MFJ MFJ-9420 User manual

Congratulations on your choice of the MFJ-9420 20-Meter
transceiver. The MFJ-9420 is specially designed to deliver
performance you never thought possible from a low power radio.
Please read this manual carefully before attempting to operate
your new radio.
FEATURES
The MFJ-9420 is an easy-to-operate,"back to basics" radio. Operate the unit with ease in minutes!
Quiet DBM front end and lots of overall gain for great sensitivity. If the signal is there, you'll hear it!
Adjust the tuner or aim the beam with pin-point accuracy. A calibrated analog S meter measures even the
smallest variations in signal strength. The meter also monitors your speech processing level on transmit.
Sharp ladder filter cuts adjacent chatter and focuses transmitter power where you need it most for great
selectivity.
Effortless tuning with custom manufactured 8:1 reduction drive ball bearing VFO tuning capacitor provides
smooth and stable VFO.
Big audio and powerful AF output even in noisy locations from a special Philips BTL audio chip and
rugged 3" speaker.
Full time RF speech processing commands speech cuts through QRM and competes with radios running
far more power. Compare with your 100-watt rig. You'll be amazed by the signal reports!
Packs the same punch as 50-Watt radios without the heavy 10 Amp power supply! Designed to run on
NiCd's or the ultra light MFJ 4110 ac wall adapter supply.
Rugged Motorola PA transmitter runs cool, and tolerates 3:1 VSWR accidentally opened feedlines or
feedline shorts.
Conservative design, with a premium plate through pc board, quality components, handsome brushed
aluminum panel, and a tough vinyl clad case ensure years of dependable service.
Available CW adapter board installs and lets you jump
to the low end to work CW.
Semi-break in keying and sidetone.
At home or on the go, you'll enjoy countless hours operating the MFJ-9420. Best of all, it's fully backed by
MFJ's exclusive "No Matter What" 1 year guarantee. If it breaks, we'll take care of it!
You'll need three things to get your MFJ-9420 on the air fast:
12 14 Volt 2 Amp Power Source.
600 Ohm Dynamic PTT Microphone
20 Meter Antenna
1. POWER SUPPLY: The MFJ 4114 AC/NiCd Portable Power Pack or
MFJ-4110 ac Wall adapter. Supply are specially designed to run
the MFJ-9420. You may also use any 13.8 Volt supply or 10-cell
NiCd pack as long as it delivers 2 A on peaks (12 Volt supplies
will work fine but at reduced RF output). Use a 5.5mm OD x

2.1mm coaxial type power plug (Radio Shack 274 1567) with (+)
connected to the center pin.
2. MICROPHONE: The MFJ 9420 speech processor circuit was designed around the companion MFJ 290
600 Ohm Dynamic Microphone. Radio Shack's 21 1172 replacement microphone also plugs in without
modification and works well. If the mike you select requires a plug, install a 5 Pin DIN connector (Radio
Shack 274 003) as shown:
Pin 3: PTT Line
Pin 4: Mic Line
Pin 1,2,5: Ground
3. ANTENNA: The MFJ 9420 will function with any 20 Meter antenna exhibiting a VSWR of 3:1 or less
(your home station antenna or the MFJ 1772 Portable Dipole should work fine). AVOID USING
ANTENNAS WITH UNKNOWN OR HIGH VSWR. The MFJ 9420 PA is not protected by high VSWR
shutdown circuitry and you may generate out-of-band parasitics if a high VSWR load is present.
IMPORTANT OPERATING NOTES:
Your MFJ 9420 features a very potent speech processor. PLEASE RESIST THE TEMPTATION TO
SHOUT OR "CLOSE TALK" INTO THE MICROPHONE IN ORDER TO BE HEARD! Instead, hold the
mic about 2"away and speak normally. The S Meter should deflect about 1/2 scale as you speak although
this may vary somewhat with antenna load. When operating in noisy environments, you may find it
necessary to turn down the mic gain control (a screwdriver adjustment located on back panel) to reduce
background noise.
