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Assembled my QMX+ and it's working.? I had a couple of issues related to enameled wire not making good connections that were resolved.? I know it's working, I've made contacts on several bands successfully, including some decent DX, all on FT8.

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What bothers me is that the output power seems to jump around wildly.? If I go to the diagnostics screen and key the transmitter, the Bias SMPS shows fluctuations in current that correspond to the fluctuations in the output power.? The Duty cycle on the Bias SMPS always shows 0%, even as the current jumps around in the red.

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I suspect I've managed to damage one (or more) of the surface mount semiconductors.? At least that's what my reading on the subject would seem to suggest.? What I need is a test that can be done with a multi-meter that would shed some light on the problem.? I can probably resurrect an oscilloscope if I need it, but I'm hoping someone can lead me to a simple voltage or resistance test that could help isolate the problem.

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My last problem was related to soldering and enameled wire which was easily resolved.? I'm hoping this issue can be resolved without major surgery as well.

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Thanks in advance for all of the good advice I'm about to receive.

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Andy

WB9YXA

This sounds similar to the issue I had - documented in this thread.? I spent time working though options with Hans and in the end he sent me a new main board. I think it was the processor that caused the issues.? Hans did have a lot of things worth testing and trying, so it'd be worth raising a support ticket if you haven't already.

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Andy, the QMX+ I recently completed shows Exactly the same symptoms as you describe in your second paragraph.

I checked all components that I installed and re-soldered literally every point on the pcb. No change. The radio seems OK in all other respects. Other than wildly fluctuating bias current and output power.?

Since I too lack the knowledge to complete a troubleshooting of the bias system, the radio is now enroute to Jeff Moore, whom I am positive will cure it.

It's a shame there is not (yet) a resource here for we non-EE PhD's to access for guidance on how exactly to check and resolve at least the common problems that these otherwise fine radios manifest. And there are a LOT of them.? Both kit and factory built rigs, based on postings here, are a crapshoot when power is first applied.

A new WIKI where the forum experts could contribute precise tips and tricks to diagnose and fix problems would sure be appreciated.?

Good luck with yours.? I'll be following the discussion with interest.

Bill? W3WJ??

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On Tue, Apr 8, 2025 at 11:34 PM, Bill Jobes wrote:

A new WIKI

But why not the current Wiki?

On Tue, Apr 8, 2025 at 02:11 PM, WB9YXA wrote:

What bothers me is that the output power seems to jump around wildly.? If I go to the diagnostics screen and key the transmitter, the Bias SMPS shows fluctuations in current that correspond to the fluctuations in the output power.?

There could be many causes for this.? Some are assembly-related, some are board smd soldering issues, and it could also be a failed component or bad port on the processor.

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Here are some of the assembly-related things to check for bias current issues, via inspection and your multimeter:

- Make sure L504 and L514 (two of the through-hole inductors you put in) are properly soldered, and not cracked.? This type of inductor is a bit fragile, and it is easy to break them with a little too much force.

- Make sure your diodes are all installed the correct direction.

- Make sure L501 (the small 330uH coil) has not been broken from the board during assembly.

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Some board SMD soldering issues to check that affect bias current:

- make sure C504, the large tantalum SMD capacitor, is properly soldered on both ends.? Also test that it is not shorted internally.? Jeff W1NC said he has found C504 soldering issues and failure in some boards he has repaired.

- verify the soldering of Q501, Q502, and Q516.? One user found that Q516 was poorly soldered.? You need to check these under good magnification.

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Hopefully one of those will hold the solution to your? particular problem.

Good luck, Stan KC7XE

On Tue, Apr 8, 2025 at 02:11 PM, WB9YXA wrote:

The Duty cycle on the Bias SMPS always shows 0%, even as the current jumps around in the red.

This is a bit different than some others have reported, where their bias smps is maxed out.

This suggests that the processor may be sensing a higher-than-needed current, and trying to reduce it.

Maybe this suggests a problem with the bias control circuit (Q501, Q502, Q516 and surrounding components), or some other signal shorted to the tx_bias line.

This really is a tricky problem.

On Tue, Apr 8, 2025 at 02:34 PM, Bill Jobes wrote:

the radio is now enroute to Jeff Moore, whom I am positive will cure it.

Please let us know what he finds.? This is a very tricky problem, with many possible causes.

Stan KC7XE

Ludwig you ask "Why not the current WIKI?"?

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Mostly because there are often suggestions to do this, do that, check this and check that, without offering specific advise on exactly HOW to execute the suggestions.

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I'm suggesting a WIKI on troubleshooting how-to procedures and techniques, not being told to take an action while presuming all readers know how to do it.?

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I will do that once the radio is back in hand.

On Tue, Apr 8, 2025 at 02:34 PM, Bill Jobes wrote:

It's a shame there is not (yet) a resource here for we non-EE PhD's to access for guidance on how exactly to check and resolve at least the common problems that these otherwise fine radios manifest.

Ludwig and others have already documented troubleshooting approaches for the most common problems in this forum's wiki.? The fluctuating power/bias current is not a common problem with a known solution that can be easily documented.

Also, keep in mind that everyone on this list is a volunteer.? I haven't added to the wiki, because I just haven't taken the extra time to do so - it does take time and effort to make a good wiki posting.? You are welcome to help by consolidating responses (like the one I just provided to Andy, and other responses to posts about the same problem) and add them to a new wiki entry for fluctuating bias current.

And there are a LOT of them.? Both kit and factory built rigs, based on postings here, are a crapshoot when power is first applied.

That is very much an exaggeration, Bill.? Keep in mind that Hans has sold many thousands of QMX/QMX+ radios.? And only a small percentage of people assembling them have reported issues - yes, when reading this list it seems like everyone has issues, but only those who do are posting them here - without counting, I venture to say far less than 10% (a generously high estimate) of all QMX/QMX+ kit builders report problems.? And some of the most common of those problems were avoided with recent versions of the hardware - so the numbers of problems have significantly decreased.

A new WIKI where the forum experts could contribute precise tips and tricks to diagnose and fix problems would sure be appreciated.?

I agree with Ludwig.? The WIKI is already here, and already has ideas for many of the most-common problems.

What I think we need is more help in making more good entries to that wiki, derived from successes recorded in the many forum posts, and perhaps a better overall roadmap pointing to the appropriate entries for different categories of problems.

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One reason QMX/QMX+ are as inexpensive as they are, is because they are user-supported.? If Hans had to take the time to write a good troubleshooting guide for all of the problems we find solutions for, he would have to charge twice as much for the radios.?? His current documentation and assembly guides are excellent, far above what some other vendors provide for much more expensive radio kits.

Stan KC7XE

On Tue, Apr 8, 2025 at 05:57 PM, Bill Jobes wrote:

I'm suggesting a WIKI on troubleshooting how-to procedures and techniques, not being told to take an action while presuming all readers know how to do it.?

Yes, this would be very nice.

Unfortunately, I don't know who would offer their spare time to provide good write-ups of how-to procedures for all of these.

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But the nice thing is, if there is a wiki recommendation that you don't know how to do, just ask here - and someone will likely jump in and help - there are tons of examples of this behavior in the message archives.? And then with a little effort from someone, the procedure could be added to the wiki.

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Stan KC7XE

I believe I found it.? L501 is missing a corner and coils are exposed.? I was so focused on solder joints I didn't see it until I got an oscilloscope involved and I was tracing the pwm signal.

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I actually have a replacement on hand, I'm going to try to swap it in for the damaged part.

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Thanks all for your positive comments.?

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73

Andy

WB9YXA?

Well expletive deleted.? Thought I was on to something.

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I looked at and touched up a couple of solder joints on the Q5xx transistors with no change in symptoms.

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Going to try looking at it again later today.

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I'll update you if I find anything or give completely up.

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Thanks.

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Andy

WB9YXA?

Update:

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I replaced L501 just because the broken corner was bothering me.? It didn't have any impact on the problem.

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I've ordered some transistors to shotgun in to the bias circuit.? I'll update the group when those semiconductors arrive and are installed.

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Thanks for listening.

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73

Andy

WB9YXA