MFJ-9420 Installation Manual
20-Meter SSB Travel Radio
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 mic 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 that 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:
50-100 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
BLOCK DIAGRAM:
RX BPF
L1-L2
VFO
RX MXR
U1
BFO /
PROD DET
U3
SW-Q5
AF AMP
U4a
AF PWR
U5
XTAL
FILTER
IF AMP
U2
AGC
Q2-Q4
SW-Q6
BUF
Q1
SW-Q8
SW-Q7
T / R
K1
VOL
MIC AMP
U4b
BAL MOD
U6
TX MXR
U7
TX BPF
L5-L6
PRE-DRV
Q9
DRV
Q10
PA
Q11
LPF
M
F S
PTT