The MFJ 9420 uses an analog VFO tuning dial which is inherently less accurate than digital readouts (on
the plus side, analog tuning reduces phase noise, lowers power consumption, and saves you money). Please
take the potential for dial error into account when operating near the edge of your authorized sub band.
RECEIVER SECTION:
Frequency Coverage: 14.150 14.350 MHz (14.000 14.100)*
Receiver Type: Single conversion superhet
VFO Frequency: 4.150 4.350 MHz (4.000 4.100)*
IF Frequency: 10 MHz
IF Selectivity: 6 dB at 2.5 KHz
AGC: Audio derived
Sensitivity: <.5 uV for 12 dB S/N
Audio: >1 Watt into 8 Ohms at 10% THD
Receive Current: 5O 100 mA typical
TRANSMITTER SECTION:
RF Power Output: 10W Average Speech (8 10W CW)*
VSWR Tolerance: 3:1 VSWR
Maximum Current: 2.2 Amps peak at 13.8 Vdc
Audio Enhancement: RF compression processor
* Requires CW adapter module
RECEIVER: Four pole bandpass filter L1 L2 preselects incoming 14 MHz signals. Active mixer U1
amplifies and converts signals to 10 MHz using a self generated 4 MHz VFO signal. Crystal ladder filter
Y1 Y6 sets message channel bandwidth (diode switching routes transmit and receive signals through the
filter and U2). IF amplifier U2 provides AGC controlled IF gain in RX and RF compression speech
processing in transmit.

The dc amplifier Q3/Q4 drives the AGC input of U2 and the S meter circuit. In RX, AGC is audio derived
from AF pre-amp U4a (switch Q2 sets slow AGC decay for SSB reception). On transmit, processor control
voltage is derived from PA level detector D9 (Q2 sets fast decay for syllabic processor).
Receiver product detector U3 demodulates incoming SSB signals (10 MHz LO is self generated by U3).
U3's audio output feeds U4a, a preamp and active LP audio filter. U4a drives ACG detector D6/D7 (input
to U4a is killed by switch Q5 during TX to disable the AGC path to U2). U4a also drives AF power
amplifier U5 through the volume control. U5, which powers the speaker during RX, is gated into standby
during TX to prevent residual feed through.
TRANSMITTER: Microphone speech amp U4b drives balance modulator U6 during TX (switch Q6 kills
U4 output to prevent mic bleed through via U6/U2 during RX). LO for U6 is derived from U3's BFO
oscillator. Q8 is used to unbalance U6 for carrier or CW generation. DSB output from U6 is routed to Y1
Y6 via switch D1/D2 for removal of LSB products and carrier artifacts. After undergoing dynamic
compression in U2, the processed USB signal is routed to transmitter mixer U7 by switch D3/D4 where it is
mixed with 4 MHz VFO (VFO signal is sampled from U1's oscillator and buffered by Q1). Bandpass filter
L5 L6 selects the 14 MHz mixer product and feeds it to pre-driver Q9. Q9 feeds driver Q10. Q10 drives
PA stage Q11. Q11 operates in single-ended class AB with bias generated by clamping diode D10. T5
matches PA output to a 50 Ohm system, and low pass filter L7 L8 suppresses harmonic content. Level
detector D9 tracks the SSB speech envelope and generates feedback to Q3/Q4 and U2. Syllabic
compression of the speech waveform optimizes average transmitter output power. The MFJ 9420 is
especially designed to operate from light-weight constant current power sources such as NiCd cells or the
MFJ wall adapter AC supply.
SWITCHING AND REGULATION: Q7 activates relay K1, which routes the antenna line and activates the
+T and +R buses (+T and +R buses power some stages, key switch nodes, and bias Q11 on during TX).
Adjustable regulator U8 sets Vcc for low level stages. Fixed regulator U9 sets U1 operating voltage to
ensure VFO stability. Crowbar diode D11 and a pc track fuse protect the radio from reverse polarity.
