM shortwave broadcast signals occurs just BEFORE full regeneration. If you hear a whistle (carrier) along with an AM signal, turn the control back slightly until the carrier disappears.

When there are a number of very strong shortwave AM broadcasts in a given band, such as is common in the early evening, you will find it possible to tune them in one after the other with the regeneration control set "way back'' and requiring virtually no adjustment. In other words, you would tune from station to station just as if using any other type of shortwave set.

When the receiver is adjusted for good AM reception, CW signals will sound like hisses. Advancing the regeneration control slightly will bring in the familiar beeping associated with CW, RTTY (radio teletype) or similar signals.

The regeneration control can also serve as a fine tuning control, permitting slight adjustments of CW pitch for the most pleasing sound, or best clarity in a SSB voice signal. After you've had some practice with using the regeneration control, it will become second nature, giving you a sense of real control over the performance of your receiver.

Tuning SSB (Single Sideband) Voice Signals

SSB signals are all those voice signals which sound like Donald Duck unless they are tuned in very exactly. They have no background carrier as do AM broadcast signals.

On modern ham radio transceivers, tuning SSB is made so easy by means of internal filters that many licensed ham operators are not aware of the basic technique for tuning in SSB signals on receivers without such filters.

The first fact to know about any given group of SSB signals is whether they are Upper Sideband (USB) or Lower Sideband (LSB). In ham radio communication, LSB is used on 1.8 through 7.3 MHz, and USB is used for all higher frequency bands (14, 18, 21, 28 MHz.)

The best band to practice SSB tuning with your receiver is the "75 Meter'' band, 3.8 to 4.0 MHz, doing so in the evening when the signals are strong and plentiful. Notice that the band is spread out on the dial more than the other amateur bands, which permits easier tuning. These are all LSB (lower sideband signals).

Think to yourself: for LOWER sideband, tune DOWN. for UPPER sideband, tune UP.

In practice, this means that you would "approach'' the LSB signal by tuning from higher frequency (right) to lower (left), from higher voice pitch to lower pitch. Here's how to do it step by step:

1. Pick out a strong, high-pitched Donald Duck voice.

2. Turn the tuning knob ever so slightly to the left.

3. If the pitch of the voice went DOWN slightly, you're heading in the right direction.

4. SLOWLY tune left, slightly more until the voice is clear.

Reverse this process to tune UP (to the right) for USB signals on the bands above 7 MHz.

The Regeneration Control often can be used to do the last touch of fine tuning to bring the voice in clearl y. If signals are exceptionally strong, it may be necessary to reduce t he RF gain level (rear panel).

SSB transmissions are used by embassies and agencies of various governments, so you might find interesting voice signals on other than ham frequencies. Check with a Shortwave Listener (SWL) or listings in Popular Communications Magazine for more details.

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