MFJ-9420 Installation Manual
20-Meter SSB Travel Radio
THEORY OF OPERATION
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 lightweight 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.