REAR PANEL
1. POWER JACK: 5.5mm OD, 2.1mm ID, (+) to center pin.
2. ANTENNA JACK: SO 239 for standard coax plugs.
3. MIC GAIN: Sets Speech amp Gain (normal setting 12:00).
4. CW ADAPT: Engages CW adapter (accessory).
5. KEY: Accepts straight key or keyer.
FRONT PANEL
1. METER: Show signal strength on RX, ALC on TX.
2. POWER SWITCH: Turns power on to transceiver.
3. MIC: 5 Pin Din Jack for Dynamic Microphone.
4. TUNE: Injects carrier for ATU adjustments.
5. XMIT: LED illuminates when transmitting.
6. TUNING: Tunes in stations, indicates VFO frequency.
7. VOLUME: Adjusts listening level.
8. PWR INDICATOR: LED illuminates when power on.
To put your MFJ 9420 station in operation, you'll need a power source, a 20 Meter antenna, and a
microphone. Here are some specific suggestions to help you get started:

POWER SOURCE: 12 15 V @ 2A (+) TO CENTER PIN
The MFJ 4114 ac/dc PORTABLE POWER PACK and MFJ 4110 ac WALL ADAPTER SUPPLY are
especially designed for your radio. However, you may use any filtered dc power source capable of delivering
12 15 volts at 2 Amps (13.8V required for full RF output). The dc power connector at the rear of your MFJ
9420 is a 5.5mm OD,2.1mm ID coaxial type jack. Extra plugs are available from Radio Shack stores (#274
1567). Always connect plus (+) to the CENTER PIN.
MICROPHONE: USE 600 OHM DYNAMIC
The MFJ 9420 speech circuit is designed especially for a 600 Ohm dynamic microphones such as the MFJ
290 (the MFJ 290 is supplied if you purchase the MFJ 9420M package). You may also use other suitable
600 Ohm desk or hand mics by installing a 5 pin DIN plug (Radio Shack #274 003). See installation
diagram below:
PTT pin 3
MIC pin 4
GND pins 1,2,5.
WARNING:
NEVER USE AN AMPLIFIED MICROPHONE WITH YOUR TRANSCEIVER. The speech processor
already has extremely high gain and may distort severely if overdriven by a "power mike".
ANTENNA REQUIREMENTS: VSWR 3:1 OR LESS
The MFJ 9420 accepts most 20 Meter antennas with a VSWR of 3:1 or less. Use your regular station
antenna or a portable 20 Meter dipole such as the MFJ 1772 ultra light. For non resonant wires, use a tuner
such as the MFJ 971. Always "rough tune" for maximum receiver sensitivity first using incoming signals
and your S Meter to find a peak. AVOID OPERATING INTO UNMATCHED HIGH VSWR ANTENNAS.
THE MFJ 9420 DOES NOT HAVE A HIGH VSWR SHUT DOWN CIRCUIT AND BADLY
MISMATCHED LOADS COULD RESULT IN TRANSMITTER
INSTABILITY AND GENERATION OF OUT OF BAND SIGNALS!
TRANSCEIVER:
STEP 1. Connect a suitable microphone, power source, and 20 Meter antenna.
STEP 2. Turn VOLUME fully counter clockwise (down) and press ON switch to power up radio. PWR
indicator should light.
STEP 3. Turn VOLUME clockwise for a comfortable listening level.
STEP 4. If you are using an antenna tuner, adjust ATU controls for strongest received signals or maximum
background noise on the S meter. Then, press CARRIER to generate a steady RF signal for final
adjustment. Red XMIT indicator should light.
STEP 5. The VFO tuning range covers 14.150 14.350 MHz and the MFJ 9420 tuning dial was carefully
calibrated at the factory. However, it is by nature an analog frequency readout that may be subject to some
inaccuracy or misinterpretation. To ensure compliance with FCC rules, we suggest the following:

(a.) Periodically check the VFO dial with a digital transceiver or counter to confirm location of band
edges.
