Power amps are pretty much smoke and mirrors, magic and voodoo all rolled into one for me. I have avoided doing power amps for quite sometime because I find them frustrating
Well, i have made some progress, I am now making 5W out of an amp designed to make 10w LOL. I still got some work to do.
So its been a busy day here in the workshop. Caps arrived today and we set about to making low pass filters for the transmitter. I started by sticking in the caps and then in turn winding the inductors for each of the 3 filters, 80, 40 and 20m bands and giving them a test by feeding in a square wave from my it makes nice distortion signal generator.
80m Low Pass Filter Under Test
The output was fed into the spectrum analyzer on the Red Potato. The spurs either side of the fundamental are crud from my signal gen, it does that when you start to turn up the output. This was a 10 volt peak 3.6mhz square wave being fed into filter and I through up a few cursors to get some levels, first harmonic is -60dBm down on the fundamental. That should be more than good enough.
I did the same for the 40m and 20m filter, again -60db down on the first harmonic, the above image being for the 40m band.
Something I was worried about was isolation on the relays. These are no uber doober premium rf relays, no no no, they are 10 for $1 Chinesium Grade floor sweepings and seconds off Ebay. So we stick the signal in the in port and connected the 100meg Hantek scope up to the other end and turned the volts per division down to the noise floor and looked for signal leakage. Nothing, maybe a couple of microvolts but that might have been an aberration. Either way, good enough for the kinds of girls i go out with.
Final board all together and soldered up.
A bit of an idea of how its all going to go together as a stack. I think things might actually work ok after all. Touch wood. Tomorrows job is to program an ATMEGA IC to work as an iambic keyer, add in some switching and buffering to get the transmitter up and working. Not sure if i am going to have an external PA yet, or just use a couple of BD139’s and get 3 or 4 watts out. Will see how we go.
So when i designed the filter boards one thing i took into consideration was that often you need to use multiple caps to get close to the value required. Now when i first started home brewing, i used to obsess about values, oh i dont have this value, oh i dont have that value, but now i really do not give a crap, as long as its in the ball park she’ll be right mate, mostly the values are not super critical as long as they are ball park.
I mean does it really matter if the corner frequency of a LPF is shifted up or down a few 10’s of Hz, or Khz for that matter because your cap values are 5% off ideal? I am using 10% caps as it is, so there can be a bloody lot of variation here. But I did manage to get pretty close to the numbers using the one range of MLCC caps that minikits.com.au most of which are also NP0 types also. So below are the schematics for the filters.
So I figured for my stacker transceiver project that I would start with the transmitter. First cab out of the rank will be the low pass filters to test out and ensure that my board design is working as it should be. Added in the switching bits to start with and jumpered out the coupling capacitor links used in band pass filters. I have even ordered NP0 monilithic caps for these and should have them in a couple of days and have this built and tested by friday.
The three filters will be for 80, 40 and 20 meter bands using 7th order Butterworth derived filters. Probably overkill for my transmitter design and power, but hey if you got em roll em.
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.
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.
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.