Paul Harden NA5N designed the Desert Ratt shortwave regen receiver in the mid-1990’s. It’s a popular radio for builders, and my choice was due to the excellent documentation and the fact that it was designed to tune using varactor diodes, rather than metal fin variable capacitors (that were used in the original Desert Ratt).
Here’s my construction journal:
March 4: Started with the LM386 and the transistor preamp, could not get it to chooch. Reverted to the breadboard. AA7EE has pin 7 unconnected. There’s a principle that you should start with the audio amp and work backwards; this doesn’t work as expected because (as I found out), I needed to bias the base of the transistor on the breadboard before I could get output.
March 6: Soldered a socket and completed the audio amp portion of the circuit. Hoping that it will work as advertised, now that everything is soldered in, once again.
March 7: Completed soldering all components. Result is motorboating. Check AA7EE: he switches pins 2 and 3 on the LM386, which I do, and it’s a relief. However, still no output from the radio. There should be around 2v on the base of the top left transistor, but it’s at ground.
My schematic drawing for the first build of the Desert Ratt 2.
March 8 (morning): Replaced the transistor, no change. Checked voltage again — tried removing various wires implicated in the issue, and found a copper ‘hair’ crossing from the ground plane to an island. Once I removed it, I started to get some fuzz indicating oscillation. Still lots of intermittent noise; soaked the board in alcohol and dried it. This is the last time I’ll cut out islands! There just isn’t enough dependable separation between the islands and the ground plane. Should I start over?
March 8 (afternoon): Popping noise seems to have gone away now that the board has dried fully. And with the evening, I start hearing stations. It chooches!
March 9 (evening): I switch wires for most of the pots as they were wired in backwards (as in loud to the left, quieter to the right). Frequency calibrated after removing a few turns from the main tuning toroid. A is 3.0-6.3 MHz and B is 3.1-7.8 MHz. Very good fidelity and dynamic range. I’m satisfied with this radio’s sound.