(b.) Avoid transmitting on the margins of your allocation unless you are certain the station you are
calling is in "in band."
STEP 6. To transmit, press the mike's PTT switch and speak at normal conversational level with the mike
held about 2 inches from your mouth. The MFJ 9420 has a very potent speech processor and will pick up
your voice at full intensity. Avoid the natural temptation to yell or close talk this will only overload the
processing circuit and hurt the intelligibility of your signal!
SPECIAL OPERATING NOTES:
The MFJ 9420 does not have automatic high VSWR shutdown and you must provide a "reasonable" 20
Meter antenna to ensure proper operation. Whenever possible, use a low SWR resonant type antenna such
as a dipole or tuned vertical. If you use an ATU (or a high Q loop such as the MFJ 1786 or Isopole 10 30),
carefully rough tune first for maximum S meter readings before activating the CARRIER switch.
Under FCC Rules, licensed amateurs are solely responsible for the frequency of operation of their
transmitters. MFJ cannot be held responsible for out of band operation due to misinterpretation or
miscalibration of the MFJ 9420 analog VFO dial.
COAXIAL FED DIPOLES: A wire dipole for the phone band should measure about 32'10" (16'5" per leg).
Feed with RG 58U or RG 8X; raise high and in the clear for best performance. If you have only one high
support, make an inverted V. The "full sloper" (sloping dipole) is also an excellent single support antenna
with some directivity.
MFJ 1772 PORTABLE DIPOLE: As an alternative to making a coax-fed dipole, consider purchasing the
pre assembled MFJ 1772 antenna. This is a no compromise 20 Meter folded dipole complete with 30' of
feedline all made from ultra light weight 300 Ohm twinlead. Connect a random length of coax from the
MFJ 1772 feedline to your rig and let the antenna's built in 50 Ohm matching network do the rest. The
antenna is pre tuned, easy to handle, and sufficiently broad banded to cover both the CW and phone band.
WARNING: CONSTRUCTING OR ERECTING AMATEUR RADIO ANTENNAS IN
LOCATIONS WHERE THEY MAY COME INTO CONTACT WITH ELECTRICAL POWER LINES
MAY RESULT IN ACCIDENTAL INJURY OR DEATH!
While spanning the globe with a small portable rig like the MFJ 9420 may SOUND difficult, it really isn't.
High power and big antennas have never been a requirement for world wide radio communication and still
aren't. Despite the vast distances involved, hams consistently reach every corner of the planet running far
less power than your MFJ 9420. How do they do it? Here are some proven tips guaranteed to work:
GROOM YOUR ANTENNA:
Inspect and clean all traps and contacting surfaces. Replace corroded hardware, dirty connectors, and aging
coax. Retune for minimum SWR in the phone band.
Raise wire dipoles high and in the clear 40 to 60 feet is far better than 10 or 20. Carefully prune for
minimum SWR. If you have only one high support, install an inverted V or sloper.
Multiband verticals (1/4 wave types) require at least 4 good 14 MHz radials add them if needed. Clean up
corrosion and adjust your top section for minimum SWR in the phone band.
Avoid "compromise" antennas. When it comes to pure operating enjoyment, every dB you gain at the
antenna pays big dividends!
USE PROVEN DX OPERATING TECHNIQUES:

Be a good listener and search out and answer CQs, or call stations that have just completed QSOs (a
practice called tailgating).
Never hesitate to call a weak station because it may be QRP or using a marginal antenna. You may be
much louder than you think!
Be realistic about diving into huge 20 meter pileups. Once a
choice DX station hits the packet clusters and the big guns move
in, calling may be a waste of time until the smoke clears.
Tune slowly and listen carefully. If one exotic station is
coming through, there may be others you can work nearby.
Pay attention to DX forecasts and gray line propagation. When
the band is hot, power differences become much less significant.
