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I've been using a Rev 1 QMX for a while now. I recently updated to the latest firmware and finally got to play with the watt and SWR meters. To my surprise they're wildly wrong. I get no power reading at all and SWR is quite a bit more than it should be. Measured into a dummy load with an external meter I'm getting a healthy power output and a 1:1 SWR but on the QMX it's reading .2W and 1.5 SWR. I did a factory reset and that didn't seem to change anything. I've gone through the SWR bridge circuit as best as I could with a continuity meter and everything seems soldered right. Under a visual inspection everything seems as it should be, no bridged solder joints, cold joints or enamel wires improperly soldered. I'm at a loss at this point as to what to do. I'd love to have the meters accurate to help dial in antennas at the park without having to bring an analyzer. I appreciate any advice!

73, Jake KF0ARE

Hi Jake,

The problem you describe indicates that the SWR/Power transformer has an open connection.? I would start with reflowing the solder on the leads of T507.? If that does not correct the issue, check for forward and reverse voltages on D514 and D515.

I do not know a "calibration" setting for the SWR sensor.? Posts have documented a known issue with SWR readings as the frequency increases.? This is a design trad-off that Hans made to keep costs and difficulty of assembly down.? Two solutions have been posted:
1 - Wind T507 with the smaller wires on the outside of the ferrite and place the single turn close to the inside.
2 - Replace the single wire with a piece of coax with the shield connected on one end to ground

The issue is that the capacitive coupling becomes more critical as you increase frequency.

73
Evan
AC9TU

Hi Jake,

My unit (12V) behaved just the same immediately after I finished building it; I connected a dummy load and rushed into the diagnostic screen to see if all was ok on TX. But judging by what was on the screen (PWR < .5 W - less than point five - and SWR 2.06) things were not too good :-))

I connected the dummy load through my old needle SWR meter and my heart returned to its place, since in fact I had 5W (give or take) in all bands, so all appeared to be good. Then I read a post where someone suggested rearranging T507 a bit, so that the 1 turn secondaries are as far as possible from the primaries, I so I tried that and the SWR seems to be accurate now.

For the power problem, however, I did not find any cure :-(? BUT I noticed that, when powered with 8.5 V, the output, as displayed in the diagnostic screen, shows a whopping 8.0 Watts! At 9V, it rises to 9W and at 9.5V, to 10W. Rising the voltage to 9.6V, the output power becomes...? .2W (point two watts). After 10.5 V, the output changes to < .2 (less than point two). Of course, all this time I checked the output power on the old banger SWR meter and all was normal.

So, I can only speculate that:

- code which calculates the power is over optimistic (based on p.p. voltage?) OR the winding of T507 is at play
- the variable which holds output power on diag. screen somehow overflows after 10W

I secretly hope it's a bug, since I'm not even remotely in the mood to take out T507 and rewind it :-))

Hope it helps, try tingling T507 a bit, it should improve the SWR accuracy.

Regards and MX,

YO3GFH
op. Adrian

Yes! I spent the morning doing a very careful and neat complete re-do with new wire on T507 making sure the wrappings don't touch and now it's perfect! Now the on board power and SWR meter agree with .01 of my external meter! Thanks for everyone's help!

73, Jake KF0ARE

Good to know everything is ok now!

So, since you've rewound T507, did you change the sense, or just the spacing? If power and SWR are correct now, it means that there's nothing wrong with firmware code (apologies to Hans), one just have to wind the transfomer properly.

73's
YO3GFH
op. Adrian

I think the first time I wound it I was very sloppy and crossed a few turns and had the B & C turns touching the A & D ones. This time I just made sure everything was nice and neat with the A & D ones occupying about the top quarter of the toroid and B & C rather centered in each binocular, careful to not having it touch anything. ?If that makes sense. It’s a brilliant tool for antenna tuning now! I can leave the VNA at home!?

Thanks for posting this.? I had the exact same issues you were seeing.? You and the good folks here convinced me to re-heat all my SWR bridge joints which solved the problem!? At the same time, I found that my power supply connector had a cold joint which had broken loose, causing the DC return to be through my USB connector.

