MFJ MFJ-8400K User Manual

VERY IMPORTANT! PLEASE READ:

Your Receiver Kit 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 the book carefully before unpacking the small parts.
Once you begin soldering parts, neither MFJ or any dealer can accept the return of this kit for any
reason whatsoever.

Note: We have designed this book and the receiver itself to assure that you can build it

successfully and enjoy using it as soon as construction is completed. If, after building
your receiver, double-checking all assembly steps and going over the preceding trouble­shooting suggestions, you are still having a problem, please contact MFJ at 1-601-323-

5869. Before calling, please be prepared to explain your exact difficulty as clearly as
possible, and please be sure you have installed the right parts in the right places! If
needed, your receiver can be repaired by MFJ provided that it has been completely
assembled, using rosin-core solder only. Call for quote.

MFJ Enterprises, Inc.
921 Louisville Road
Starkville, MS 39759 USA

MFJ will verify repairs on any unit requiring more than the minimum service fee covers with the
owner of the unit by phone.

MFJ-8400K FEATURES

[ ] Covers entire 2-Meter Ham Band, from 144-148 MHz.
[ ] Easy to tune -- smooth capacitor with 8:1 planetary drive
[ ] Super-sensitive, pulls in weak stations with small antenna
[ ] Easy on the ears with MFJ's exclusive TAILFREE™squelch
[ ] Extra-large 3" built-in speaker for full-bodied sound
[ ] Unsquelched-audio Packet Radio monitor jack
[ ] Runs on internal 9V battery or optional AC adapter supply
[ ] Uses state-of-the-art Motorola FM-receiver IC chip
[ ] Easy to build and align -- a great "first kit"
[ ] Perfect way to learn about Ham Radio and get a license

Please read this manual carefully before you start assembly. Also, please inventory all parts
BEFORE you begin.

VERY IMPORTANT GUARANTEE INFORMATION, PLEASE READ:

[ ] MFJ's KIT RETURN POLICY: MFJ's respected guarantee covers your MFJ-8400K

specifically to the extent that you may return the entire UNASSEMBLED kit for credit or
refund.

Once you begin soldering parts, you assume responsibility as the radio's manufacturer. Neither
MFJ nor its dealers can be held accountable for the quality or outcome of your work. Because of
this, MFJ and its dealers cannot accept the return of any kit-in-progress or completed kit for any
reason whatsoever.

[ ] MISSING OR DAMAGED PARTS: If you discover missing or damaged parts when you

inventory your kit, call MFJ at [phone] or [phone] for assistance.

ABOUT THE MFJ-8400K INSTRUCTION MANUAL:

The MFJ-8400K FM 2-Meter Receiver and its instruction manual were developed by Rick
Littlefield (amateur radio call K1BQT) exclusively for MFJ. This manual is Copyrighted (c)
1993 by MFJ, Inc. All Rights Reserved.

MFJ Enterprises, Inc.
P.O. Box 494
Mississippi State, MS 39762

Printed in the United States of America

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INFORMATION FOR RADIO BEGINNERS -- BUILDING THE MFJ­8400K:

Building the MFJ-8400K gives you valuable construction experience plus a great opportunity to
work with the latest state-of-the-art components. "State-of-the-art" doesn't mean difficult to
build. Like many modern electronic products, The MFJ-8400K is electronically sophisticated yet
physically simple. This is because most of the complex circuitry is self contained within a single
IC (integrated circuit). You'll add a few peripheral parts to make the receiver IC work properly.
You'll also add a sensitive preamplifier to pull weak stations and an audio amplifier IC to drive
the radio's speaker. The Motorola FM-receiver IC will do the rest!

The fun isn't over when your kit is done. You'll quickly discover the MFJ-8400K is a "serious"
receiver with superb SENSITIVITY (ability to pull in weak stations.) Tuning in stations is easy.
This is because we use an expensive ball-bearing reduction-drive tuning capacitor rather than a
cheap potentiometer for channel selection. MFJ's TAILFREE™ squelch circuit, especially
designed for monitoring repeater stations, is another plus that makes the MFJ-8400K enjoyable
to use. Finally you get a BIG 3" built-in speaker for robust natural-sounding speech -- something
no other kit and only few expensive commercial radios provide.

