MFJ 8100W User Guide

MFJ-8100 World Band Receiver

MFJ-8100 World Band Receiver

Table of Contents

For Beginners Page 2
Just a Bit of History
Page 2
Back to Today and the Future
Page 3
A Simplified Explanation of How it Works
Page 3
For Experienced Hams, Enthusiasts and Engineers
Page 4
Receiver Controls and Connections
Page 6
Understanding and Using the Regeneration Control
Page 7
Tuning SSB Voice Signals Page 8
Using and Enjoying Your Receiver
Page 9
Setting Up A Useful Shortwave Antenna
Page 9
Your Receiver Audio Circuit
Page 10
About the 5 Tuning Ranges of Your Receiver
Page 11
Shortwave Listening in General
Page 12
In Case of Difficulty
Page 12
Notes for Radio Hams & Experimenters
Page 13
Conclusion Page
14
Some Helpful Terms & Abbreviations Page
15
Sample SWL Log Page Page 17
MFJ-8100 Parts List Page
18
PC-Board View Page 19
Schematic Diagram of MFJ-8100 Receiver
Page 20

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MFJ-8100
World Band Receiver

Instruction Manual written by Dan F. Onley, K4ZRA
Copyright  1993 by MFJ Enterprises, Inc. All Rights
Reserved.

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MFJ-8100
World Band Receiver

For 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.

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

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MFJ-8100
World Band Receiver

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 precision 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)

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MFJ-8100
World Band Receiver

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 8100 Receiver circuit
works. If any terminology used in this book is unfamiliar to
you, please check the Glossary.

For Experienced Hams, Enthusiasts or Engineers

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MFJ-8100
World Band Receiver

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 refine the regenerative
concept as much as possible, using 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

regen 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

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MFJ-8100
World Band Receiver

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 uH) 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.

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MFJ-8100
World Band Receiver

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

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MFJ-8100
World Band Receiver

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 L.E.D. 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 (R9)
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 R9 at its maximum setting. If you are using a
very good antenna (a long, high outdoor wire or ham antenna),
keep R9 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.

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