Solved. Q502 was missing.

Hey all! Got my QMX+ rev 3 last week and have had a blast assembling it over the past few days. Finished it last night, booted just fine, and was able to tune some CW on 10m with a big grin on my face ?

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The thing of it is - after a few minutes, it froze. The LCD contrast went a bit darker, audio output stopped, and all front panel controls had no effect. Unable to power off by holding the VOL encoder, I had to cut power at the source. A short time later I was able to boot back up, browse some bands, tune some CW, and then it freezes again. Same symptoms: LCD goes higher contrast, audio stops, all controls stop, have to cut power.

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I'm pretty new to debugging PCBs but figured I'd dive in as best I could to gather some info, and here's what I've been able to piece together:

I've tried connecting to the serial terminal to check the on-board diagnostics, and that seems to be the most consistent way to trigger a freeze. Without trying to connect a terminal, I can usually get about 2-5 minutes of normal operation between power cycles, but as soon as I attempt to connect the terminal, it freezes.

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Like I said, I'm new to a lot of this but eager to learn so forgive me if there's obvious debugging steps I've missed ? ??. But yeah, not quite sure where to go from here! Would greatly appreciate any help or pointers!

On Tue, Apr 1, 2025 at 06:01 AM, Braden Glett wrote:

I've heard others recommend two silicon diodes. Is there an advantage to using those, versus a cheap little voltage regulator module?

An advantage of diodes is that they are a simple 'linear' solution, but yes, as others have noted, they will generate heat.? Each diode drops about 0.6V, and during transmit the radio consumes less than 1A, so the power dissipated in each diode is less than 0.6W.? So small 1A silicon diodes will work fine.? If you are doing CW or SSB, they won't heat up much due to the duty cycle of the TX.? With FT8 there will be more heat, until they cool down between cycles.

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I personally don't use diodes, I use a converter or a USB-C PD power-bank and trigger cable.? For your situation with a good bioenno battery (if you like to use that), just get an inexpensive buck/boost converter.? The 3 amp ones made by Drok are good, but there are lots of other brands.? You can just get a small one without the meters (to save cost/space), and wire it into your cable between the rig and the battery.? Set it to 12.0V (measure with a meter before connecting to your radio), and you will be fine.? These won't generate as much heat as the diodes.? But they are switching supplies, so they do generate some switching noise.? In my use, the switching noise has not been enough to affect any operation (can hear a bit on 160m and 80m, but not bad, and ferrites on the power line would reduce it).? And I recommend the buck/boost rather than buck only, because it will keep the radio voltage at 12V regardless of the battery voltage.

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And again, as others have noted, the 10ths of a volt are indeed displayed, as dots under the volt number, one dot per 10th.

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And thank your lucky stars that you had voltage protection enabled, otherwise you would be repairing internal circuitry that burned out due to over-voltage, likely burned-out finals and driver chip and maybe power supply protection diodes.

Stan KC7XE

yes and yes

search prior messages for winlink and/or vara

Hi, has anybody tried the new firmware with VarAC and/or Winlink??

Does it work??

On Tue, Apr 1, 2025 at 04:05 PM, Azgael wrote:

The Voltage at the Drain Q101 (Gate Q102) is 9.7v, the voltage at Gate Q101 is 2.6v,

Hello Azgael, 9.7 V is wrong for this gate voltage. It looks like Q101 is broken.

the voltage at Source Q102 is nearly 12v and at Drain Q102 is 0V (With 2.3v difference Source to Gate I was excepting to have approximately 12V).

Yes, it's also wrong. It looks like Q102 is also not ok.

If you remove Q101 use the chance to measure voltage at LIN_REG_EN. The voltage should be near "+12V" for your Rev. 2 QMX+. (For some Rev. of QMX the voltage is only 50% of "+12V". For some Rev. a voltage devider is used and for some not. Check schematics for your Rev. to find the right value.)

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73 Ludwig

On Tue, Apr 1, 2025 at 09:01 AM, Braden Glett wrote:

Do the diodes consume power by converting it into heat, as a resistor does??

Yes, so don't bury them under layers of tape/insulation or they will overheat!? W is still E x I.

73, Don N2VGU

I see this with our house indoor ceiling fans. ?they all have motor speed controllers that generate lots of RFI..;-( ?plus... being in an HOA where the homes are smaller and closer... lots of people have solar panels / controller AND those darn decorative outdoor LED's on their houses... so lots of RFI ... but 40 kHz spacing is most likely a speed controller

I have the issue too since before the SSB FW release.? It has been suggested (by Jeff Moore here /g/QRPLabs/message/136646) that this issue could be PCM1804 related.? I bought a replacement but then life and complications from eye surgery have so far prevented me from attempting to swap it out.? FYI.