Operating just before the band goes out at night may yield strong
signal reports from exotic locations with little QRM.
Don't be shy about calling. The MFJ 9420 has very potent audio
and on air tests indicate its signal is competitive with higher powered radios. If your antenna is good, most
stations won't
even know you are running low power!
US AT
800 647 TECH (800 647 8324): Your MFJ 9420 is fully backed for 1
year by MFJ's "NO MATTER WHAT GUARANTEE". After that, MFJ
Customer Service Technicians will be there to help you keep your
rig in top shape for as long as you own it. However, before you
call us, please check through this list just to make sure the
problem isn't something simple you can fix yourself!
1. RADIO DOES NOT POWER UP
Is power plug okay, broken supply wire?
Is supply or battery functional?
Is reverse polarity fuse open?
2. NO SIGNALS RECEIVED
Is antenna disconnected or broken?
Is TUNE switch stuck on?
Is the band dead?
3. RADIO DOES NOT TRANSMIT OR KEYS ERRATICALLY
Is microphone and connector okay?
4. ERRATIC TRANSMIT
Is SWR high (above 3:1)?
Is power source voltage low or "soft" under load?
5. AGC IS INEFFECTIVE OR S METER IS NOT ZEROED
Is Regulator okay (10.0 V on TP1)?
Is AGC okay (4.3 on TP2?)
If TP1, TP2 okay and AGC works, rezero meter via R51.
6. RADIO DRIFTS IN FREQUENCY
Is radio sitting in Sun, on hot surface, in cold draft?
Was radio moved from extreme cold to warm (condensation)?

If you suspect condensation, allow radio to fully dry out.
7. POOR CARRIER SUPPRESSION
Was radio moved from extreme cold to warm (condensation)?
Was radio exposed to rough handling or shock?
To null, unplug mic and ground pin 3 to key radio. Adjust R36
and T3 for minimum carrier (monitor using a second receiver).
8. LOW TRANSMIT AUDIO:
Is the mic a 600 ohm dynamic type?
Is the MIC GAIN set too low?
9. NOISE ON TRANSMIT AUDIO:
Is MIC GAIN set too high in a noisy room?
Is mic next to a power transformer or motor?
If these checks don't uncover the problem, or if you don't feel
qualified to make the prescribed adjustments, please call us for
help at 800 647 TECH (800 647 8324).
FIELD ALIGMENT PROCEDURES FOR THE MFJ 9420 TRANSCEIVER:
SPECIAL TOOLS, PARTS, TEST EQUIPMENT:
AC Power Supply, 13.8 Volts @ 2 Amps
Voltmeter
Non inductive Alignment Tool kit
Frequency Counter
Wattmeter with 50 Ohm Resistive Dummy Load
14 MHZ Signal Generator or other weak signal source
General Coverage Receiver with digital readout
INITIAL TEST SET UP:
A. Remove transceiver cover.
B. Connect 13.8 Volts Power Supply to Power Jack.
C. Connect Microphone.
D. Turn on unit.
VOLTAGE CHECKS AND ADJUSTMENTS: (use voltmeter)
A. VOLTAGE REGULATOR: Set R64 for 10.0 V at TP1.
B. RECEIVER AGC: Set R6 for 4.3 Volts at TP2.
VFO CALIBRATION:
A. Tune VFO dial to 14.250 MHz (mid band).
B. Connect counter probe to U7 pin 6
C. Adjust L3 for 4.255 MHz readout.
BFO FREQUENCY CHECK AND ALIGNMENT:
A. Tune across 14.250 MHz. test signal (use generator).
B. Identify 2 peaks in passband by watching S meter.
C. Carefully set VFO dial to lower frequency response peak.
D. Adjust BFO trimcap (C61) until CW note is 600 Hz.
E. Recheck. First peak should be at 600 Hz, second peak around 2000 Hz. Pitch of CW tones should go
DOWN as you tune UP in frequency, indicating USB operation.
