Poor Man's VFO
I am reminded of a classic Road Runner cartoon, where Wylie Coyote wonders why the Road Runner didn't fall when he sawed the cliff out from under him, and refers to the laws of physics. To which the Road Runner holds up a sign, which states that he never read the laws of physics, and then darts off with his trademark "beep-beep".
Such sets up the premise for this project. If you were to relate to me today, the fine details of what I was about to do back in 1975, I'd have said; "that'll never work". Well, not knowing that it would never work back then allowed me to build it and "make it work", as it were. Sometimes the truth really is stranger than fiction....
It was the fall of 1975 and, like I was prone to do, I was experimenting with my radio. Specifically, I was attempting to expand frequency coverage in a valiant attempt to gain some "private" escape channels. At the time, I was running a Lafayette Comstat 25 tube-type base station and I already had channels "22A" and "22B" built-in, but I wanted to access the RC channels and anything else I could get to. To accomplish this goal, I was attempting to shift the frequency of the synthesizer crystals. During the course of my experimentation, I had discovered that by varying the capacitance across a crystal, it caused it to shift frequency slightly. Well, following a naively simple train of logic, I thought that if a little capacitance moved frequency a little, just think what a lot of capacitance will do. So I scrounged though my junk parts box looking for a variable tuning capacitor. I found a suitable part and summarily scavenged it from a portable transistor AM radio. I then soldered a pair of wire leads to it and placed it across one of the synthesizer crystals in the Comstat. Imagine my disappointment when the oscillator abruptly stopped and the receiver went dead. It was obvious that I had used too much capacitance, and it looked as if I could not move the crystal very far before the oscillator stopped. At this point, my project was starting to look like a failure. But hey, setbacks like this only make people more determined. Before I gave up completely, I decided to give it yet another try in a slightly different direction. I had read somewhere (Or maybe Ralph had discussed it at one of our Explorer post meetings) that frequency can be controlled by parallel inductor/capacitor tank circuits. So with that basic theory in mind, I mounted my previously scavenged tuning capacitor into a plastic box, took a nearby BIC ball point pen and proceeded to wrap several turns of stiff, insulated "Bell Tell" wire around it, and then place this homemade inductor in parallel with the tuning cap. I then connected the resultant circuit across the crystal by means of two short parallel leads. Imagine my surprise when the oscillator continued to operate, which was easily verified by the receiver's normal noise, and the transmitter still operating. But it was soon obvious that I wasn't in Kansas anymore Toto. As far as I could tell, I had shifted frequency quite a bit more than before. So what frequency was I now on? Being a 15 year old, with little pocket change, didn't allow for such luxuries as a frequency counter (Which were big, bulky, and expensive Nixie tube affairs back then). So I needed to do things a little more crudely. After some scanning around the familiar channels by turning the channel selector and then playing with the tuning capacitor, I was able to determine that by just placing the "VFO" circuit across the crystal in the minimum tuning position, it dropped frequency by about 5 Khz from the channel I was on. Tuning the capacitor to the maximum position, it would drop an additional 6 KHz for a total drop of 11 Khz. That was neat but, of course, I wanted more. Adding more turns of wire to the pen, and another retry, showed that I could now drop a maximum of about 20 Khz before the oscillator finally had enough and stopped. That was more than enough range to access the "RC" channels, and up to a 2 channel drop below Channel 1. Not bad for a couple of crude parts, which cost me nothing. Stability was another matter however. Since it was attached to the crystal by two parallel wire leads, any shift in the position of those leads would shift frequency. I could improve this to some extent by keeping the leads as short as possible, and not moving them, but I could not eliminate the drift completely. Fortunately, I was not dealing with SSB, so precise frequency stability was not as important. Nonetheless, this "VFO" allowed me to go places that many others couldn't at the time. It came in handy when we needed a more private channel. But not too many in our group, outside of Steve, Jimmy, Whitey, and Al had the capability to "slide" between channels. Still it was nice to have a "private" place to escape to when required, and it drove my Channel 15 neighbor crazy trying to find me when I was bleeding him over, until he acquired his own VFO equipped radio in the next few months.
I tried duplicating and adapting the VFO circuit to work on solid state radios, but had no success. In all cases the oscillators would stop. I guess I got lucky by having a very forgiving tube rig. A brief audio clip where I briefly used the poor man's VFO to slide off frequency can be heard here. I used this circuit for the better part of the next year, until I was able to achieve similar results with the clarifier circuit in my newly acquired Midland 13-885.