Karl K5KHK wanted to build the receiver from junk box parts. But he did not have the needed AF transformer in his junkbox. So he designed an AF amp that would not need a transformer. That's great, but the circuit was a lot more complex than the one we designed for use by beginners. And we advised others to search for the needed transformer and to use the circuit we had prescribed. Thus the original "Honorable Mention" category. But this is no big deal. Karl has made great contribuitions to the radio art, and his circuit will, I think, be of use to those who want to build a different AF amp AFTER they complete the basic receiver.
NOTE FROM 28 March 2024: Karl found the right kind of AF transformer! He went the extra mile and re-built the audio amplifier using the transformer. FB Karl. You Honorary Mention status has been removed! Welcome to the full Hall of Fame!
Here it is with the simple AF amp that we are using:
Karl writes:
First Light! I know, I am a rebel and cannot follow instructions, but I wanted to do this from my stash (OK, mixing worlds here, that's what fiber artists call their junkbox of fiber, but I did join my wife this morning at a fiber fest). I do not have an output tranformer (I do have a few 600:600Ohm transformers). So here are my three answers:
1. Biggest challenge was to find a way to replace the output transformer. I did have a few baggies of BD139/140, so I researched small amplifiers wiith a complimentary output configuration and found one in John Lenk's "Handbook of Simpified Solid State Circuit Design", which apparently goes back to a Motorola application note (based on ancient transistors). After some noodling and heavy LTSpice'ing, I had something that looked like it would work, and it did, even though without anything connected, it blasted me with the local AM talk radio station (and some RF oscillation). A few strategically placed 0.01uF capacitors tamed the beast. What also helped with the design is that I found a schematic online without much comment (besides one who said "this will not work") that was based on the same design with more modern transistors.
2. I learned a lot about how to keep strong RF from messing with my AF amplifier, and that without having to put it into a shielded box.
3. I've started on two modules for Pete's PSSST transceiver when he first published it, but stuff got in the way, and I am motivated to pick it up again.
So again, the complete receiver was build with parts from my junkbox. This means that I had to make substitutions. I have for example a 5.1V Zener diode for the oscillator. It does produce enough power to drive the mixer, even with that lower voltage. I had smaller toroids on hand (-37 vs. the -50), so I had to do some math for the BPF to figure out how to wind them. I had no silver micas, and also no NPOs of the correct values, so a couple other values added up to almost exactly what I needed.
Thanks Karl, and congratulations!
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