CARRIER NULL: (use a second receiver to monitor beat note)
A. Short mic pin 4 to ground to kill mic pickup.
B. Key mic and monitor signal slightly off frequency.

C. Alternately adjust R36 and T3 for minimum carrier.
RECEIVER SENSITIVITY CHECK:
A. Connect weak 14.250 MHz signal source to antenna jack.
B. Tune in signal source for maximum S meter reading.
C. Touch up L1, L2, T1, T2 for max SIG Meter reading.
TRANSMITTER BANDPASS FILTER AND COMPRESSOR ALIGNMENT:
A. Connect QRP Wattmeter with dummy load to ANT Jack.
B. Turn R61 full CW, Press Carrier Switch (SW2).
C. Carefully touch up L5 and L6 for peak output.
D. Turn R61 CCW until RF output power drops by 10%.
If radio fails to operate properly after following these
procedures, call 800 647 8324 for help.
INTERNAL ADJUSTMENT LOCATIONS:
V REG: Regulator voltage
AGC: U2 AGC threshold
M ZERO: Panel meter zero
BFO: Carrier osc. freq.
VFO CAL: VFO freq. adj.
IF1,IF2: IF ampl. peak
L1,L2: RX front end peak
L5,L6: TX filter peak
CAR BAL,T3: Carrier null
PA BIAS: PA idle current
SP PROC: TX ALC level
MIC GAIN: TX audio level
TUNE SW: Full carrier out
PWR: Power on
VOL: AF Ampl. gain control
VFO TUNE: Sets operating freq.
FUSE: Reverse polarity protect
*If reverse polarity track fuse opens, install a miniature 2.5 A
pigtail fuse or a 3/4" hair pin loop of #32 wire.
PARTS PLACEMENT, MFJ 9420:
DC VOLTAGE CHART TROUBLESHOOTING GUIDE:
For advanced troubleshooters, the following are typical DC
voltages found in the MFJ 9420. Voltages were checked with
handheld probe type DVM to minimize effects of lead inductance
and to limit the potential for RF feedback.
Vcc = 13.8 (Supply Voltage)
TP1 = 10.0 (LM 317 Output)
TP2 = 4.3 (AGC Bias Voltage)
INTEGRATED CIRCUITS: (TX checks made with mic. keyed, no audio)
RX TX
Pin U1 U2 U3 U4 U5 U6 U7

1 1.4 10.0 1.4 5.0 9.9 1.3 1.4
2 1.4 10.0 1.4 5.0 2.5 1.3 1.4
3 0 0 0 5.0 0 0 0
4 3.9 3.3 4.0 0 2.9 4.9 4.7
5 3.9 4.9 4.0 5.0 4.9 4.9 4.7
6 4.9 3.3 5.0 5.0 0 5.7 5.8
7 4.5 0 4.6 5.0 0 4.9 5.2
8 5.0 10.0 5.1 9.9 4.9 4.9 5.8
BIPOLAR AND JFET DEVICES:
D/E = Drain/Emitter
S/B = Source/Base
G/C = Gate/Collector
* = Test in TRANSMIT mode
** = Test with TUNE switch depressed
*** = DO NOT TEST gate of Q9 may cause generation of parasitic oscillations.
Q1 Q2 Q3 Q4 Q5* Q6 Q7* Q8** Q9 Q10
D/E 9.8 0 9.0 9.7 0 0 3.5 0 13.0 0.25
S/B 2.3 0.7 3.9 8.9 0.7 0.7 2.8 0 0 1.0
G/C 0 13.2 0 4.3 13.0 0 0 9.0 *** 13.7
Q11 (MRF477 PA Transistor): To avoid RF damage to DVM, check
collector voltage in receive mode only full supply voltage
(13.8v) should be present. Check base (bias) with PTT pressed,
no audio this should indicate 0.6v. Emitter is grounded.
SCHEMATIC DIAGRAM, MFJ 9420:
PARTS LIST: