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I agree, but my view was that if cheap replacement regulator PCB's are available at 2 for $10 (as they are/will be) many people (on the basis that the mainboard was probably OK ) would choose to fit a new board over a miniature fault hunt that left the risk of undiscovered damaged components for a future date. .?
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A crowbar circuit would add cost and complexity to an already dense pcb layout and it could cause further damage to the regulator by effectively shorting the output to ground when activated.? ?That could save the main PCB at the expense of possibly do more damage to the regulator.? At one time, there was discussion by some folks to redesign the switching regulators to do a soft turnoff if the regulator output voltage is exceeded. If there were any future redesign, that might be something to pursue.? But I believe the Rev 2 and 3 boards with the 5 watt zener on the 5V regulator solved most of the switching regulator related problems and failures are now fairly rare. It's a grand balancing act between achieving low cost and compactness vs. added circuit complexity and its associated costs and further challenges to fitting in within the already densely packed PCBs.
-Steve K1RF
------ Original Message ------
Date 4/17/2024 8:30:08 AM
Subject Re: [QRPLabs] #qmx PCB Rev 2 recommended modification
I'm starting to think that a discrete 'crowbar' power management circuit operating immediately if 3.3 exceeds 3.5V or 5 exceeds 5.2V or the input exceeding (say) 12.8 might be a good use of board space in a future version.? This could be made to operate sooner and better than low voltage Zeners allow?
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I'm starting to think that a discrete 'crowbar' power management circuit operating immediately if 3.3 exceeds 3.5V or 5 exceeds 5.2V or the input exceeding (say) 12.8 might be a good use of board space in a future version.? This could be made to operate sooner and better than low voltage Zeners allow?
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[To reduce potential confusion, I've deleted my original response (that Hans subsequently pointed out was in error)?to Razvan's question (post #117718) and reposted the response here corrected.]
For the Rev 1 PCB?there are a few?optional?modifications that could be done, none are necessarily recommended.
?
I've replaced the stock Zener diode on the 5V smps board with a 5W version as a precaution. Not easy as one has to remove the original, then add a couple insulated leads to the pads since the 5W Zener will need to ride above that spot in order to fit. I'd recommend just purchasing a set of replacement SMPS boards from QRP-Labs as the path of least resistance (not measured in Ohms).
?
If you need USB-C to USB-C connectivity to a PC, there are 2 smd resistors that will make that type of connection work more reliably. Again these are tiny components needing careful very fine tip soldering to the USB connector pads, which are very closely spaced. If you instead just use a USB-C to a USB A cable (very common), things will work fine. I recommend this one, as it effectively isolates RF from getting into ones PC (likely more common an issue with end-fed antennas):?
?
If you?need?the functionality of the PTT Out jack to be "grounded" (ex: to trigger an SDR switch if using an external SDR as a panadapter), then a jumper wire needs to be installed across two points on the opposite side of the board from that jack. The 5V PTT Out works fine without this mod. (This particular mod applies to Rev 1, 2, & 3 QMX main boards).
?
The only other mod I've made is removing the power coaxial jack, and replacing that with about 6 inches (15 CM) of good grade silicon insulated wires terminated with power pole connections. This helps avoid a power disruption if the QMX is jostled while operating. An intermittent power disruption could cause, in some rare cases, the older 5V SMPS board to be damaged.
?
So that's the list. Again, these are not necessarily "recommended", as they are entirely optional.
-- 73de Roy - KI0ERLittleton, Colorado USA
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Thanks Hans for that necessary clarification!?
I hadn't had enough coffee yet when I wrote that post (#120902) apparently (the best excuse I could come up with).? I had actually swapped the Zener out to the 5W version on my 5V smps board (from early Rev 1 QMX kit), but not on the 3.3V board as I mistakenly indicated (because I had read the discussion concerning the characteristics of the 3.6V 5W Zener diode a while back)
I concur that swapping the Zener diode on the 5V smps is tricky since the 5W Zener does not fit directly on the board; it's a bit of a risky proposition.?
In short, there's no "recommended" mods for the Rev 1 QMX kit, unless one needs the grounded PTT out function.? That mod is quite straight-forward to do.
--
73
de Roy - KI0ER
Littleton, Colorado USA
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Nobody should ever replace the 3.6V 500mW diode on the 3.3V SMPS board with a 3.6V 5W one. This is unnecessary and doesn't work. All PCB revisions of QMX have a 500mW diode here. It's important because the voltage characteristic curve of low voltage general such as 3.6V is not steep. A 3.6V 5W zener will pass a significant current when 3.3V is placed across it. This will result in higher heat dissipation and higher current consumption of your QMX. Additionally it probably doesn't provide any additional protection you're looking for.?
The 5.6V zener on the 5V SMPS board is a different story. On Rev 1 boards and some Rev 2 it was 500mW. We later did change it to 5W. The curves for 5.6V zeners are steep so 5W zeners work as well as 500mW ones. A 5W zener diode is more robust in the event of an unlikely input DC power glitch. Having said that I *still* wouldn't recommend replacing a 5.6V 500mW zener on your 5V SMPS board if you have that older board. Remember if it ain't broke don't fix it. Removing that tiny 500mW zener and replacing it with a larger package 5W one is not without risks. I'd suggest keeping the 500mW one; in the very unlikely event that you blow it, you can replace it with a 5W one then if you wish.
