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I've been following QRP Labs since before its inception and praise the quality
of design, build and support for the kits, as well as the innovative
inspiration that brought them to market.
However, I'm looking at buying a QMX+ but I'm seeing lots of folk with issues
with the PSU modules and LPF's.
Is this just a small 'vocal' minority of builders having issues or has Hans
innovated a shade too far with the PSU's and pushed the limits of LPF
tolerances, introducing a measure of unreliability?

I think I have bought just about every kit that QRPLabs have released, in many
cases multiple examples, and really appreciate the value for money and the
support that the kits bring but I must admit my nervousness about aquiring a
QMX+.

comments? brickbats etc welcome. 73 Mike

On 14/01/2025 14:09, Mike via groups.io wrote:

Is this just a small 'vocal' minority of builders having issues

Mike,

Without a proper survey how can anyone tell?
On the whole do successful builders use the group much less?

Thousands get sold, I'd expect most failures, for any reason, to be reported.
Most failures seem to get sorted providing the owner persists.
There is a core of members that diagnose, some offer physical help.
What more can you ask for an advanced project?

There are no doubt simpler, easier to build kits if that's what you want.

73 Alan G4ZFQ

To me the biggest issue is vulnerability to a badly matched antenna along with the failure mode if that happens. I have seen a fair number of people report failures owing to that sort of situation. My QMX died when the dummy load I had on it for testing opened up, probably due to a physical shock along the way. The final's transistors were fine, but the band pass selector chip died. I implemented one of the fixes to it when I repaired it, but that would not prevent the same thing happening again if I were to transmit into an open or short. I would much rather blow the transistors, which would be a much easier repair. I built mine at 12V, but run it at 9V to reduce risks a little.

The next failure mode is a vulnerability to serious damage if the power input is interrupted for a short period and is then restored. It seems to wreak havoc on the thing. I have not had that problem to date. IIRC correctly, my QMX has a 5.5/2.1mm jack. I think a 5.5/2.5 jack would be better as it would make it essentially impossible to use a power supply with the wrong plug on it which would be one way to have that sort of thing occur.

JRJ

cube1us <cube1@...> wrote:

My QMX died when the dummy load I had on it for testing opened up,
probably due to a physical shock along the way.

I killed finals in my QDX this way (fortunately only finals). I also had
some intermittent antenna connections in my QMX and all they did so far
was lighting up the S (SWR protection) indicator. Maybe I just got
lucky...

I wonder what can be done to really protect the circuit from that. Some
transil at the output (what about its capacitance)?

I implemented one of the fixes to it when I repaired it, but that would
not prevent the same thing happening again if I were to transmit into an
open or short.

What fixes exactly?

The next failure mode is a vulnerability to serious damage if the power
input is interrupted for a short period and is then restored.

This, in my opinion, is the biggest design flaw of the QMX (I don't have
QMX+, only QMX and QDX).

The buck regulator control loop goes through the MCU, there are some hacks
in the software to minimize the issues (transients) Hans wrote about
(there's an article on his website about them), but the whole concept in
my opinion is a mistake. There are dedicated chips for that, with line
regulation much better than the MCU can provide with its ADC and PWM. The
frequency of these chips can be controlled (to avoid birdies) as well.

I replaced PSU boards with linear regulators in my QMX (QMX+, as far as I
learned, doesn't have separate PSU boards, but it can be done there as
well).

I also recently had a problem with the synth chip (there's my thread about
it on the group, it manifested as frequency spikes 100 Hz apart, on every
band), I had to replace it with the genuine SI5351A.

Another flaw of the QMX (but not QMX+) is how tiny it is. I know that one
of the design goals was to fit it into the QCX case, but I feel it's more
tiny than it should be. It's very tightly packed, there are almost no
clearances on the PCB. It makes it easy to make mistakes, solder bridges,
etc., and hard to fix things if they need to be fixed.

Other than that, great entry-level radio, if someone isn't afraid of
soldering and has some skills in electronics. QDX was my first HF rig,
after years of rag-chewing on UHF it allowed me enter the world of HF. I
thought about hacking CW into it, but QMX was conceived in the meantime,
so I bought it and switched to CW (if QMX+ was available at the time, I'd
have chosen QMX+). I'm honing my CW skills and planning to buy some more
expensive, all-mode, factory-made rig (probably FT-891), but I never
regretted buying these two radios. A great value for a fairly low price.

Jeffrey W Moore via groups.io <jeffreymoore@...> wrote:

Several folks have asked which tip I use. ?Then I realized that I
misspoke. ?I use a 2.4mm wide chisel tip from Hakko. ?T15-D24

Isn't it too big for the SMD parts?