Friday, January 31. 2020
Construction report QRP-Labs.com 50W PA
In June last year, when I talked to Hans, G0UPL, ordering his QCX kit at his booth at the Ham Radio fair in Friedrichshafen, he mentioned that there would be a 50 W PA kit coming out soon, and that stuck with me. Meanwhile, I noticed indeed that QRP is just no fun when no one is answering your calls—sorry for having to say this. I like to think of this as a kind of “Law of the Conservation of Effort”: If a DX contact is established, and one of the stations is QRP, then the other one is doing all the work and effort (in terms of operating a respectable receiving antenna and/or “having to crawl into the speaker”). Well, thinking of my main goals that led me to the QCX, which were
- assembling a TRX completely on my own,
- getting a lightweight TRX, no matter its power,
- having it only support CW,
wouldn’t there be a better follow-up step than
- assembling a PA kit,
- a lightweight PA such that QRP is no longer an option (hi),
- having it only support CW (not sure about this, though, but at least it’s marketed as “the QCX PA”).
So, I kept close eyes on the QRP Labs site in the recent months for signs about that PA, and finally, at the beginning of December, Hans announced that PA on the mailing list, and only about one hour later my order was through. I was one of the lucky ones that could include his enclosure kit before it went out of stock.
QCX mods
While I waited for the kit to arrive, I had time for preparation: The QCX needed a PTT mod to be able to provide a PTT signal to the PA; this is achieved by patching a stereo jack into the circuits (plus creating an outlet in the QCX’s enclosure) and adding a pull-up resistor. I also flashed the latest firmware release using an Arduino Uno as programming device, which might be a topic for a different blog post. The harder question to answer was how to get a cheap but still qualitative 20 V 5 A power supply for the PA. Luckily, I already possessed the Lenovo UltraSlim 90 W (20 V 4.5 A) notebook travel power adapter, which even included the option to use a wall socket or a standard automobile 12 V plug. Therefore, I could even go from a standard 12 V lead-acid battery. I only needed a non-standard DC barrel adapter, which took some extra time to arrive. While I was at it, I purchased a 3S LiPo battery pack with nominal 11.1 V to power the QCX itself with something lightweight; note that I measured up to 13.1 V fully charged, so using a fresh 4S pack might fry the QCX.
The PA kit arrived after Christmas, and I was again looking forward to the assembly process, which promised to be not as lengthy as that for the QCX. It took me about ten hours nonetheless, mainly due to assuming errors where there had been none. I will now go over some specialties that I had encountered during the assembly.
Transformers
When I wound the toroids for the QCX, I learnt that left-handers should take some extra care to not accidentally end up with mirrored windings—as mentioned in my previous assembly report. This is not important from an electric point of view, but helps putting everything into place. So, I took some extra care to look at the pictures in the PA assembly manual to exactly copy the windings. For the first three toroids, that strategy worked out. But not so for the (bi- and trifilar) transformers: There are two pictures for each transformer, and guess what, the windings are different in each picture! In the first picture of T2 in the middle of page 15, if the two ends point at you, the left end is at the bottom and the right end at the top. But at the top of page 16, the left end is at the top and the right end at the bottom. Luckily, for T2 this is not so important, because the top will be at 12 o’clock and the bottom at 6 o’clock, so the bottom end will bend away a bit. The first picture of T1 is at the bottom of page 17, shown with left end at the bottom and right end at the top. At the top of page 18, this is the other way round—and this is indeed the way T1 should be installed. Because otherwise—and of course just like in my case—the bottom wire will stretch somewhat diagonally from 12 o’clock through the toroid down to 6 o’clock. I just ensured that that wire is bent a bit in an L-shape, such that the last centimeter stays close to the ring. But I guess that’s a mere detail.
I used version 1.00f of the manual for the assembly and stumbled upon a few typos which are meanwhile fixed in the most recent versions, so I won’t list them here. But just one thing, when soldering T2 and T1, exactly follow the manual paragraph by paragraph, because otherwise you cannot perform the mentioned continuity tests in between. I soldered everything at once, and suddenly—and of course—“everything” was now linked and grounded. That was odd for me at first, and I made measurements here and there and took a closer look at the wiring diagram, to finally conclude that everything was okay; I’m still too much used to the DC world and need more experience with RF and AC.
And that was already the most difficult part, assembling the other components didn’t pose a real challenge anymore. Only the (almost) last step of sliding the PCB into the enclosure was a bit fiddly, because—and luckily I noticed—I accidentally pushed the silicon pads aside that should be isolating the big transistors against the heat sinks.
Here’s again a little photo show of my progress:
Tuning
I delayed reading the next chapter about circuit explanation and continued directly to the adjustment process, which was also straight forward: measure the current with a DVM while turning a trimmer potentiometer. For this process, I powered the PA with just 12 V to have a little more buffer in case of errors. In that case, I measured the base current to be 83.1 mA during PTT. Then I dared to feed it 20 V, and everything looked okay, so I also fed it with some RF into the 20 W dummy load. An infrared camera helped me prove that nothing was about to explode, so it was time to get on the air!
Practice
During my first tests on the air I noticed that the notebook travel adapter I used wasn’t really the best choice, because it generated “ocean wave” sounds in the receiver when I powered it by a 12 V battery: Whenever I generated PTT (and no RF, that is, the QCX was in practice mode) for a certain time span, I heard an “ocean wave”-like rise and fall of white noise which took exactly the same time. Later, that behavior disappeared, maybe because the power adapter got hot enough. (I noticed that it might be a good idea to patch a power meter between PA and antenna to notice whether there’s actually power going outwards or not.) Luckily, that “ocean wave” doesn’t show up when I power the adapter from the wall socket.
My first contacts using the 50 W PA went to the Netherlands, Cyprus and Spain, and the RBN skimmers also heard me in Finland, Estonia and even Russia, and that with my home-brewed wire loop of about 27 m of wire on the west side of the house. I notice, I reached a real milestone for myself by
- going on the air from my home
- with a self-made and permanent antenna
- using a TRX and a PA that I soldered by myself,
- and all of this in morse telegraphy!
How top this? This is at least my winter mode. When the higher bands open up again, I will come back doing real DX using my hex-beam. Starting with ham radio at the end of a solar cycle isn’t such a bad timing, because there will be positive surprises and improvement of condx for years and years—even if the old OM slobber over the good old times when they worked one Japanese after the other using just a “rusty nail” as antenna.
What is my next project, then? Well, the home-brewed wire loop needs a home-brewed sheath current filter. And I want to measure the RF through the PA using my old 1970’s oscilloscope I got from an OM, what I already used to debug the QCX. I want to get deeper into real RF home-brewing, but I don’t quite know yet how. We’ll see. Another important thing for me is to pass the border to HST... I wish I were half as awesome as these kids!
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