Have you ever been bored out of your mind even when you had plenty of things to do and little time to do them? The child’s whine, “I’m bored. There’s nothing to do!” has never been my problem. My whine has always been, “Damn-it-to-hell-and-back, there’s too much to do!” It’s a much bigger problem when most of the things to do are perfect storms of complexity, uncertainty, frustration, struggle, disappointment, discouragement, bafflement, and unrealistic expectations. Sometimes, I freeze up, or experience a kind of mental vapor lock. Over the decades, I’ve found in such a funk that what I need is an injection of accomplishment feedback to my non-inverting inputs–my eyes, ears, tongue, and sense of touch (I’ll leave what my single inverting input is to your imagination). So I need to get something done and working (or cooked and eaten, written and mailed, laundered and folded, etc.), and I need it quickly.
So in such a mental state the other day, as I was rather aimlessly browsing YouTube hoping to find a video on yet-one-more piece of esoteric knowledge I couldn’t live without, Alan W2AEW’s video on the Michigan Mighty Mite (hereafter M3) came up–a one-transistor, crystal-controlled “transmitter” built up on a piece of copper-clad using five discrete components, hand-wound coils, and a variable capacitor. I knew I had all the needed parts on hand, and in a previous funk I had organized them in such a way that I could gather everything in just a few minutes. I had an already-cut piece of copper-clad board just the right size, and I had just purchased new soldering-iron tips which needed to be baptized in tin and lead. “By hell! The M3 is just the thing!,” I exclaimed. In addition to helping to break my vapor lock, I could once again experience what Bill N2CQR and Pete N6QW call the “Joy of Oscillation” (hereafter JO2). I found a circuit for the M3 from George KC6WDK’s website, and I proceeded to round up the parts.
I had a number of variable capacitors in my Junque Box I could use. From the M3s I’d seen, and from the value called out on the schematic, I supposed I would need either an air variable or a polyvaricon. I had several of each type I could use, though in the back of my head was the dim thought, “Why do I need a variable with such a wide range? To get the nominal frequency of the crystal, only a single as-yet-unknown capacitance is needed, and if pushing or pulling the frequency a bit is desired, putting in a 365pF air-variable or even a 200pF polyvaricon would be gross overkill.” As I said, though, it was a dim thought, and not in the forefront of my stream of consciousness.
I wound the coil primary and secondary using the instructions on the webpage. For the coil form, I used one of the dozens of pill bottles I’d been saving for years (along with any other plausible coil form of paper, plastic, or ceramic that fell into my hands). For wire, I used 20 AWG copper wire from–wait for it–Michael’s Crafts. They’ve got lots of different gauges of enameled copper and aluminum wire in their beading department. The enamel is not quite as durable as genuine magnet wire, but in ordinary handling it works well, and it’s almost as tough to scrape or sand off as the real stuff. As I usually dope my coils with two coats of clear nail polish, I don’t worry about durability.
The primary winding of the finished-and-doped coil measured 32μH from top to bottom, and the tap was 7.7μH up from the bottom. At this point, that dim thought I had earlier came to the front of my mind. Looking at the schematic, I confirmed that the variable capacitor sits across the entire 32μH. “It’s a tank,” I said to myself, pleased to be fluently conversant with RF slang. It has to be resonant at the crystal frequency (or, within narrow limits, the crystal can be made to resonate with it). Whipping out my laminated reference sheet, I found the formula for a parallel LC circuit:
f = 1 / (2π√LC)
with f = 3.5795, L and C in Henries and Farads. I needed to rearrange the formula to solve for C and plug-in 0.000032H for L and then . . . instead I pulled out my cell phone, brought up my EverythingRF app, entered the L of 32μH, and the answer for C was 62pF. As it turned out, I had several old 60pF mica-ceramic compression caps on hand, so I wired one across the primary coil with a 33pF disk in parallel to give me 30pF up and down from the calculated resonant capacitance. So in reality, a variable cap isn’t needed at all if the purpose is simply to experience JO2. A combination of disk caps (NP0 type for S&Gs) in parallel and/or series would get the thing working, and there’d be no hand-wringing over the need for a hard-to-find variable (even though they’re not really hard to find at all).
Instead of the 2N3904 transistor usually called out for this circuit, I used a 2SC710. I have a few hundred of them, and they were developed for RF use while the 3904 was not (but works well anyway). This was actually my first use from the big bag I bought several years ago. The 2SC710 is a Japanese part number, and there have been several manufacturers, most of them not indicated on the part itself, and with some variety of pin-outs among the various versions. You don’t need to ask how I know, I will tell you.
