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.
