This page is a continuous work in progress. It will document common faults with QMX builds. Further down this page, I am keeping a log of faultfinding on assembled QMX transceivers at QRP Labs offices, as I find the faults and fix them. Of course a large number of the assembled units work correctly immediately; this page is about those that don't. 


If you can use a current-limited power supply set to about 200 or 250mA for first power-up, and set to a lower voltage such as 6.5V or so, then this will reduce the pain and fire and destruction that could occur if there is a fault. Read the whole of this page before starting troubleshooting. 

  1. If QMX is built carefully and correctly, it MUST work. Faulty components are very rare. A faulty PCB (broken trace, short etc) is even more rare - except for the PCB Rev 1 fault noted below, the Q103/Q104 drain short in the 5V SMPS board. 
  2. Read the manual, read it again, and again - did you miss something? Are you SURE you didn't? 
  3. Remember the terminal login has lots of tools to help diagnose hardware faults and optimize the hardware setup.
  4. Remember the QRP Labs forum has a lot of knowledgeable and friendly people who can help. 
  5. Described below are some common faults; and below that, a log of the QMX that I have fixed here in the QRP Labs office. 
  6. A VISUAL INSPECTION can often find many kinds of faults!

Faultfinding procedure is always:

  1. Basic operation: Get the unit showing something on the screen, text, responding to button pushes, connecting the USB terminal etc. Until you have this level of life force activity, there is no point going any further. 
  2. Transmitter: next is to get the transmitter working on all bands and producing approximately the expected power output. This will prove the low pass filters are working, and the band switching circuits are basically working. There's no point trying to debug receiver issues until the transmitter is working. 
  3. Receiver: finally, you can get to debugging receiver issues (if any). 

Don't skip these steps!

QMX does nothing, the screen is blank, but shows up as a USB flash drive when a USB cable is connected

Great! This is exactly what it is supposed to do! QMX is supplied without firmware. So the first thing that needs to be done is connect a USB cable, download the latest firmware, and install it. This is all covered in the documentation so you know the saying: RTFM. 

The backlight is lit, but nothing shows on the display

Again - make sure you RTFM... did you read that you need to turn the contrast trimmer potentiometer on the main board, all the way anti-clockwise to start off with? Backlight being on is a good sign that all is well with the CPU and the SMPS boards. But you maybe just didn't adjust the trimmer potentiometer. 

If you do see a row of black blocks on the top row, and the bottom row empty, then it means that the contrast adjustment is correct; but for some reason the processor isn't able to talk to the LCD module. We did have one instance on the QRP Labs forum, where a constructor (no doubt familiar with the QCX-mini construction) simply failed to solder in the 2x2-pin header on the right hand side... 

No or low power out, on some or all bands

By far the most common errors are soldering problems in the RF parts of the circuit on the PCB. 

  • Remember the enameled copper wire, is, well, err, ENAMELED and in order to make electrical connection, you will have had to remove that enamel to expose the bare copper which you can solder to. The thin wire, you can burn off the enamel, scrape it off, sand it off, etc.; the thicker wire will certainly need to be scraped or sanded. 
  • The 6-layer board is very good at conducting away heat from the joint you are trying to solder; particularly on ground connections. For example on the grounded leads of LPF capacitors - it takes a bit more heat than you're probably used to, to make a good connection here. If you don't make a good connection, you will have an incorrect filter shape, and that can mean some power out but not as much as you expect.
  • Are all the 1N4007 "PIN diodes" installed with the correct orientation? 
  • The 47uH inductors could break if installed roughly
  • Inspect all nearby SMD components in case something got broken while soldering or cutting a nearby through-hole component lead. 

USB-C connectivity issues

NOTE: This applies to Rev 1 boards only. Rev 2 and above already have the 5.1k resistors on the board. 

Andrew KI7FXL and Ryan K5DMA had issues with getting QMX to be recognized on laptops (mac and windows) which have only USB-C connectors. A lot of people are using QMX on computers with old-style USB-A (full size) USB connectors and a simple USB-A to USB-C cable. But apparently there could be issues with USB-C (unless you dance around with various combinations of USB-A adapters, hubs, blah). See the full forum post and discussion here. The solution is two 5.1K resistors, implemented as below. Note that these need not be 5.1K SMD 0603-size, it could be done with through-hole resistors with care. 

