QMX bench log

Do all of this before any soldering. It takes an evening and catches the two Rev 1 problems that can destroy the board.

• Read manual sections 1 to 2.1 twice, then the PA transformer manual
• Check the parts against the BOM (manual section 2.3). Expect the 22pF capacitor ("220") to be unused and C403 not fitted
• Count the nylon hardware: 5 hex spacers (11mm), 12 screws (6mm), 2 screws (9mm), 6 nuts. One builder's kit was missing a nylon bolt, so keep spares
• Look at the 2x4 and 2x5 header strips; one builder found rough cuts that left a pin unsupported
• Tape the 15 through-hole capacitors to a card and label them with the table below
• Break the boards out of the panel: grip each tab with needle-nose pliers and twist
• File every rough edge smooth, especially on the two small SMPS boards
• Keep the two tiny spacer boards that break out of the display board panel: the one with the 2x4 holes (for the long header pins, 2.19) and the small square one (under the hex spacer, 2.26). They are easy to lose
• Snap off the 5mm rail of spare board material if there is one
• Test the bare 5V SMPS board for the Q103/Q104 short (below)

Capacitor identification (80-20m)

Bare-board Q103/Q104 test

Solder can bridge the large drain tab of Q103 (the big AOD403) to the drain pin of Q104 (the small transistor next to it). Look at the two under magnification, then set your multimeter to continuity and touch the Q103 drain tab and Q104's drain pin. A beep or zero ohms means the short exists.

If it does, heat Q104 and slide it about 1mm away from Q103. Q104 is a small SOT-23 part, and the drain is normally pin 3, but confirm that on the Rev 1 layout PDF because I couldn't see the manual's marked photo. Repeat the test right before first power-up.

These are the easy parts, but they set up everything after, so orientation and clearance matter more than speed.

1. Capacitors (2.5). Fit all 15 per the diagram. Ground pins take extra heat. Capacitor leads may need bending from 5.08mm spacing to the 2.54mm pads. C525 must not overlap the outline of the adjacent connector, or leave it until later. Keep every lead off-cut.
2. 1N4007 diodes D508 to D513 (2.6). The white stripe is the cathode and must match the stripe on the silkscreen. Stripe in the left hole for D508 and D511, the right hole for D509 and D510, the top hole for D512 and D513. Bend one lead 180 degrees so the body stands upright.
3. 47uH inductors L505, L507, L509, L514 (2.7). Same upright mounting. Orientation isn't critical, but handle them gently because they can break.
4. BS170 PA transistors Q503 to Q506 (2.8). Flat faces flush against the board, pressed down with the M3 10mm bolt, washer and nut. Take static precautions.

Tips from other builders

• Sit the capacitors and diodes a fraction proud of the board, so there's room to move toroid wire around later.
• One builder suggests checking where L507 will fit before fitting C514, C515 and C517, since their board's LPF toroids then wouldn't sit as shown. Their revision may differ, so treat it as a prompt to check, not a rule.
• D512 and D513 sit under the right-hand rotary encoder. Another builder had to bend D513's lead sideways, and taped the encoder, so the encoder body can't touch it. Think about this now, and leave those two diodes leaning away from the encoder position.

Checkpoint

Most build failures trace back to this stage, usually enamel left on a wire or a toroid in the wrong place. Do one component at a time, test continuity after each, and don't move on until it passes.

Wire budget

The 0.33mm wire is almost all used. By my sums from the manual's lengths, the toroids need about 353cm of the 365cm supplied, so cut to the stated length and don't waste any.

Wind in the direction that puts the wire ends near their holes: start from the top, down through the hole, then around and over, accumulating turns anticlockwise. Each pass through the centre counts as one turn.

1. T501 output transformer, 3:2 for 12V (2.9 and PA transformer manual)

The primary is always 3 turns with a centre tap. The secondary is 2 turns for 12V. Never fit a 3-turn secondary on a 12V build, because it will likely destroy the PA transistors.