73 Hans G0UPL
For the Rev 1 PCB there are a few?optional?modifications that could be done, none are necessarily recommended.
I've replaced the stock Zener diode on the 3.3V board with a 5W version as a precaution. Not easy as one has to remove the original, then add a couple insulated leads to the pads since the 5W Zener will need to ride above that spot in order to fit. I'd recommend just purchasing a set of replacement SMPS boards from QRP-Labs as the path of least resistance (not measured in Ohms).
If you need USB-C to USB-C connectivity to a PC, there are 2 smd resistors that will make that type of connection work more reliably. Again these are tiny components needing careful very fine tip soldering to the USB connector pads, which are very closely spaced. If you instead just use a USB-C to a USB A cable (very common), things will work fine. I recommend this one, as it effectively isolates RF from getting into ones PC (likely more common an issue with end-fed antennas):?
If you need the functionality of the PTT Out jack to be "grounded" (ex: to trigger an SDR switch if using an external SDR as a panadapter), then a jumper wire needs to be installed across two points on the opposite side of the board from that jack. The 5V PTT Out works fine without this mod.
The only other mod I've made is removing the power coaxial jack, and replacing that with about 6 inches (15 CM) of good grade silicon insulated wires terminated with power pole connections. This helps avoid a power disruption if the QMX is jostled while operating. An intermittent power disruption could cause, in some rare cases, the older 3.3V SMPS board to be damaged.
So that's the list. Again, these are not necessarily "recommended", as they are entirely optional.
--
73
de Roy - KI0ER
Littleton, Colorado USA
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On Thu, Feb 8, 2024 at 01:12 PM, DL2ARL wrote:
Hello,
are there any recommended modifications for the Rev 1 PCB version?
I repeat my unanswered question here. Reading the postings in this group, I understood that at least the Zenner Diode on the 3,3V Switching Power Supply Board had been changed against one with a higher power rating. Is this the case? What else should be done for the Rev 1 PCB Version? Tnx es 73 de dl2arl
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On Thu, Jan 4, 2024 at 01:27 AM, Hans Summers wrote:
I have written an article about the recommended PCB Rev 2 modification
Hello, are there any recommended modifications for the Rev 1 PCB version? I wish I could search myself for this issue in this group, but the search function is a pain... Bear with me if the question I ask is "brain challenged" 73 de Razvan dl2arl
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I've got both versions (80m-20m, 20m-10m). My low Band trx was deaf after 4 weeks working good with 100 QSOs on CW and FT8, also DX ing. I removed the ic402 and installed the smd 1N4148....really tiny.
I have to test, if the diode solderings are good, but I managed to fix the BPF Switch. See the pictures. I HAVE HAD SOME QSOs to USA on FT8 after the substitute. I hope that I could solve the problem. I used a carpet cutter to cut the single pins of the IC and than removed each pin by using the soldering pen separately. It works.?
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My QMX Rev 1 board was shipped in July of 2023. At that time I inventoried the parts and set it aside as I continued to see a multitude of issue and builders having problems. There are plenty of threads about the QMX and I know after my QMX was shipped a Rev 2 PCB was released and that is what this thread began with. I see that there is no revision needed to the Rev 1 board but I am thinking of going ahead with the build and want to make sure I am following the right path. I had downloaded the manual QMC Rev instructions1.00f and then replaced the pages needed to bring it up to 1.00G. I see there is now instructions 1.00j. Without printing off the entire 66 page instructions or going page by page to figure out which pages need replacing, is there anywhere I can find something like I used and refered to replace just the pages with changes going from f to g? It would save paper and toner or at least simplify it tremendously. Any help will be appreciated.
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Hi Stefan,
" lower IR" should be no problem, lower IR is better for reverse operation.
Look . The worst value for 1N4148 is 50 ?A @ 20 V 150 ¡ãC but only 0.025 ?A @ 40 ¡ãC.
"lower URRM" should also no probleme, see posts above.
73, Ludwig
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Hi folks, now that I got my filter sweeps fine after replacing Q508, I want to do the Diode mod. However, I have no 1N4148 lying around. What I got at hand that comes closest is a BAW76, specs (and the 1N4148 specs in brackets):
What worries me the most are the much lower IR and lower URRM. Any comments on that? - Will that work or fry the diode and possibly more? Thanks a lot and Vy73 Stefan.
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Stephan, I'm glad it works! You may wish to tack that wire to the PCB in a few points. It might move around and cause damage otherwise.
72 & 73 JZ
Dear all,
Hooray, the QMX rev. 2 is on the air. The protective diode (1N4148, package SOD323) has been soldered in and the IC402 has also been replaced.
Soldering SMD components is a bit of an adventure for me. When desoldering the IC402, I tore the PAD for PIN14 off the board. It was unlucky that the line goes directly to the processor and could not be reached at another component nearby. Luckily, this line arrives at PIN51 on the processor, which is easily accessible at the corner. Not nice, but it works.
