开云体育

On Wed, Apr 16, 2025 at 01:19 AM, WB9YXA wrote:

As it stands right now, the SWR jumps around erratically, but the BIAS SMPS now looks 'normal'? 30.5 mA and 4% Duty cycle.

?

?

The SWR will jump around erratically if there's no output power because the SWR calculation will be dividing two small numbers.

?

4% duty on the bias SMPS seems small.? I'm seeing 15% with a 12V supply. ? AIUI the bias current is determined by measuring the ADC _BIAS voltage.? I don't recall hearing how this is converted to a current but it probably assumes that the circuit around the bias generation is correct.

?

One thing to check is the output power independently of the QMX, measuring the peak voltage on your dummy load for example.? If you are seeing power then there could be a problem with T507 because this is what measures the power in the QMX.

?

And I suggest that you do your tests at a lower voltage, 7V and 500mA max for example.? This will help your QMX to survive while you sort out what the problem is.

JP501 is in place.? I decided I did a less than admirable job on T507, so I took it out and rewound it.? Still in the same place.

?

The error condition is "Supply Prot", which I assume means it's not happy about the status of the power rail(s).

?

As it stands right now, the SWR jumps around erratically, but the BIAS SMPS now looks 'normal'? 30.5 mA and 4% Duty cycle.

?

I'm calling it quits for the night.? I'll take this up again tomorrow.

Andy, the main problem is the SWR Protect state, which defeats transmit.? The tx voltage is a little low, but ok.? Did you perhaps disturb the JP501 jumper, or T507?

Well, one step forward, one step back.? When I unsoldered Q516, it came off the board in two pieces.? Replacing it cured the erratic behavior.

?

However, while waiting for the part, I decided to touch up some solder joints.? As luck would have it, I made the problem worse, I now have zero output power.? The only other symptom is that when I try to key the transmitter with a hand key in CW, there's no side tone in the headphones.

?

@

Stan, as requested, here is Jeff Moore's summary of his findings on my rig that was exhibiting wildly fluctuating bias current and simultaneous fluctuating output power.

?

Jeff found under microscopic exam, that the Q516 transistor had been damaged either in manufacturing or shipping. He also found and replaced a faulty TCXO.

?

I commend Jeff (W1NC) for his amazing skill and generous support of our QRP Labs community.? For the mere price of parts, postage, and a voluntary donation so he can assist other hams, a rig sent to Jeff is returned in the best possible operating condition.

?

Here, in Jeff's words, with his permission, are his findings on my radio:

?

" ?

Here is what I found and did:

?

1) Disassembled all boards, scraped off old flux, cleaned with 99% IPA

The reason I do this on every rig is that it forces me to look closely under the microscope at every solder joint and component. ?

Often solder splashes or cold joints can be hidden by the dried up old flux.

2) Inspected under 10x - 20X microscope

Display / Control board

Nice job!

The Main PCB had no issues seen

3) Powered up at 6.1V and installed rev 27 of firmware

Factory reset - all good

The reason I use 6.1 volts is because it’s the lowest voltage that will allow the rig to operate (except for TX) and will minimize damage is something is terribly amiss.

4) Tested on bench at 6.1 V Controls, buttons, encoders, AF, image scan, A/D scan and rf scan functions - all found to be good

5) Shut down rig and increased voltage to 12v.? Tested LPF & TX functions on all bands at 12V.?

Power out fluctuated and bias current all over the place

Diagnosed to a faulty Q516 transistor - You can see a crack in it under the microscope.

Replaced Q516

Now power out is constant and bias current steady at 30ma

LPF shapes and cutoff frequencies all good

TX bias current is correct for every band (30ma)

TX power reduction (to about 1-2 watts) works well for LPF scans - This is an important test

SWR bridge works well for all bands

6) Measured minimum discernible signal on all bands - < .02 microvolts - very good

7) Measured frequency accuracy - not close to setting

Measure TCXO output - found a very odd and varying waveform

Replaced TCXO - This is a tough chip to replace

Now frequency accurate to within 1 Hz

8) Measure spectral purity on all bands - easily exceeds FCC requirements and meets QMX design goals

9) reassembled into case and retested everything one last time - now 100% RX, TX all modes, all bands

?

Total time spent - about 3 hours"

?

I hope this info sharing helps others who may encounter this bias current misbehavior.

?

?

72 and 73,

Bill? W3WJ

Update:

?

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

?

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

?

Thanks for listening.

?

73

Andy

WB9YXA

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

?

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

?

Going to try looking at it again later today.

?

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

?

Thanks.

?

Andy

WB9YXA?

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.

?

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

?

Thanks all for your positive comments.?

?

73

Andy

WB9YXA?

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.

?

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.

?

Stan KC7XE

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.

?

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

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

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

?

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.

?

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.?

?

?

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

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 02:11 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.

?

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.

?

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.

?

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

Good luck, Stan KC7XE

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

A new WIKI

But why not the current Wiki?

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??

?

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.

?