Low 20m Receive sensitivity on early QMX builds

A parasitic resonance was discovered in the bandpass filter system. It appears to afflict QMX not QDX even though both have the same receiver design. So must be a component or board layout difference. It was resolved via this modification documented here. The modification requires a change to the L401 bandpass filter inductor, and changes to the Band Configuration.

The operating manuals in the QMX documentation on the main QMX page show a screenshot of the Band Configuration screen, assuming that you made this modification. Firmware version 1_00_003 and above automatically set up the Band Configuration screen at first power up, assuming that you have made this modification (mod #2 on the page). 

If you have an early QMX build and you built it the old way, or you installed firmware earlier, then you may need to make sure you do this modification and that the Band Configuration screen matches the one in the operating manual. The band configuration screen can be edited manually to be like the one in the operating manual, or you can do a Factory Reset to put it to the default values. 

Low 20m output power on early QMX builds

There was an error on the schematic and in the build instructions; basically the C525 and C516 capacitors were swapped. The current assembly manual is correct: C525 is 270pF and C516 is 180pF. These are the two capacitors closest to the PTT output connector. Removing and re-installing them can be a little tricky. To remove a through-hole capacitor be prepared to use plenty of heat and a powerful soldering iron because the 6-layer PCB will dissipate heat rather too well, making desoldering difficult. A useful trick is to use a wire hook or a fishing wire to grab the capacitor between the legs, then apply heat from the other side while pulling it out carefully. Cleaning the pad holes is also difficult and an easy solution to this is not to bother, just tack the capacitors across the appropriate pads on the bottom side of the PCB. 

Q103/Q104 drains short on the 5V regulator board

A proportion of QMX 5V SMPS boards have a solder short circuit in manufacturing (PCB assembly) between the large drain tab of Q103 and the drain pin of Q104. It's an error that passed board checks but occurs on a small percentage of boards during the assembly. It's recommended to use a DVM to check for this short BEFORE applying power to the board. 

If the short exists, then it MUST be fixed before applying power; with a hot air gun you can heat up Q103 (the large AOD403 MOSFET) and move it slightly away from Q104. Even without a hot air gun, it is possible to desolder the small transistor Q104 and move it 1mm or so further away on its pads, so there is no longer a short from the drain pin to the large drain pad of the AOD403. 

IF you unfortunately applied power and if this short circuit exists, then you have probably destroyed or severely damaged the 1N4148 diode D101, unless current limiting of your power supply saved you. Because when you press the left encoder button to switch the radio on, one side of D101 is grounded, and the other at the supply voltage; massive current flows and pffffftttt a little puff of smoke perhaps, and for sure a bad lingering smell, and D101 is no more. So in that case you would need to replace D101. It doesn't have to be a 1N4148, that is non-critical; but it DOES need to be the correct way around. 

Faultfinding log

This is a log of QMX transceiver faultfinding done here on assembled QMX orders prepared in QRP Labs office. 

1. Nothing happened at all, no current consumption; pressing left rotary encoder to switch on did nothing. Fault very easy: just improperly soldered pins at the left rotary encoder.

2. At first power up, current consumption as usual, but did not appear as USB Flash drive; problem: short circuit at solder pins of USB connector, removed solder bridge, all OK. 

3. Seemed to kind of work... but on TX, power output on 80, 20 and 30m bands was very low and the current consumption on these bands was very high; RF BPF sweeps were also abnormal. A tiny solder whisker was found which was shorting out D509, so the 40m LPF was therefore always partially in circuit, disrupting filter performance on other bands. 

4. Nothing happened at all on pressing the left rotary encoder. Investigation revealed, the row of 4 pins on the bottom side of the 5V SMPS board (larger of the two boards) had not been soldered at all! All OK after soldering. 

5. The unit powered up but after the splash screen, the display just showed "1." on the top row and nothing on the bottom row. There was a solder blob short from both of the two pins of the button connection, of the right hand encoder, to the metal body of the encoder. Therefore the button was shorted. 

6. Q103/Q104 short-circuit. Checked pre-power up. Resolved, could install firmware. Next problem: low output power and weird RF sweeps on 80m, 60m. D508 was reversed. After fixing that, everything was OK. 

7. Unit would not power up at all. No current consumption. I followed the input supply voltage, through the board to the SMPS connectors, through Q103, and as far as the Source pad of Q105. On pushing the ON/OFF button, the voltage got as far as the Source pad of Q105 but not onto the pin! Close inspection revealed a factory soldering issue at the source pin; there is a blob of resin sitting between the pin and the solder pad, and actually insulating the pin; there is no electrical connection. It was easy to solve via hand-soldering. 

