QMX+ 160–6 m Multi‑Mode Transceiver – Build Tutorial
Completed QMX+ 160–6 m transceiver on workbench

QMX+ 160–6 m Multi‑Mode Transceiver

From unboxing to first QSO – a builder‑friendly step‑by‑step guide

1. Pre‑flight checklist

Category What you need Notes
Documents Latest assembly manual for your PCB revision (e.g. Rev 3 doc 3.05) and matching operating manual Download from the QRP‑Labs QMX+ product page.
Tools 25–30 W temperature‑controlled iron, flux pen, solder wick, fine side‑cutters, tweezers, #1 Phillips, TORX T6, ESD strap, magnifier, bright lighting Six‑layer board & large ground planes need extra heat – plan accordingly.
Test gear Current‑limited bench PSU (12 V / 1 A), DMM, 50 Ω dummy load, watt‑meter or RF power probe, frequency counter or calibrated SDR, USB‑C cable Optional: oscilloscope or spectrum analyser for PA check‑out.
Consumables 60/40 or Sn‑Ag‑Cu 0.7 mm solder, isopropyl alcohol, cotton buds Keep the board squeaky‑clean as you build.
Before you heat the iron:
  1. Clear a static‑safe bench and brew decaf – shaky hands ≠ happy solder.
  2. Inventory every part against the BoM; contact QRP‑Labs immediately if anything’s missing.
  3. Read the manual twice, then watch a build video or two for extra context.

2. Board‑level assembly (main PCB)

  1. SMPS daughter‑boards – align headers perfectly; re‑flow any dull factory joints.
  2. Low‑profile through‑hole parts – resistors, small caps, diodes. Work left‑to‑right, top‑to‑bottom.
  3. Toroids & transformers
    • Wind exactly to the turn count in the manual; keep turns tight and evenly spaced.
    • Strip enamel by dragging the wire through a molten solder blob before inserting – guaranteed bright copper every time.
    • Verify continuity with your DMM.
  4. Power‑train devices (MOSFETs, regulators) and all IC sockets.
  5. Heat‑sensitive parts last – crystals, TCXO, microphone capsule (if fitted).
The six‑layer PCB soaks heat. Builders report best results with a slightly larger chisel tip, plenty of flux and quick, decisive soldering. Wick excess solder before adding the next part.

3. Front panel & LCD

  1. Tack‑solder the 16‑pin LCD header, plug the LCD in dry and verify it sits perfectly flush; then complete the joints.
  2. Install encoders, push‑buttons and 3.5 mm jacks on the front side only – double‑check orientation; mis‑placing them is the #1 de‑soldering horror story.
  3. Thread the front‑panel PCB onto its spacers and tighten just finger‑snug; you’ll remove it again for cleaning.

4. First power‑up & firmware flash

  1. Set your PSU to 12 V, 0.5 A current‑limit.
  2. Hold the left encoder shaft while applying power; the LCD backlight should come up and the current settle around 80 mA RX.
  3. On first boot choose QMX + 160–6 m so the MCU loads correct band data.
  4. Connect USB‑C to your PC, run qld‑flash (Windows/macOS/Linux) and load the latest firmware (≥ 1_02_002). Progress appears on the GUI; the rig reboots automatically.

5. Alignment & self‑test

Step What to do Expected outcome
REFCAL Menu → Tests → Cal TCXO → zero‑beat WWV 10 MHz or use the optional GPS module Reading within ±0.5 Hz
PA bias Menu → Tests → Bias adjust; set each band for 0.22 A at key‑down 3–5 W into dummy load
Band sweep Use the built‑in signal generator; monitor on a separate receiver Spurs ≤ –50 dBc
SWR bridge TX into 50 Ω load; SWR should read ≈ 1 : 1 Confirms bridge wiring

6. Button up the enclosure

  1. Clean the board with IPA, blow‑dry, then mount the chassis rails.
  2. Slide the PCB into the extruded case; use nylon washers under the BNC and GPS SMA to avoid scratching the panel.
  3. Install the dev‑board or QLG3 GPS now if you ordered them.

7. Getting on the air

  • CW / keyer – internal iambic keyer with memories; menu‑select paddle or straight key.
  • Digital – one USB cable to WSJT‑X/JS8Call/etc. Choose the built‑in sound card, set CAT to Kenwood TS‑480, 115 200 baud.
  • (Soon) SSB – firmware 1_03 adds full voice‑mode with CESSB processing; mic plugs into front‑panel 3.5 mm TRRS.

8. Popular mods & extras

Mod Parts Why / when
Internal 3 S 18650 pack & BMS 3 × 18650, TP4056‑3S 11 V rail = full 5 W portable; fits behind LCD.
ATU‑10 nano ATU‑10 kit + ribbon Slips into spare volume; use AUX PTT line.
Fan bracket 40 mm whisper fan Keeps PA cool on FT8 marathons.

9. Troubleshooting cheat‑sheet

No LCD backlight
  • Missed 12 V rail or reversed LCD header
  • Verify +12 V on pin 2 of LCD
High TX current, < 1 W out
  • Toroid leads not fully stripped or PA bias mis‑adjusted
  • Re‑strip leads and realign bias
USB not recognised
  • Solder bridge on MCU header
  • Inspect under magnification and continuity‑test adjacent pins

Still stumped? Search the QRPLabs@groups.io archive – hundreds of builders have logged their fixes.

10. Final sanity & safety

  • Always TX only into a 50 Ω load or tuned antenna.
  • RF burns hurt – the PA heat‑spreader can reach 70 °C at full duty.
  • Obey your local licence conditions; 5 W is QRP, but it can still cause interference.