1. Straighten the 0.60mm wire. Pass it through both holes from the top left; that is turn 1.
2. Pass it back through the top hole, left to right, for 1.5 turns.
3. Pass it back through the bottom hole, right to left, but leave a small loose loop. That loop becomes the centre tap.
4. Go through both holes again to finish 3 primary turns. Leave about 1cm sticking out.
5. Secondary: right to left through the bottom hole, then left to right through the top hole. Repeat once for 2 turns. Cut leaving 1cm.
6. Bend all wires down to meet the PCB holes. Cut the centre-tap loop. Those two wires should be longer than the secondary ends. Twist them together.
7. Test before soldering: all primary points connected, both secondary points connected, and no continuity between primary and secondary.
8. Scrape the enamel from the four ends first, then the two centre-tap wires. Cut to about 2mm. Solder with plenty of solder, 10 seconds on the ends and about 15 on the centre taps.
9. Check nothing protrudes more than 2mm below the PCB, then re-test the five joints as the manual's step 17 describes.

If you later get a drill, the twisted RWTST version is the recommended 12V design, but it isn't needed for 80-20m.

1. L401 tapped band-pass inductor (2.10)

1. Cut about 60cm of 0.33mm wire. Wind 28 turns on the red T50-2 toroid, with a loop between turns 19 and 20 for the tap.
2. Squeeze the tap loop into a sharp point so it fits the large tap hole.
3. Fit the start, the 19-turn tap and the end of the winding. No wire goes in the hole marked X.
4. The two holes labelled 28 must be jumpered together. The end of the winding goes in the larger of them. On the forum, the key point was that the two 28 holes are shorted and the end can connect either way.
5. Pull wires tight from underneath, cut to 2mm, solder 10 seconds each.
6. Check from the underside: pins 3, 4, 5 and 9 of IC3 have continuity to each other, and pin 6 does not. One builder put the 19-turn tap in the X hole and caught it only because of this test.

1. L502 (2.11)

10 turns of 0.60mm wire on an FT37-43 (dull black). Scrape the enamel, don't tin the wire, cut to 2mm, solder 10 seconds.

1. Six low-pass filter toroids (2.12)

All are yellow T30-6 cores wound with 0.33mm wire. Spread the turns evenly with a small gap; slightly tighter is a little better, but don't snap the wire.

• Count out loud. A wrong turn count is hard to find later.
• Cutting the two leads to different lengths makes them easier to insert.
• 28 turns on a T30 is tight; one builder couldn't keep all turns neat. Keep them even and don't over-tighten.
• Test continuity end to end on each toroid. Poor connections here cause low power on whole bands.

1. T401 trifilar transformer (2.13)

1. Cut three 25cm lengths of 0.33mm wire and twist them tightly together, about 60 twists per 20cm. Keep them under light tension.
2. Wind 10 turns on an FT37-43. Leave about 2.5cm of tails.
3. Untwist the ends. Tin the last few mm of each. Find the three pairs (A to A, B to B, C to C) with the multimeter, and label the ends with marker or coloured tape. Forum members link audio sweep faults to T401, and there's no DC test that proves the winding is right.
4. Insert with the orientation as the manual's diagram shows. Check continuity again at the board before soldering.
5. Cut and solder one wire at a time, so the toroid doesn't fall out. There are SMD parts close by.
6. Check that A, B, C, IC402 pin 7, and IC403 pins 7 and 9 are all connected.

1. T507 (2.14)

1. Wind 10 turns of 0.33mm wire through each hole of the BN43-1502 binocular core, and thread a 25mm length of 0.60mm wire once through each hole.
2. Fit the 0.60mm single turns at B-B and C-C, and the 10-turn ends at A-A and D-D. Scrape the enamel off the ends, don't tin them.
3. It sits in the PCB cut-out and should poke out about 1mm below the board, so the right encoder clears it. Adjust the 10-turn windings on the top side before soldering. Wind the 10 turns in the same direction as the other toroids.

Checkpoint

The SMPS boards are, in the manual's words, the least forgiving part of the build: one short here can destroy the whole radio. Go slowly and inspect everything under magnification.

