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Chris, G5CTH

Chris, is this with or w/o Zeners?
--
73, Dan? NM3A

The short is somewhere on the main board, verified by pulling out the PSUs and testing each board individually. I wish the PSU was the problem since it's easier to just order another power module than to troubleshoot a very dense main board with a bunch of very tedious labor put into it that I'd rather not repeat... As it is I'm going to borrow a thermal camera from a friend and see what heats up when I try to power up. I live pretty close to a digikey warehouse so I can get pretty quick shipping on replacements.

I'm willing to bet the failure went something like this -

* Bench supply goes into current limit, drops input voltage
* Duty cycle of buck regulators increase
* Bench supply comes out of current limit when TX ends, input voltage comes back up
* Known issue with CPU failing to respond to transients in input voltage fast enough to prevent rails from spiking
* Protection zeners fail open? I don't see any shorts in PSU boards
* Op amps are probably fine since they're all 30 volt parts, but the Tayloe detector, ADC, DAC, and LCD are all liable to have been killed by this.

I would propose the following modification to the power circuit to help prevent this issue:

https://photos.app.goo.gl/shkveUjM5gU6WNqP8

This would shut down the pwm in an overvoltage event much faster than the cpu can.

The ultimate solution would be to use an off the shelf buck converter IC with a frequency synchronization feature. An IC whose control loop runs as fast as its pwm will naturally have better impulse response than a CPU running a control loop at 1khz.

I think it's just a matter of being cautious.

I use an Elecraft T1 to tune but use a tuning indicator (TI) that
ensures no more that 2:1 swr is presented to the rig. I have forgotten
to put the TI in tune mode more than once. All it takes is 1 bad
mismatch and maybe I kiss my finals goodbye. I'd rather not replace
them if I can avoid it. Since Hans puts it in big bold red letters to
not go over 12.0v I'm gonna listen... maybe necessary, maybe not...

Until I'm fully comfortable that these rigs are indeed robust, I'm
erring on the side of caution.

-mike/w1mt

Why would 5W at exactly 9V, for the 9V version be ok, but 5W at exactly 13V, for the 12V version, be dangerous? FWIW, I’ve ran my QMX off a LiFePO4, transmitting with as high as 13.8V, but mostly in the 13.2-13.3V range, with no problems. But to be fair, I’m using a manual tuner. ?But again, it borderline feels like what I’m reading, on a near daily basis, is running more than 4W at 12V is a sin.?

The right encoder / button problems are likely a bad ground - upper left pin on the control board if I remember right.? ?Recheck that one, that six layer? ground takes a lot of heat.??

Yes - I noticed that too late! My bad, the QMX should connect to XCVR. 72 Sandy KB3EOF

Great setup at low cost. One thing just wonders me: is the QMX connected to the ANT side of the tuner?

Ahhhh, 13.8v. That explains it. I target 4w at 12.0v when building
(matches the graph in the manual). Hans has warned against running
more than 12v on page 4 of the manual -"Do not power a QMX with a
higher voltage than you built it for."
Anything more than that risks the finals, especially when using an
automatic tuner. I personally use a 3s Li-Ion battery and when fresh
it's 12.6v and I'm a little nervous. So far so good.


I use 0603 for the limiting resistor cause that's what I have. No
problems with heating.

Enjoy!

-mike/w1mt

Hi Mike,

Nope, I built for 12V.? Changed L513 back to 10T and squeezed turns.? Played with L512 to optimize 40m (and 60m).? Now have right around 6W on all bands with 13.8V PS.

120 ohms looks good for display brightness.? I only had 0805, the 33 ohm is a 1206, but the 120 ohm 0805 does not get warm or hot so think it is fine.

I'll say it again, what a fabulous rig!

Thanks and 73,

Robert, WA2T

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I've started playing with a spectrum analyser, this is what I have so far.

There's a bit of scruff round the signal and the second and third harmonics are about -60 dB which seems pretty good, better than my Pixie.

And at this stage L513 had 9 turns instead of 10.

At present I'm using a crude 38 dB attenuator and was running about 1W.? I've a 10W 20dB attenuator coming and that should allow me to run my QMX at full power.

Not used a spectrum analyser before so a bit of a learning experience, there must be a way of getting a screen shot onto a SD card.

?And I have some Zeners, 1N4756A.? Hans, I could send you a couple if it helps.

I'm taking my time over this, not applied more than 10V to my 12V QMX, that gives 2 to 2.5W, astonishing how far that gets.

Chris, G5CTH

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Sure thing Dan - here is a sketch and photos. It is a simple OCFD and I optimized it for 40-30-20 meters using antenna modeling software (MMANA). The 4 state tuner tunes it easily on those three frequencies (and above). Sandy KB3EOF

I had the exact same initial boot sequence experience. After power cycling and pressing the left encoder button, the magic smoke was let out. I went through the first 2 steps of G5CTH Chris'? troubleshooting steps and determined that PS1 is damaged. I checked the main board interconnects from PS1 and sure enough I had a short to ground. Turns out I had a very fine [sic] solder bridge across C201 on the main board, it came off easily without the use of an iron. No more short to ground on pin 6 of JP101.?

It would appear that as a minimum Zener D106 on PS1 is damaged as a result of the short, maybe more. The QMX in its present state is able to respond to a power on key press and enter terminal applications mode.? So it is need of some repair on PS1 or replacement.?

Steve W6WU

What is your input voltage? 9v? How are you measuring power out?

Yes I would go back to 10T on L513. I haven't built a 9v unit and I
suspect yours is 9v. Likely, the L513 tweaking is probably only for
the 12v version.

I put kapton tape in several places to prevent shorting. Most
importantly on the power jack and left encoder. Also, on the exposed
pins for the power modules. That's about it.

Enjoy the rig!

-mike/w1mt

Thanks for getting back Kees.

I flooded the area with flux and then used a T12 - JL02 tip. It is about the size of a tooth pick - hi! It must have worked since the firmware update was successful!

Still having some issues with the control board push buttons and right encoder. BUT at least the QMX is booting up.
--
73, Rick
VE7TK

Website:

Hi Group,

I just finished my QMX build, Perfectly functional right out of the gate, pretty happy.?

While very densely packed and easy to screw something up, I gotta say BRAVO Hans!? What a fabulous kit!

To Mike, W1MT,

Anyway, I used 11T on L513 during the build after reading this thread.? The problem I am having is that on 20m, I am getting over 7W.? The rest of the bands are all right around 5W.? I'm thinking I should take the extra turn off L513 and go with 10T.? Thoughts?

Off to do the R306 mod for LCD brightness too high.? Any other mods you recommend Mike?

Thanks and 73,

Robert, WA2T

Hi Cliff,

I appreciate your having taken a look. B1 to B2 does indeed have continuity, as noted. The results for those checks were duplicated, and remain consistent from the first round of checks (through which there is a hand-drawn green strikeout in the image, since more enamel was removed from the solder joints at the transformer, before the checks were all performed again).

(and the image is actually directly out of Hans’s manual, with the numbering and results from my board added.)

Thanks again for any further ideas!
Garrett KC3UNP

Could you post a sketch of your antenna?? Thanks? Dan

An addition that I should have included in the prior post:

In my estimation, simulation is good to help work out issues with a design.? Actual measuring should be done for an individual radio.? If you rely on measurements only, you can get into a parts variation problem.? Is your measurement sample large enough to verify the outliers in a production run?

The above is from experience in taking processes from R&D to production.? The results are not always as expected.? This is from food industry experience, but I believe the concept applies.

73
Evan
AC9TU