WHAT YOU'LL HEAR ON 2-METER FM:

Two meters is especially popular because it provides an abundance of "on-air" meeting places for
the hams in your area. Located in the VHF (Very High Frequency) portion of the radio spectrum,
2-meter FM is relatively free from atmospheric interference and noise -- and is especially well
suited for MOBILE UNITS and small HAND-HELD RADIOS. You'll hear friends discussing an
endless array of ideas and topics (an activity called "rag chewing"). You may also hear hams
performing public service -- coordinating a road race, reporting a traffic accident, or tracking a
dangerous weather situation. Many families use ham radio for personal communication, staying
in touch with each other throughout the day. Some even access the phone system from their
automobiles and make phone calls using a special device called an AUTOPATCH. Others
communicate via personal computer using special PACKET RADIO signals. Listening in on 2­meters, or "reading the mail," is a great way to improve your technical knowledge, since hams are
always sharing hobby-related information.

Most of the stations you'll hear on 2-meter FM will be transmitting through REPEATER
STATIONS. Repeater stations make 2-meter FM more useful because they extend transmitting
and receiving range.

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FM REPEATER STATIONS, WHAT THEY DO:

VHF radio waves normally travel line-of-sight.* When mobile and handheld stations attempt to
communicate directly with each other over longer distances, hills, buildings, and the curvature of
the Earth eventually get in the way -- blocking or absorbing the signal. This limits reliable VHF
radio contact to a few miles. Repeater stations provide a way to overcome this limitation.

A repeater consists of a receiver, transmitter, and antenna system located as high as possible
above the surrounding terrain -- usually on a mountain top, high-rise building, or tall tower.
Because the antenna is positioned well above local ground clutter, distant signals have a direct
path to the repeater which is free from signal-robbing obstructions. From this ideal listening
post, weak mobile and handheld signals that wouldn't be audible at ground level come in loud
and clear.

Repeaters extend communicating range by retransmitting (or repeating) everything they pick up.
The repeater rebroadcasts on a second channel -- one sufficiently removed in frequency so it
won't interfere with the repeater's receiver. Since the transmitter is powerful and the antenna is
high in the air, the repeated signal blankets a wide area. This "boost" can enable a tiny walkie­talky with a normal point-to-point range of 1-2 miles to reach out 40-50 miles and beyond!

* VHF signals sometimes "bend" and travel beyond the usual line-of-sight path, following the
contour of the Earth. Technically, this condition is called Tropospheric Bending or Atmospheric
Ducting (depending on the exact cause). However, you'll usually hear it called "skip" or "lift" by
hams. Warm air trapped aloft is the most frequency cause.

REPEATER INPUT AND OUTPUT CHANNELS:

Each amateur repeater station has two assigned operating frequencies. The repeater's INPUT
FREQUENCY is the channel where it listens for incoming signals. The OUTPUT
FREQUENCY is the channel where it retransmits what it hears.

Two-meter repeater input and output channels are normally spaced 600 KHz apart. This
STANDARD SPLIT provides enough channel separation to prevent the repeater's transmitter
from interfering with its receiver. It also provides a standard spacing between transmit and
receive frequencies that manufacturers can program into their two-way radios.

When you tune around the 2-meter band, you'll be looking for repeater outputs to monitor, since
these provide the strongest and steadiest signals.

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REPEATER OWNERSHIP AND CONTROL:

Like most amateur stations, ham repeaters are privately owned and operated by individuals or
radio clubs. Most repeaters are "open" -- anyone with a ham license can use them freely.
However, repeater clubs may collect annual dues from members to raise money for new
equipment or to pay for site rental. A few repeaters are "closed" and can only be accessed with a
special encoded tone which is kept secret from non-members.

Virtually all repeater stations are unmanned and fully automated by a microprocessor unit called
a REPEATER CONTROLLER. The controller unit, which tells the repeater's transmitter and
receiver what to do, also periodically sends the transmitter's call letters in MORSE CODE or
digitized voice to identify the station. In addition, the controller may provide a number of special
functions like AUTOPATCH (permits making phone calls), TIMEOUT (cuts off people who talk
too long), COURTESY BEEP (lets people know when the channel is clear), and much more.
Some functions are automatic, which others are "called up" via the built-in touch-tone pad found
on most modern 2-way FM radios.

With thousands of repeaters on the air from coast to coast, 2-meter FM operation is both fun and
practical nearly everywhere in the U.S. Indeed, most of the conversations you monitor on 2­meters will be coming through repeater stations built and maintained by the hams in your local
area.

HOW YOUR MFJ-8400 WORKS -- MADE SIMPLE:

A. Your radio's antenna picks up all kinds of VHF stations -- including fire, police, FM
broadcast, and aircraft. The MFJ-8400K uses a BANDPASS FILTER to get rid of unwanted
signals.

