The Heathkit VF-1 VFO (Part 1)
New life for an old classic
History
The Heathkit VF-1 VFO (variable frequency oscillator) was an essential accessory to novice class U.S. amateur radio licensees as they earned their upgrade to the coveted general class license.
Starting on July 1, 1951, the U.S. Federal Communications Commission (FCC) created a license for beginning hams, the novice license. While this topic is far beyond the scope of this article, one of the primary limitations in novice operating privileges was the requirement that new hams use fixed-frequency crystals in narrowly defined portions of the ham radio bands. Another term for this limitation was to be "rock-bound". Novice hams could not spin the dial and make conversations, they had to communicate on frequencies for which they owned crystals. While nearly every one-time novice remembers the era fondly, they also dreamt of the day when they could spin that a and use any frequency they were entitled to use. For this, one needed a general class license and a VFO.
Heathkit produced several radio kits with the Novice in mind. Transmitters for novices had no tuning dial, Rather, there was a prominent two-pin socket on the front panel of the transmitter where the newly minted ham would insert his (or her) crystal. Heathkit also produced a kit for an external VFO that, when plugged into that same socket, gave the operator the ability to put those crystals in the drawer and never look back.
The first such VFO kit was the VF-1. Released in 1952, the magical box with its iconic backlit green tuning dial was an instant hit. Heathkit continued to provide this kit until 1961 when the HG-10 VFO took its place.
Reputation
The VF-1 had a reputation for instability, drift, and chirp. While the design had some shortcomings, many of the problems were the direct result of shoddy assembly, poor solder joints, and insufficient power from the various Heathkit transmitters to which it was to be mated.
One thing I have not seen adequately emphasized is the cumulative effect of every flaw. The VF-1 has two operating ranges - one for 160M through 40M and another for 40M through 10M. In either case, the lowest frequency will always be the most stable one. The reason is simple: the other frequencies relied on the harmonics from the base frequency. For instance, range one provided oscillation between 1750kc and 2000 kc, or the 160M band. The 80M band relied on the doubling of the 160M band. This means the dial bandwidth is cut in half. It is halved yet again for the 40M band. Similarly, range two oscillated in the 40M band and was then halved for 20M. The next harmonic worked on 15M, and the next for 10M. In each case, the dial bandwidth is shrunken. Furthermore, any flaws that produced barely noticeable instability on the base frequency would be thusly doubled, tripled, or quadrupled with each successive band. A small chirp, drift, or jiggle on 40M would be four times worse on 10M.
My Efforts
This post chronicles a modern ham's efforts to collate and apply all the wisdom accumulated through the years. I hope to add my own $.02 wherever possible, which assumes the conclusions I reach are unique. I find that possibility unlikely.
I shall break this down into three main areas: mechanical stability, electrical stability, and power stability. While some have replaced the vacuum tubes and converted them to solid-state, my efforts are to retain at minimum the oscillator tube and its associated high voltages. On that latter point, experimenters are hereby warned that the voltages in all tube equipment can be lethal. Precautions must be taken to prevent life-threatening electrical shock.
Mechanical Stability
Heathkit did an admirable job of developing a kit that inexperienced builders could assemble with little or no formal electronics training. While efforts were made to introduce structural rigidity, the VF-1 could not hold a candle to the commercially produced Johnson 122 or Globe 755 of that era. That being said, thousands of hams built them and thoroughly enjoyed the experience of both assembling and using the VF-1.
With minimal effort, the VF-1's mechanical stability can be improved upon in significant ways. Here are a few I have learned.
Chassis
I know this seems trivial, but every mechanical connection in the VFO must be secured with a lock washer and checked for adequate tightness.
Coil Slugs
Because the finishing coil slug adjustments are made with the case on, many hams neglect to adequately tighten the jam nuts on the two ceramic coils. It is not enough to use pliers here. One must use a 5/16" wrench to tighten them such that the slugs can only be adjusted with some exertion once the chassis is returned to its case.
Furthermore, even where the jam nuts are tight, the heavy slug on the end of the rod can still jiggle inside the ceramic coil form. Efforts to dampen the gap between the slug and the inside of the coil form should be attempted, whether with felt, velcro, closed-cell foam, or any other suitable material. The author is currently experimenting with this mitigation.
VFO cage / Green Plastic Strip L Brackets
The green plastic strip is mounted via two screws into the top of the VFO cage. I recommend that the builder inserts the screws from the bottom up, using a lock washer on the screw itself. Instead of passing it through the L bracket, I recommend using another lock washer followed by the original two nuts. The VFO cage should be aligned and the screws and nuts tightened firmly, without the L brackets. Afterward, the L brackets should be mounted to the remaining threads of the screws using another pair of 6-32 nuts. No lock washers are needed here.
Coil Mounting
I feel Heathkit erred in the instructions on this point. The illustrated instructions show the builder adding a lock washer and the solder lug between the coil bracket and the chassis. This technique results in a less secure and rigid connection, thereby introducing more instability. I recommend that you mount the brackets directly to the chassis and put the lock washer and solder lugs on top of the bracket.
Furthermore, I recommend cutting off a small piece of perfboard and inserting it between the base of the ceramic coil and the chassis before tightening the mounting screws, This step will lock the forms in place even further. Remember everything you do will pay dividends on the higher bands as the effects are multiplied.
TO BE CONTINUED
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