I was reminded by a video by VK3YE about a PA chain he is working on using BD139’s that I had sometime ago thought about using them in a class c amp. I had seen something similar in a schematic for a circuit by Diz at Kits and parts. When you can get a good 5w from a pair of transistors that cost 20 cents and a couple tv baluns, why settle for less in your qrp rigs.
I finally got around to boxing up the 10w CW module i bought off ebay. I mated it up with a SI5351A and Arduino, a home brew keyer and put it on the air tonight to give it a test. I was Txing only as I am yet to make a matching receiver to go with this, but that will be the next task.
Rear panel and the internals.
Video thanks to Steve VK3MEG who was listening out and recording my transmission.
Smd soldering is not always the easiest thing to do. I am certainly no fan of doing smd with a solder iron, sure some of the PRO guys can solder even the tiniest components with an iron, I am certainly not one of them.
So I bought myself as fancy smd reflow solder station to do some smd boards i have with past and hot air. And let me assure you, it was money well spend as it was just so easy.
The process is very simple, add the tiniest amount of solder paste to the pads, then place the component onto the pads, the paste will hold them in place, add hot air and watch the magic happen.
As the flux burns away, the solder coalesces and wicks to the pads and the component tabs, surface tension then pulls the components into alignment and down onto the boards pads. 9 times out of 10, leaving a perfectly solders part. See C4 and 5 in the above images.
All those other things you are scared of like tiny smd transistors become cake and nothing to be feared any more. I certainly won’t be bothered with kits and projects that contain smd anymore, even ones with multi pin devices should be super easy now with hot air and solder paste.
The above is a 10w CW transmitter kit from ebay. The schematic can be found here on my blog: http://robs-blog.net/2016/04/11/10w-radio-shortware-cw-telegraph-transmitter-7-023mhz-kits-hf-power-amplifier/
So we got it built, connected it up to an swr power meter and dummy load, as well as hooking the scope up to the output also and keying it up for the first time we get a nice 23.25 V RMS output, or 10w, I did not expect that it would deliver to the specifications, but it does, and that is kind of amazing for a cheap Chinese kit off ebay. This is in xtal mode, I still need to try it in VFO mode and see how much drive is needed to get the 10w out. But for now, I am pretty happy with how this thing works.
Ok, so mostly I am a DDS and Arduino kind of guy, they just work, they don’t drift and they are super simple. But sometimes, you get that “Retro” kind of nostalgia happening and a desire to build something rock bound. I have a ton of crystals in matched pairs for 7mhz, enough to cover the whole CW portion to 7.050. So i thought, what the hell, I will make a Super VXO, and incorporate it into a transceiver if it proves to be stable enough.
So i went looking for a suitable circuit to modify to suit my parts bin and needs. And i came across this one above. Varactor tuning is nice, as getting quality air variable caps and reduction drives is impossible, so varactors are must. The only thing i changed is the inductor as I do not have any Toko IF coils, and the only other thing i changed was to add an unbalanced to balanced transformer to split the signal in to, one for the receiver and the other for the transmitter. The original article and associated info for this Super VXO can be found HERE!!
Laid the board out in Sprint Layout, and after ironing out the bugs with the help of Ben VK6FBLJ who put his eyes over the layout and noticed all the errors, the gerbers were then converted to gcode for routing.
Could fit 2 boards on the one sheet of FR4 the above picture is the boards on the mill deck still after routing and v-cutting in two.
And this is the finished board after being cleaned with steel wool and a coat of PCB lacquer being applied. They have come up rather nice looking. More to come on this, after they have been built and tested to see how stable they are.
Most of the parts are installed, I threw in a pair of 10mhz xtals for testing with, they will not be the what is used in the final board, but are good enough for testing purposes. Just have the inductors to wind, 10 turn pot to wire up and the 8v regulator to install and it is ready for smoke test. Oh and i was able to find a MV2109 varactor in my semi conductor box, so, no cheeping out and using a silicon rectifier.
Well i cannot get this thing to start oscillating. Have tried just about everything i can think of, will post back when i work out what the case is.
Failure is always an option 🙂
So, i need a 1w driver amp to drive a pair of IRF510’s in a 10W final PA. Having played with a number of my own designs, and failed miserably, i decided that it was time to borrow from others something that is known to work. The following is the amp chain from The Beach 40 by VK3YE Peter Parker.
I first laid out the board in Sprint Layout.
I then routed the board out on the CNC machine.
Then built the board and proceeded to smoke test it.
I have an obvious clipping issue because of 2 much gain in the 2nd stage where i reversed the bias and gain resistors by mistake, a quick fix on those should fix that ugly waveform. I also do not need almost 3w, I only need 1w, so i will also be increasing the gain resistors to bring the output back closer to the 1w i need. All in all, a successful experiment.
So after tinkering with the gain on the first stage, i have the thing looking nice and clean and giving 0.8w, smack bang in the middle of the 0.5 to 1w drive the final amp requires.