73, Stephan
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Dear all,
Hooray, the QMX rev. 2 is on the air. The protective diode (1N4148, package SOD323) has been soldered in and the IC402 has also been replaced.
Soldering SMD components is a bit of an adventure for me. When desoldering the IC402, I tore the PAD for PIN14 off the board. It was unlucky that the line goes directly to the processor and could not be reached at another component nearby. Luckily, this line arrives at PIN51 on the processor, which is easily accessible at the corner. Not nice, but it works.
73, Stephan
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Hans
Thank you for this,? Makes great sense and I can get back to worrying about something else :-)
?
The U3S kit remains an interesting part of the range to me as it can work over 160 - 10m with a reasonable 'WSPR power' and while operating stand alone with GPS it can do the useful XXX/P /A /M? etc WSPR callsigns (among other useful modes and twists) and quite sophisticated programmable band switching schedules for months on end.? It may be under-rated in my view.? ?
I'm not sure how many build the receiver too, but to me it's both a useful indicator of good conditions when others signals come through strongly and in my mind it makes it a? 'proper' transceiver too.? (Built with 'both' detector paths it has continuous audio monitor to a small internal speaker and switchable Audio output or I/Q output to the 3.5mm socket by rear panel switch. )
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I've never heard of a MUX failure in a U3S Receiver module ??
The product was launched in August 2016 and even though it is not such a significant product as QMX, 1,832 kits have been sold and I am certain if there was any design issue that it would have been uncovered by now. 7.5 years and 1,832 kits are both relatively large numbers in the context. So I would say no change is necessary. But if you changed C7 from 100nF to 1nF it would be harmless. I would not do it on the usual basis, "if it ain't broke don't fix it".?
There are plenty of key design differences between QMX and the U3S Receiver module, including but not limited to: - The MUX at risk in the QMX is the BPF switching MUX, not the QSD MUX. The receiver module is a single band affair therefore with no BPF switching MUX. It's a totally different ball game, and the QSD presents a low impedance off?the LO frequency, so it is thought (and measured).?
- There are 82-ohm resistors in the signal path on both the RF inputs (probably unnecessarily - but they are there).?
- The Rx/Tx MOSFET is upstream of the BPF; the BPF in question has some dB of insertion loss too
- The BPF is a double tuned parallel resonant circuit type. This means it is made of two parallel LC tank circuits each of which has low impedance off resonance. This contrasts with the series resonant circuit in the QMX BPF which is a single resonant circuit; a series resonant circuit which is has high impedance off resonance
- As pointed out, the power output of a U3S is much lower than a QMX PA (though this would not be the case if the 5W PA kit option was being used).?
One or more or even ALL (who knows) of the above could conspire to prevent the kind of destructive parasitic oscillation build up seen in QMX.?
Good question Bruce as there definitely are common features.
I have not seen fails attributable to the related problem posted here on the forum. It may be that I have not been looking, or that the U3S transmitter isn't powerful enough to do harm, or that the difference in receiver BPF structure is saving the day. Dunno. As this receiver module is used in diverse ways, anything is possible.
Hans, or someone else with direct experience, will need to answer your question. I'll be around for simulation help if it is needed.
On Fri, Jan 5, 2024, 1:06?PM Bruce Akhurst < bruce@...> wrote: As someone has to ask :
Is there any reason to reduce the antenna capacitor to 1nF (or fit a diode)? on the U3S receiver which also covers the lower bands and also has a 100nF input cap, FET mute/inhibit system, plug in bandpass filters etc.
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Good question Bruce as there definitely are common features.
I have not seen fails attributable to the related problem posted here on the forum. It may be that I have not been looking, or that the U3S transmitter isn't powerful enough to do harm, or that the difference in receiver BPF structure is saving the day. Dunno. As this receiver module is used in diverse ways, anything is possible.
Hans, or someone else with direct experience, will need to answer your question. I'll be around for simulation help if it is needed.
On Fri, Jan 5, 2024, 1:06?PM Bruce Akhurst < bruce@...> wrote: As someone has to ask :
Is there any reason to reduce the antenna capacitor to 1nF (or fit a diode)? on the U3S receiver which also covers the lower bands and also has a 100nF input cap, FET mute/inhibit system, plug in bandpass filters etc.
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As someone has to ask :
Is there any reason to reduce the antenna capacitor to 1nF (or fit a diode)? on the U3S receiver which also covers the lower bands and also has a 100nF input cap, FET mute/inhibit system, plug in bandpass filters etc.
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OK! That is great? Adrian!
73 JZ KJ4A?
80m and 40m with the antenna. Will try 20m tomorrow, now it's a liitle late for 20 with 5 watts :-)
I tried with a dummy load on all bands, and runned all sweeps, they're like before the mod, no changes.
Regards,
YO3GFH
op. Adrian
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80m and 40m with the antenna. Will try 20m tomorrow, now it's a liitle late for 20 with 5 watts :-)
I tried with a dummy load on all bands, and runned all sweeps, they're like before the mod, no changes.
Regards,
YO3GFH
op. Adrian
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Bruce Akhurst
DL2ARL
Tom Meyer
DD6DO