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Hello, I just completed the QMX build and passed the smoke test. I have the following power output per band with 12.1V input.

80m 3w, 60m 4w, 40m 1.7w, 30m 3w, 20m around 1 watt.

I have tinkered with L511-L513 spreading and compressing turns with little effect.

Any thought on what to do next?

73 John N8JJO

As suggested in some other thread, I also have re-done L513 (for 20m) and L512 (for 40m) with one more turn, which added +0.5w to those bands. Now getting about 3.5 to 4w with 12v (my kit-built qrp power meter doesn't seem to be accurate, so I cross compared with my K2 to guess the rough range).

73,
Yoon KM6KJI

I compressed turns on L513 as much as possible, and it made no difference in 30/20m output. So your suggestion is to rewind 512 and 513 with an additional turn?
I'm surprised at the difference in power output between bands. It is otherwise operating perfectly in every mode although 40 meter RX seems just a bit deaf. No problem making FT8 contacts on 20 even at 1 watt but I would like to get this as close to spec as possible, I assembled this for a friend.

It seems like most folks are getting 4-5 watts so I wonder if I have an issue elsewhere.

Any and all tips are welcome.

73 John N8JJO

John, have you had a look at the RF band sweeps? they are available through serial terminal and are most useful to identify issues in your LPFs.?

72/73 de Enzo M0KTZ

Here are the RF sweeps to dummy load, which don't look great.

Hoping for some insight.

73 John N8JJO

John N8JJO,

The first thing that I see is all of the levels are low.? This indicates either LPF selection issues reducing the signal, the LPF (L401) is not installed per the instructions, or T401 has problems.? I would start with verifying that the correct LPF is being selected.? This is most likely because there are transmitter issues as well.

Verify that only two LPF selection diodes have close to zero volts on the banded side.? That is more difficult to verify on the QMX as the band is next to the board for some diodes.

You could also try reflowing all the LPF toroids, as a poor joint with the toroid is the most common issue.

If the LPF selection checks out, verify that Q508 is switching correctly.? There should be approximately the input voltage in transmit mode and close to zero when receiving.? Also, verify on the drain side to be sure.

Please start with the above and let us know what you find.? Be careful as the board is rather cramped, and it could be easy to short something that causes another failure.

73
Evan
AC9TU

Evan, thanks for your suggestions. I am fortunate that I have the proper tools to desolder/solder and to verify these voltages carefully, and hopefully without incident.
I will report back when I have the results. Much appreciation sir!

73 John N8JJO

John, as all the bands have pretty low power levels, it might be worthy having a look at the output transformer (T501, he binocular one) for shorts or scratched enamel or poor soldered joints. I would also have a look at the inductors in the LPFs, as I had a similar issue on 20m which boiled down to the enamel on one of the two ends of the inductor not having been removed properly. Resoldering the joint did the trick.

My2Cents

72/73 de Enzo M0KTZ

Enzo thanks for those tips. I will rewind T501 in any event as I'm not happy with the look of the thing, even if it does pass continuity tests.

73 John N8JJO

Well I decided to start with L401. Rewound per latest manual. Had a heck of a time getting continuity, that enamel doesn't want to burn off (Hakko set at 750F and using Kester 235). I finally did get them. RF sweeps attached. A lot more gain now. No difference in TX power of course. Will go through the rest of the items tomorrow.

73 John N8JJO

Let's try that again with attachments this time..

Although I still plan on rewinding T507 and T501, I reflowed the 6 toroids and T507/T501. Using a slightly wider tip on the Hakko seems to be the trick. I was using a pinpoint previously.

Power output 80m 3.5w, 60m 4.5w, 40m 4w, 30m 3.5w, 20m 2.5w. So it seems we're onto something here.
The voltage input is 12.5V. Perhaps we need to adjust inductance on the toroids?

73 John N8JJO

Well I have completed the all of rewindings, and I added 1 turn to L512 and L513. I have 4 watts + on all 5 bands and RX sweeps look acceptable.

I'm calling it done. The poor board has handled a lot of heating and I'm satisfied where it's at now. Although I enjoyed building the original QCX and the Mini, I probably won't build another QMX, the density is too tight for me.

Thanks to everyone who sent tips, I appreciate it very much!

73 John N8JJO

That's great news John. Getting 4W at 12V seems to be standard/usual for the QMX. I wouldn't go to the lengths of rewinding the LPF toroids for a fraction of a dB, as trat's unnoticeable in a QSO. I hope you'll enjoy your new toy as much as I am HI

72/73 de Enzo M0KTZ