Again pulling out my smartphone, I googled for a datasheet. Still not thinking at maximum gain, I clicked on the first promising link, and I found the pin-out for the 710’s TO-39 case is E-C-B, with the flat facing the viewer. Naturally, I wired it accordingly, but no JO2. Now a bit more alert, I confirmed all connections were correct and sound. I was about to lookup other versions of the schematic to make sure the fault wasn’t from that source, when I realized I hadn’t tested the transistor or the crystal. I don’t usually test passive components, but I think it’s a good idea to test active ones when practical. Pulling the 710 out of the circuit, my cheap-as-dirt (hereafter CAD) transistor tester confirmed the device was okay, with a β of 90, a Vbe of 754mV, and a pinout of B-C-E. “Okay, there’s nothing wrong with the transi . . . . wait, B-C-E?!” So I soldered it back in place, with the base lead on the left this time. The circuit oscillated, but still no joy. My CAD Sanjian frequency counter indicated a frequency at the third overtone of the colorburst crystal:
After tightening down the compression cap as far as I could, I got it to jump down to the second overtone:
It’s important to note the frequency was not by pulled continuously down to the second overtone. It actually pulled only a few-hundred Hz before it jumped down. Many crystals can operate (stably) only at their fundamental frequency. This one was stable at the second and third overtones. I wonder if it would be at the higher ones?
Okay, that was a very interesting diversion, but I still needed to get it to work at 3.579545 MHz. I wasn’t long finding the problem: I had left one end of the 33pF capacitor I wanted parallel to the trimmer unsoldered. A puff of rosin smoke later and proper JO2 was achieved! A little tweaking of the trimmer got it dead stable on the nominal frequency. All I could say at that point was SOSINTS! (“The Smell of Success is Never Too Sweet”–pronounced sō-sǐntz).
At the risk of splitting hairs (another of my hobbies), it’s worthwhile noting that overtone I isn’t exactly the same thing as harmonic. A harmonic is an exact integer multiple of the fundamental frequency. So for this particular crystal as trimmed to its nominal, that would be 7.159090 MHz for the second harmonic and 10.738635 MHz for the third. What the counter actually read, though, was 7.158627 (a difference of only 463Hz down) and 10.555258 MHz (a whopping 183.377 KHz greater!), respectively. The second overtone as counted was within the probable pulling range to the second-harmonic figure, but the thing is I couldn’t get it to trim to that before it jumped abruptly to the fundamental. That is, I could not get the oscillator to run at the second harmonic, only the second overtone. With regard to the third overtone, there’s no way any amount of trimming could account for a 183KHz increase in frequency. As soon as I tried to move it more than a few KHz downward, it jumped down to the second overtone. Apparently, the main cause of this behavior and the reason for a distinction between harmonic and overtone is that the motional capacitance (“Cm”)—the effective series capacitance of the crystal itself—is not the same when operated in overtone mode as in fundamental mode, and the Cm plays an unavoidable part in the resonance characteristics of the crystal. See http://hyperphysics.phy-astr.gsu.edu/hbase/Music/otone.html and https://www.electronics-notes.com/articles/electronic_components/quartz-crystal-xtal/overtone-operation.php.
You’ll see from the photos that since my purpose here was to get something done and working with a minimum of fuss, I didn’t spend a lot of time on non-essentials. You’ll also see I had to resort to some infelicitous wire routing. I ended up running the capacitor-side connection to the crystal across the board and under the capacitor itself. It’s only two inches (50mm) long, so it won’t matter at this frequency, but it looks a little goofy. Additional awkward connections were made unavoidable by the way I wound the coil, dressed its leads, and super-glued and coil-doped everything solid. I fed the lead from the top of the primary coil back down through the form, perpendicular to the windings, to emerge from a tiny hole at the foot of the form. I had started the winding by threading the wire through two closely-spaced holes, also at the foot, to keep the wire from pulling out as I wound. I left an inch sticking out for connections. Without thinking carefully enough, I made the lead from the top of the coil emerge to the right of the bottom lead when it would have made for much easier circuit connections had it emerged on the left. It proved to be less of a problem when I decided to abandon an air variable or polyvaricon in favor of a compression trimmer. For a while, though, I thought my version of the Mighty Mite was going to have to play a game of Twister.
In the end, it came out nicely enough. I put some memorial inscriptions on it because someday it will end up in the K7TFC museum (a battered U-Haul box in the attic). As pleased as I am with my M3, the real success here was that I managed to cure both by boredom and my vapor lock with a little accomplishment feedback that took only about an hour and a half to complete. Give it a try sometime if you find yourself in a funk.