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Building a u3s for a friend, built several of these with no issues.?

This one is a nightmare, came with a dud screen and some wrong parts, put a different screen on and correct parts but it refuses to tx.

I've programed wspr on 6 bands, 10 00 cycle time, gps works, time and location. Power is from a 7805 for logic and 7809 for pa, big leccy across input and caps fitted to output and input pins.

As soon as it goes into tx it either resets or screen goes momentarily blank then carries on to next time slot. Tried different 13.8v psu's and battery, no joy. Screen also loses back-light sometimes.?

I've checked ALL joints, with and without relay board, buzzed everything out, now run out of ideas.

Rf getting in somewhere?

Any ideas?

Andy

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The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.

Andy,

What about bypassing the 7805 for a beefy 5V supply and/or monitor current drawn when it goes into Tx?

Those symptoms suggest power supply problems.

Dud processor? Swap with one of your known good ones?

Remove PA FET...does that solve problem of reset.

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Hope it helps!

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BTW, there's not much mention here about the U3S anymore. Lately I just teamed one of mine up with QRP-Labs' 10W Linear amp and running it overnight on 80m. I was hoping to use it on 160m over the winter, but I wasn't happy with the match my random wire antenna provided. Maybe not your cup of tea, but I very impressed with that linear amp.?

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73 de WD4AH

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-Al

Yes, I’d certainly put a scope on the 5v rail and see what it’s doing when it goes into Tx. ?I’m assuming you are testing it into a dummy load.

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Regarding the U3S with the 10W amplifier into a poorly matched antenna. How about putting a 6dB attenuator between the amplifier and antenna? Worst case is the amp would see 1.6 VSWR into an open or short and you’d still have 2.5W output. According to the QRP Labs testing the amp appears quite intolerant to a poor match.

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I had to repair my wife’s father’s U3, when I blindly plugged it into my bench supply, not realising that he ran it directly from a 5V supply, unlike mine which I had built for 13.8 with a 5v regulator. Whoops.?

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cheers Nick G0OQK

Andy

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This has all the symptoms of drawing too much current when it transmits. I experienced this on something. If you attach a probe to monitor voltage where it feeds the microcontroller, it is likely dropping below its threshold. A fault in PA circuit or an inadequate power supply will each do this.?

I use a linear 5 volt vintage supply. It also insures it won’t spike the voltage on turn on.?

Of course if you have another u3s available to compare, that helps.?

I haven't seen a bad tiny regulator IC before, but can't rule it out.?

Be careful.?

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Curt

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Thanks for replies here and off line, I'm sure power supply is solid, plan is to put the new build mcu into my good u3s to see if it's OK, if so the bad unit is then going to a friends where we can scope the power rails.

Initially I thought it was a si5351 problem, tcxo fitted, but that works fine in the good unit, on the lcd some of the D pins were not pulsing, but that has resolved itself somehow. Which makes me think its more of a mcu problem.?

Andy

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The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.

Yes, I have the 10w amp and a set of lpf's after it and have used it but found it more satisfying to see where less than a watt gets, have reached south pole and new Zealand from an attic dipole running 0.1w.

Amp is on a dusty shelf.

Good fun.?

Andy

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The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.

And fixed.

Problem? I am running a 7805 for logic and a 7809 for pa. There is a 4700uf electrolytic across the input, 0.33uf caps on both input leads to regulators and 0.1uf caps on the outputs. In sheer desperation I removed the output cap from the 7805. U3s is now sat there chuckling away and chucking out rf into a dummy load.

Thanks for all the input, especially Mike Pencoys.

Off for coffee, Christmas cake and a hair Transplant.

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The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.

On Tue, Dec 17, 2024 at 10:27 AM, M0RON wrote:

And fixed.

In sheer desperation I removed the output cap from the 7805. U3s is now sat there chuckling away and chucking out rf into a dummy load.

That seems odd. Isn't that the usual recommended cap to use on the output of the regulator?

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Glad you got it fixed!

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-Al

Hi Al, yes it is, I am now running the 7805 with no cap on the output pin. All I can suggest is that the cap was taking voltage to ground when going into tx. It was behaving like an intermittent fault throwing up several different faults on the u3s, it was a good brand name cap, I will probably replace it by another from the same strip and it will be fine. These things happen but it was a pig to find, however I have learnt a lot in the process so other than a lot of time spent on it no harm done.

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The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.