Building a QMX Transceiver from QRP Labs

Notable Features

• 3 Versions: 80, 60, 40, 30 and 20m --- or --- 20, 17, 15, 12, 11 and 10m -- or -- 60, 40, 30, 20, 17 and 15m
• CW, FSK Digi, and SSB (USB/LSB) modes
• 3-5W output at 12V supply (can be built for 4-5W at 9V supply)
• Solid-state band switching and transmit/receive switching under CAT control (built in USB Virtual COM Serial port)
• High performance embedded SDR SSB receiver with 60-70dB of unwanted sideband cancellation
• Built-in 24-bit 48ksps USB sound card
• Built-in test signal generator and testing tools
• Switched mode regulators receive current 80mA, Transmit current 1.0-1.1A for 5W output with 9V supply (around 0.7A for 5W with 12V supply).

Initial Build Recommendations

• Use ≥ 60 W soldering iron
• NEEDS- digital multimeter for continuity checks. Have soldering wick/solder sucker on hand, sharp diagonal cutters, magnification
• Sort all components from their bag, tape similar components to a poster board and label
• Read instructions carefully before doing. Then read again.
• Use a PCB vise

Board Parts- Main, Display, Control, SMPS, Spacers

Components-toroids and capacitors vary based on kit you select

Ceramic capacitors and diodes

47 µH inductors

Installed appearance

BS 170 PA Transistors

BS170 transistors function as the Final Power Amplifiers (PAs). They take the low-level radio frequency signal from the transceiver's exciter and amplify it to the target output power for transmission through the antenna

Binocular Transformer T501- major decision time! Will you power your radio with 9 V or 12 V?

Continuity checks must pass before proceeding

Toroid Time

Low pass filter toroids

• They allow your intended frequency to pass cleanly to the antenna while blocking (attenuating) unwanted higher-frequency harmonics

T401

T401 is part of the receive circuit for image rejection; it filters out unwanted frequencies that convert to the desired intermediate frequency (IF); eliminates distortion and interference

T507- I botched this one

BNC antenna connection

The SWR bridge- this binocular toroid is next to the antenna connection and enables sampling of the ratio of forward to reverse power for SWR measurement

Switch Mode Power Supply Boards

• Step-down voltage conversion: They step down your primary input voltage (9V /12V) into clean, regulated 5V and 3.3V rails.
• Reverse polarity protection: They help prevent (further) damage if you accidentally hook up your power supply backwards.
• Firmware failsafe controls: They operate under the direct control of the QMX's internal microcontroller. This allows the radio's firmware to manage start-up sequences, regulate power, and apply self-protection mechanisms (like shutting off if there is a voltage spike or dangerous fault)

Completed Main Board

With Control Board

Inspect boards carefully for solder bridges/whiskers before applying power!

Completed Assembly With Display Board Ready to Go in Enclosure

Post Build Testing- Terminal emulator

• INSTALL FIRMWARE FIRST
• QMX features a terminal emulator, I used PuTTy serial connection to the COM port that shows up in device manager upon plugging in the radio
• Can use internal signal generator or RF (on dummy load) to test individual components as well as overall function of the radio
• SSB calibration and mic gain adjustment
• Allows for easier entry of preferences or message memories to be stored on radio

SSB Operation

• Enabled via a firmware update 2 May 2025
• Uses polar modulation for efficiency; less heat generation vs linear amplification and less power consumption
• Commercial microphone not available- build your own

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Great Features and Performance in a Minimal Footprint

Transceiver, battery (9 V), key, speaker

US-5273 Robinson Hollow State Forest 5/17/26 18:59-19:58

• 20 m activation @ 5 W
• ¼ wave vertical antenna
• 10 CW contacts
• 7 SSB contacts
• With spotting

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QSO Maps

POTA activation

Hunting from home

Run WSJT-X modes via CAT, or use your phone for FT8/4 with iFTx App

OEM lightning to

USB camera adapter