Connectors (2.15)

1. Fit X101 (DC jack), X202 (paddle), X401 (audio), X201 (PTT) and X502 (BNC). The USB-C connector J201 is already fitted, but touch up its joints and check for bridges between the closely spaced pins.
2. Hold each 3.5mm jack at exactly 90 degrees while you solder; they try to twist. If you plan to use the enclosure, bolt the right-hand panel to the BNC while soldering so everything sits square.
3. Clip excess leads so nothing stands more than 1.5mm proud, and no body overhangs the board edge.
4. Optional PTT mod: with X201 fitted, solder a jumper wire from pin 2 to pin 4. It only matters for the grounded PTT output, and nothing breaks without it.

Female header sockets (2.16)

Fit the 2x5, 2x4 and 2x2 female sockets. One builder fitted male pins where the female sockets belong, which is a painful mistake to undo. Keep them square.

SMPS boards (2.17)

1. Prepare both boards with smooth edges. Do not solder their 2x4 and 2x3 female sockets yet.
2. Fit a 9mm nylon screw per board (head on the underside of the main PCB) with a nylon nut on top as a spacer.
3. Place each SMPS board over its screw and secure it with another nylon nut. Take care around the nearby 2x5 header, the BS170s and the delicate SMD parts.
4. Mate the 90-degree male headers (2x4 and 2x3) with their female sockets, and push them into the main board without soldering.
5. Trim about 1mm off each female header pin to reduce the risk of shorts.
6. Check everything is square. Solder the male headers to the main board with the plastic suspended off the surface by about 1mm, so the SMPS boards sit level. Solder quickly with a lower temperature if you can.
7. Solder the top sides of the female sockets on the SMPS boards.
8. Unplug the boards, pulling on the large AOD403 transistors with a fingernail and never on the 330uH inductors. Solder the bottom rows of pins.
9. Inspect every joint with the loupe: dry joints, bridges, whiskers.
10. Reinstall the boards and tighten the nylon nuts without over-tightening. Push the bolt near the PCB corner away from the corner, or it will interfere with the 11mm spacer later.

Checkpoint

This stage is mechanical, and the clearances are tight. Test-fit the boards before you solder anything that can't be undone.

Display board (2.18 to 2.21)

1. Fit the LCD module from behind, through the rectangular cut-out. Bolt it with the M3 6mm screws, with equal gaps top and bottom.
2. Drop the saved capacitor off-cuts through the 16 holes with their bottom ends sitting on the bench. Solder them on the top side and trim.
3. Turn the board over, make sure the LCD sits flat, then solder and trim the other side.
4. Fit the 2x5 male header from below, with the short pin ends through the board. Solder one pin first, check alignment, then the rest. Fit the 2x2 male header on the right the same way.
5. Fit the four 11mm nylon hex spacers with 6mm screws. Turn each so a flat side is parallel to the PCB edge, so no corner overhangs.
6. Trimmer R305 (20K, LCD contrast): cut off its four plastic feet so it sits flat. It's too tall otherwise for the enclosure.

Controls board (2.22 to 2.26)

1. The 2x4 male header has long pins. Slide its plastic body down until it sits flush on the socket. Put the small spacer PCB between the header body and the underside of the controls board. Solder, then trim the excess pin length.
2. Rotary encoders: cut off the two large mounting lugs, not the pins. Bend each of the five switch pins 180 degrees toward the front. Fit the locating tab in its hole, with the nut on the silkscreen side. No washer. Solder the five pins on the underside. The centre pin may need a wire off-cut to reach its hole. Use a small tip, and keep solder from the centre pin away from the board edge, since it can touch the metal case.
3. Check each encoder's button pins for a solder blob to the encoder's metal body. That fault made one radio show "1." on the display and another switch itself on.
4. Tactile buttons: solder two diagonal pins, check they're square, then the other two.
5. Electret mic: polarity is critical. The pins must sit exactly in the silkscreen circle.
6. Fit the last 11mm hex spacer on the controls board with an M3 6mm screw from the front, with the small square spacer on the screw.