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Jeff, I appreciate that. I did look at that and wonder! I went back and checked the build guide because I knew it was in the instructions for the QMX. I avoided trying to remove that display at all cost. Thankfully I do not think that was the case here. I do see that the alignment is flexible enough that it certainly can be an issue. I'll get in touch with you about making my QMX work if I cannot figure it out soon. As it is now, I think my QMX+ will be my primary radio. I've been on the air and it TX's on all but a few high bands. Low power, but I'll see if I can get it improved.?

Just leaving this here for anyone fault finding

Just dealt with my second ?MS5351M 'subtle failure' in a couple of years

First was in a QCX +TCXO where after 20 minutes or so running the Transmitted WSPR tone degraded into wide band noise that amazingly did still include enough of the right signal to get a WSPR report and show the usual power output to the antenna.? ?Several people around the world helpfully pointed the problem out as their spectrum displays were in a mess and I very quickly shut it down ......? ?
A spray of cooler on the 5351 paused the problem.? Fixed by replacing the MS5351.


Second just now was in my U3S with receiver option (SI5351 board and QRP Labs TCXO) that was starting to emit random audio noise in receive at a high level once warmed up.? ? The CLK1? 56.3824 MHz Park /Receive LO carrier in this case was sniffed for but not recognisable on another receiver but a noise spectrum a few KHz wide was filling the vacancy.? ? A quick sense check of removing the BPF, swapping power supplies, moving/cleaning? interconnector pins, switches? etc did nothing, and the transmit signal (CLK0 14MHz remained pure).

Again cooling off the 5351 area restored operation for a few minutes . .? ? and replacing the 5351 oscillator board completely (for convenience) fixed it.? ?(Not dug deeper.? As the WSPR TX modulation was always fine and the regulator seemed happy enough I think its the 5351 again. . )

Both radios have been used in 24/7 beacon operation and their only faults so far in 10,000's of hours have been misbehaving MS5351's.? ?

So just leaving this note that they can fail subtly, don't assume part working means all working . .??

This may have been the audio out processing, it was set to default, setting it to off seems to allow normal operation with no need to play games with the thresholds.

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Thanks again Ludwig,

The Voltage at the Drain Q101 (Gate Q102) is 9.7v, the voltage at Gate Q101 is 2.6v, source Q101 is 0V and the voltage at Source Q102 is nearly 12v and at Drain Q102 is 0V (With 2.3v difference Source to Gate I was excepting to have approximately 12V).

73 Azgael

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Check out the channel today. Looks like Hans has some serious competition.

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https://www.youtube.com/watch?v=1pERdo3uUos&t=12s

On Tue, Apr 1, 2025 at 03:01 PM, Braden Glett wrote:

I sure wish the voltage readout on the radio had one more digit to show voltage to tenths of a voltage,

Braden, but you may see exactely this. Please read Operating manual section 5.16 Display/controls menu.

And if you are in over voltage protection the displayed value is blinking.

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73 Ludwig

On Sun, Mar 30, 2025 at 10:55 PM, @Ludwig_DH8WN wrote:

If the controller doesn't work the gate of Q101 is pulled to 50 % of "+12V",

To be right, there was an error in my previous post. If the controller doesn't work the gate of Q101 is pulled to (or near) the voltage at "+12V", not only 50 %.

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73 Ludwig

Stan, turns out it was the voltage protection. So, thank you. I sure wish the voltage readout on the radio had one more digit to show voltage to tenths of a voltage, as other QRP rigs do.?

I've heard others recommend two silicon diodes. Is there an advantage to using those, versus a cheap little voltage regulator module? Do the diodes consume power by converting it into heat, as a resistor does??

I might use 7 rechargeable lithium batteries, as a possible 10.5v solution, to see if that works. Any thoughts on that?

Thanks again!

On Tue, Apr 1, 2025 at 12:00 PM, Azgael wrote:

I measure the LIN_REG_EN and I get 2.6v when the button is pressed meaning at gate of q101 i have now 2.6v

Azgael, if not alredy done please remove the black wire.

At least from 2 V up at gate of Q101 the voltage at drain should be near 0 V. You see 2.6 V at the gate, enough.

Please check voltage at drain of Q101 (or at gate of Q102). If voltage at gate of Q102 is lower than voltage at source of Q102, in worst case 2 V or more, voltage at drain of Q102 should be close to voltage at source of Q102. Which voltages do you see?

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And voltage at drain of Q110 should also be near 0 V.

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73 Ludwig