I think the lesson here is that just because you're seeing continuity in circuit doesn't mean you actually have continuity through the path you are trying to test! And to any future people wondering about why your meters are off just take another crack at T507 very carefully and you'll be in business.

On Tue, Dec 26, 2023 at 10:08 AM, Jake Larson wrote:

just because you're seeing continuity in circuit doesn't mean you actually have continuity through the path you are trying to test!

Also, be certain to probe the PCB pads/traces and not the bare ends of the wire which only checks the continuity of the wire not the joint.?
I scrape the enamel off cleanly before soldering rather than rely on soldering heat to remove the enamel.
73, Don N2VGU

...aaand it would be cool if the manual had a word or two about the sense of T507 windings, wink wink, nudge nudge :-))

73, Adrian YO3GFH

Hi, I am re-attaching the pictures of my solution to the SWR-bridge meter issue which I posted in this thread:
/g/QRPLabs/topic/101942509#112149

--
73 Bojan S53DZ

Thanks, Bojan!

Do you remember the sense of the windings? I'm asking because, in my case, the SWR problem was solved by combing T507 a bit, but I still have vastly inaccurate power readings in the terminal diagnostic screen (see a few posts above). I *think* that the sense of the second half of T507 might be the cause, and I haven't found a description for how T507 should be wound, sense-wise. My head hurts :-)

Regards,

YO3GFH
op. Adrian?

Hey Adrian,
I woud it according to the Assemby Manual. As I remember I wound each half the same. So if you see the Manual picture with the right hole and the one turn on the left side of that hole then I started on the right down and wounded CW to the right top. Then I turned the toroid 180 and did the same with another winding. (I think).
--
73 Bojan S53DZ

...aaand I'm officially a Silly Person (to put it politely).

After reading the QMX (and also the NorCal powermeter, which has a similar bridge) assembly manual again, and staring at the pictures ... it dawned on me... my 1 turn windings are not 1 turn each... but two :-)
Soo, the "great power" mistery solved...

I AM getting old :-)

Regards,
YO3GFH
op. Adrian (old fart)

I have been struggling trying to find:

1. The "right way" to wind this T507
2. Why power and SWR was vastly different from other rigs

This thread comes closer to the solution than any so far. So I took my QMX apart last evening to find windings touching! So made the windings to look like your pictures. Today the tests will be repeated and will post here.
--
73
Karl
KI4ZUQ

Very low power indicated on all bands while SWR was high on 80 meters tapering to none on 20 into a dummy load! Power was 4-5 watts by measuring voltage at the dummy load.

My interim solution is to turn off power and SWR metering until I can devise or hear of some corrective approach.
--
73
Karl
KI4ZUQ

Have decided to remove T507, rewind and reinstall. Now I must ask HOW did you clear the through-holes of solder?

I have a hot iron solder sucker but it is way too big. Have also solder wick and have used to clean up the pads but still have thru-holes with solder in them.

I will wait to see your answers before I drill out the holes....
--
73
Karl
KI4ZUQ

Firstly please don’t drill the holes out.
It’s a multi layer board and there may be inter-layer connections, I don’t know for sure but just saying.

I soldered a capacitor in the wrong place on one of my kits and managed to successfully remove it and get the holes cleaned out. I used solder wick with a little flux, but I first added some fresh solder to the holes to give the solder wick something to grab on. I had to do it a couple of times on one hole, trying from both sides, but it worked.

Another way is to not clear them but to introduce the fresh wires whilst heating the pad/hole (preferably and easier from the top side if access allows). Hold the wire relatively close using some fine forceps and push the wire in and through when the solder is wet. Then follow up again with your iron and a dab of fresh solder to re wet the joint.

Or, if you need, solder the new wires to the top side, I did that to one toroid on my QCX Mini 40M, where I was changing turns to optimise power. Though could be difficult to get to all on that transformer.

73.
Rowan.