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.
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