Fit and join (2.27 and 2.28)

1. Plug the controls board into the main board by its 2x4 headers. Fix it with an M3 6mm screw from underneath into the spacer pillar.
2. The black metal LCD retaining tab nearest the 2x5 header clashes with a 330uH inductor. Bend it about 45 degrees; one builder had to bend it nearly flat.
3. Join the boards by lining up the 5-pin headers at top left and the 2-pin header at top right. The rest falls into place.
4. Check D512 and D513 against the right-hand encoder, as in Stage 1. A scrap of insulating tape over the encoder body is cheap insurance.
5. One forum builder recommends this fit check early, before committing to later parts.

Enclosure (2.29)

The enclosure is optional. If you have it: slide the display board into the guide rails, bolt the right-hand panel to the BNC with its washer and nut, then follow the manual's photo sequence. Fit the knobs last, with a small gap to the front panel, and put the volume knob's grub screw on the shaft's flat.

My suggestion: bench-test the radio unboxed on a non-conductive surface first, so you can reach the board. Fit the case only after the sweeps pass.

Checkpoint

The radio has no firmware from the factory, so the first power-up should show zero current and a USB drive. Anything else means stop and check.

Final checks before power (2.31)

• Inspect the whole board under magnification for dry joints, bridges and whiskers
• No short between the left encoder body and the DC jack
• No solder bridge between the rear pad of the DC jack and the nearby corner pin of the 2x4 header to the 5V board. The gap is very narrow, and a bridge here will fry the radio permanently
• Q103 and Q104 not shorted (repeat the Stage 0 test)
• 2x4 and 2x3 headers and the SMPS edge connectors all soldered, with no bridges
• Both SMPS boards plugged in and secured
• Controls board seated and secured; the left encoder's long press switches the radio on
• Contrast trimmer fully anticlockwise
• Supply set to about 7V with a 250mA current limit

First power-up

1. Connect the USB-C cable to your PC and the supply to the DC jack. With nothing pressed you should see zero current.
2. Long-press the left rotary encoder. The radio should appear as a USB flash drive named QMX, with a blank LCD and no backlight. That is firmware update mode.
3. Download the latest stable firmware from the QRP Labs QMX page, unzip it, and drag the .QMX file onto the QMX drive. The radio reboots and the display comes up.
4. If you see a prompt for the band version, choose 80-20m. Don't skip it, because that item becomes read-only later; a factory reset is the way back.
5. Turn the contrast trimmer slowly until the text appears. A row of black blocks with no text means the display is not talking to the processor. Check the 2x2 header on the right.
6. Power off with a long press of the left encoder, then raise the supply to 12V. Don't hot-plug a 12V supply onto a radio that was running at low voltage, because sudden voltage steps have killed radios.
7. Expect around 80 to 90mA on receive at 12V. A forum builder measured 89mA on 80m rising to 95mA on 20m.

Terminal tests

Connect PuTTY (or similar) to the QMX serial port. The built-in tools let you test without extra equipment. Work in this order, because there's no point debugging the receiver until the transmitter works:

1. Basic life: display, buttons and encoders respond.
2. Transmitter, into a dummy load only: short key-downs on each band at the 12V supply. You should see about 4 to 5W and about 0.7A for 5W. Run the LPF sweep and SWR sweep, and use the Diagnostics screen, which shows the reason if there's no RF.
3. Receiver: run the RF filter sweep and image sweep on each band, and the audio filter sweep. Screenshot all of them as a baseline.
4. Compare 20m with 30m and 40m. If 20m is far lower than the others, check the L401 joints and that the Band Configuration matches the modified wiring. A factory reset restores the default.

Rev 1 items to do now

• Test the USB connection with your own laptop. If it isn't recognised, add the 5.1k CC resistors or use a USB-A to USB-C cable
• Decide on the PTT jumper (optional; see Stage 3)
• Save the EEPROM file from the QMX drive as a configuration backup