B. Desired two-meter signals pass through the filter without being affected. They are boosted by
a LOW-NOISE AMPLIFIER (LNA). The LNA (or preamplifier) is especially designed to
improve reception of weaker stations.

C. A VARIABLE FREQUENCY OSCILLATOR (VFO) is used to select the 2-meter channel or
frequency you wish to monitor. VFO frequency is controlled by the radio's main tuning dial.

D. The FM Receiver IC transforms radio signals into usable voice signals. To accomplish this,
incoming radio signals are CONVERTED, FILTERED, and AMPLIFIED at two successive
INTERMEDIATE FREQUENCIES (10.7MHz and 455KHz). The receiver's DETECTOR then

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extracts the audio (voice) component from the radio signal and sends it to the radio's AF
amplifier for further amplification.

E. The detected AUDIO signal passes through a VOLUME CONTROL to the AF AMPLIFIER
IC which powers the radio's speaker.

F. FM receivers generate random noise (an annoying rushing sound) when no radio signal is
being picked up. A special circuit called a SQUELCH senses when this is happening.

G. The squelch shuts off the AF Amplifier IC, keeping the radio silent (and conserving battery
power) until a signal appears.

HOW YOUR RADIO WORKS -- FOR EXPERIENCED HAMS AND
ENGINEERS

Please refer to the schematic diagram shown on page 26.

A two-section bandpass filter at L1-L2 rejects out-of-band signals and matches the 50-ohm
antenna line to Q1. Q1, a low-noise preamplifier biased for minimum noise figure, boosts
incoming signals by approximately 20 dB. The untuned output of Q1 is fed to U1, a monolithic
dual-conversion FM-receiver IC. The first mixer of U1 down-converts two-meter signals to the

10.7 MHz 1st-IF. LO is provided by a Colpitts tunable oscillator running in the 130MHz region.
All LC tank components are selected for temperature and mechanical stability and U1 is voltage
regulated by U3 to ensure electrical stability. FL1 establishes a 100 KHz bandpass window for
the 10.7 MHz 1st-IF. Although some conversion gain is realized at 10.7 MHz, most of U1's
overall gain is focused in the 455-KHz 2nd-IF to limit IMD potential. The 2nd-LO is crystal
controlled by Y1, and 2nd-Mixer output is filtered for message-channel bandwidth by FL-2, a 20­KHz FM-data bandwidth ceramic filter. After amplification and limiting, the 455 KHz signal is
fed to a quadrature detector for audio recovery. U1 then amplifies the audio signal to quasi-line
level.

Unsquelched audio output from U1 is fed to packet-monitor jack J2 and also routed to AF
Amplifier U2 via volume control R13. U2, a gated device, boosts audio output to speaker level
and provides a squelch gate for voice-signal monitoring.

U1's RSSI output drives the squelch circuit. RSSI level is sampled across squelch sensitivity
control R12 and fed to a comparator. Any time the sampled RSSI level drops below a fixed
reference set by R5-R6, the comparator triggers U2 into it's low-current stand-by mode, shutting
off the speaker and reducing receiver power drain. When an incoming signal -- or a change in
the squelch setting -- causes a rise in RSSI voltage, U2 is triggered on. This TAILFREE™
squelch circuit responds instantaneously to the presence or absence of signals, eliminating
squelch lag and annoying noise-bursts characteristic of hysteresis squelch.

The receiver may be powered by an internal 9-volt battery or an external power source. Vcc to
U1 is regulated at 5.0 volts by U3, while Q1 and U2 operate at the input source voltage. The
MFJ-8400K operates on any well-filtered DC source between 6.5 to 15 volts.

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RECEIVER CONTROLS:

1. POWER ON: Applies power from the internal battery -- or from an external power source -­to receiver circuitry.

2. TUNE: The channel-selector knob for tuning in stations. Dial pointer indicates approximate
frequency of reception.

3. SQUELCH: A control used to blank out background noise when no signal is present.

4. VOLUME: Adjusts voice signals for a comfortable listening level.

5. POWER JACK: Accepts external power sources such as an AC adapter or 12-volt
automotive electrical system. Radio's 9-V battery is disconnected whenever a power plug is
inserted.

6. ANTENNA JACK: Accepts the connector-mounted whip antenna or coaxial lead-in cable
from an external 2-meter antenna.

7. PACKET MONITOR: Provides unsquelched audio for a TNC (Terminal Node Controller -- a
radio MODEM device). Signal level at this jack is not affected by the volume control setting.

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