8. No output power on 20m and 30m. Simple problem of badly soldered copper enamel at L513. 

9. Unit comes on without button being pressed. S304 rotary encoder button connection shorted by solder blob to S304 body. 

10. Very low output power on 40m. But 60m was OK, though they use the same LPF. So I reasoned there must be a shape problem with that LPF (not toroid enamel), caused by wrong component values; investigation showed C524 (390pF) and C521 (56pF) had been interchanged. 

11. I plug QMXs into my keyboard which has a convenient desk-level USB port. The LED backlights on the keys went out! I inspected the USB connector and there was a small solder whisker (constructor error, not factory error) shorting the USB power signal to ground. 

12. Low power output on 20/30m, and weird waveform shape. 40, 60 and 80m were normal. Investigation showed the OFF voltage (meaning when band 20/30m was NOT selected) at the junction of L510/L513 was a little lower than the other bands which were about 11.5V when off; here we had about 10V when OFF. The ON voltage was nearer to zero volts, as normal. Furthermore the SMPS PIN diode bias was seen to be at about 18% duty cycle for 20/30m but around 13% for 40/60/80m; indicating that for some reason a higher PWM duty cycle was necessary in order to achieve the target current flow.
It seemed likely that Q511, L509, D513 or D510 were faulty and I started replacing them one by one. Some component seemed to be partially failed, not completely, because the voltages were somewhat normal; but something was leaky and non-performant somewhere. Replacing Q511, L509 and D513 did not cure the radio. Replacing D510 did it.
Conclusion: D510 was faulty.

13. 2 units the same: No reception on 40, 60 and 80m. RF sweeps on 20 and 30m were normal, and Image sweep too. This is a very simple matter of a soldering problem (wire enamel incompletely removed) at the BPF toroid L401, the connection for the 40/60/80m tap (end of the toroid winding).

14. Radio did not switch on, zero current consumption on pressing the left rotary encoder button. Investigation revealed that the PWR_ON signal was present at the 2x4-pin header pad going to the controls board, but not at the PWR_ON pad of the 2x4-pin header going to SMPS PCB #1 (5V and soft on/off switch). So there was a track breach. On inspection the track was seen to be burned, the solder mask had burned off and parts of the track showed bare copper, parts blackened. So it had suffered a catastropic high current. There must have been no current limit in place on the power supply.
The track was repaired using a wire. 
On pressing the left button, very high current consumption was seen (hitting the PSU current limit). 
The two SMPS boards were replaced, and now there was NO current limit, but also still no activity at all. 
The LIN_REG_EN signal voltage was too low, the processor on booting up, should set that to +3.3V while using the linear regulator 78M33. When the processor is not yet booted up, the signal should be high, enabling the linear regulator. It should not be pulled low by the processor when the processor isn't booted up. This is a sign of a probably fried processor. 
Further investigation of the suspect +5V SMPS board PCB #1 revealed a solder bridge inside the 2x4-pin female connector, joining together THREE pins, PWR_ON, PWM_5V and V_IN. 
This explains everything since:
1) Pressing the ON button, which connects PWR_ON to ground, therefore shorts V_IN to ground, passing unlimited current through that trace and burning it. 
2) The V_IN voltage is connected to PWM_5V, therefore sending V_IN directly to a processor pin (PWM_5V), exceeding its Absolute Maximum ratings for processor pins and killing the processor. 
Conclusion: this QMX is toast, unrepairable, and we will use it for spare parts. 

15. Low power out on all bands, but not zero.
On connecting the terminal emulator, using the Diagnostics screen, it could be seen that the supply voltage measurement, at the top left corner, was shown in a red colour, and reading only 4.9V even though the supplied voltage was 12V. The SMPS were all operating normally. 
My suspicion was that a component in the circuit which measures the supply voltage, R203/R204/C204 was at fault or missing. Sure enough visual inspection showed R203 (47K) resistor was physically blackened and damaged, presumably due to a soldering iron misplacement during assembly of nearby through-hole components; replacing R203 restored the supply voltage measurement to 12.1V reading. 
The reason for the low power output is that the DAC which drives the PA amplitude modulator chooses its maximum voltage drive level depending on the supply voltage. In this case since the measured supply voltage was low (4.9V) the PA amplitude modulator was setting a very low maximum output voltage.