April 10, 2010

I love building receivers

There is something very, very cool about building receivers.  You start with a handful of parts, and in a few hours (or a few evenings), you flick on the power switch and (if all has gone well) you hear signals.  Of course, quite often I have to spend a few more hours troubleshooting things before I can get my receivers to work, but that can provide very helpful experience.

I've been wanting to build another regenerative receiver, just for grins and giggles, and hearing Bill, N2CQR, speak on the Soldersmoke podcast about how he suspects regens are possessed and possibly evil made me want to try another one of these guys again.  I have built a couple of different regen rigs--one was a regenerative W1AW code-practice receiver built around a 3.579 MHz color-burst crystal, and the other was a Ten-Tec multi-band regenerative shortwave receiver kit.

I had found an old article by Charles Kitchin, N1TEV, titled "A Simple Regen Radio for Beginners," from the September 2000 issue of QST, that didn't require any tough-to-find parts (except for the air-variable capacitor), and after about 15 minutes of going through my parts bins and drawers, I had gathered up just about everything I needed.

I started by building an enclosure--well, it's really not an enclosure, because the back, top and sides are open to the air.  I suppose it's more of a chassis, of sorts--really just a base and a front panel, made of double-side copper-clad PC board.  I used a 150 pF variable capacitor for the main tuning control, and I wound the coil on a plastic pill bottle.  The article called for 22-gauge solid insulated wire for the coil, but I didn't have any around, so I used some enameled wire I harvested a few years ago from a TV degaussing coil.

I drilled holes in the front panel and mounted the pots and the tuning capacitor, then began breadboarding the circuit with the schematic at my side.  My aim was to finish the receiver in one evening, so I worked quickly (more quickly than I should have), and my layout admittedly leaves a lot to be desired.  And, of course, it didn't work the first time I turned it on.

N1TEV kindly included a few voltages at various test points in his schematic, so I checked those, and found that proper voltage was not getting to the transistor, though it was not apparent why.  I suspected a bad diode, so I replaced it, but to no avail.  I gave it up for the evening.

So the next evening I decided I was going to make this radio work one way or another, and I started poking around and found that the voltages that were incorrect the day before were magically correct.  (How does that happen?  Probably a bad solder connection or a short somewhere that was remedied by my poking around.) I turned the power on and started tuning through the entire range, to see if it would be possible to hear it oscillating on my store-bought shortwave receiver on the bench.  The good news is that I did hear it--the bad news is that there was still nothing in the headphones, so I turned my attention to the audio part of the circuit.

I had used an old LM386 amplifier chip that I had salvaged from another project--it occurred to me that it might actually be dead, so I replaced it with a new one, and when I powered back up I began hearing things in the headphones.  I connected three or four feet of hook-up wire for an antenna and just draped it across the bench, and was able to hear dozens of signals.  WWV came in loud and clear, along with a number of shortwave broadcast stations in a variety of languages.  Sweet, sweet success!

Tuning a regenerative receiver is definitely an acquired skill and it takes some getting used to, but eventually you get the hang of two-handed tuning (one hand on the main tune control, the other on the regen control).  My radio is really sensitive to hand capacitance, meaning that once I tune a station in, I have to keep my hands on the controls, because moving them away changes the tuning.  If I had gone to the extra trouble of using insulated shaft couplers, I might have avoided this problem.  (As you can tell from the photograph, I could also have built this thing with a lot more care given to lead length and layout, and that might have helped a bit.)

But hey--it was a fun project, and it worked, and there was great satisfaction in being able to troubleshoot it successfully.  I think my next receiver project will be some sort of direct-conversion rig, and after that, I'll turn my attention to a superhet design--I have a whole bunch of surplus crystals that I might be able to use.

I hope you're melting solder in some useful (and fun) way!

73 de aa0ms

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