A tale of two VFOs

I am always a little surprised at the constant learning experience afforded by this hobby. My assumptions are usually incorrect, but the discovery of what is actually going on is always entertaining and instructive. Consider the story of the two VFOs.

At the end of my last post, I was pretty convinced that my refurbished HG-10 VFO was not going to cut it in my Novice tube station. The station was keying oddly, with transmitter output wandering up and down, although the output from the VFO was steady. At this point, I was keying the VFO and the transmitter simultaneously, using the old two diode trick. I had more ambitious plans about keying, but this comes a bit later on in the story.
About the same time as the discovery of weird transmitter keying (by the way, the T-60 is totally well-behaved when using a crystal instead of the VFO), I happened across an eBay listing, only hours old, for a Knight V-44 VFO in good condition.

This, if I haven’t already mentioned, was the VFO that sparked all this VFO interest. I had seen one at auction that went for a really silly, high bid. That’s when I started looking for HG-10s. Anyway I jumped on the V-44 at its very reasonable price, and sat back waiting for its arrival. During that wait, I got my hands on some V-44 documentation, and studied up.

The v-44 has a self-contained high voltage power supply, and uses a really clever design of putting the power supply at the top of case, with the tuned circuits below, so the heat from the supply won’t add to thermal drift. This gives the cabinet a kind of cool “portrait” rather than “landscape” aspect ratio. It’s rather charming.

Knight V-44 VFO

Knight V-44 VFO

Since it contains a built-in high voltage supply, I knew I would have to re-cap it. I also found it interesting that the manual warns the V-44 not be used on or near grounded surfaces such as a metal table – adding a three-wire plug and a fuse was a must. So the laundry list of repairs was:

  • new filter cap in the HV supply
  • three-wire cord, with chassis ground and an inline fuse.

This was not too tall an order, and while the VFO was still in transit from its former owner I ordered and received the necessary cap from Mouser. I was all set.

The timing of the arrival of the V-44 was about a week before I would be traveling to the upstate QTH (site of the novice station) for about ten days. During this time I would be there on my own, leaving plenty of time for ham radio adventures. So it seemed really important to get the repairs done before this trip. Really, really important.

Of course, there was always the normal myriad of responsibilities: family, work, social… so it seemed that bench time was a scarce commodity during this lead up to the trip. I allowed myself to fall victim to the worst (and possibly most dangerous, but happily not in this case) foible that affects amateurs of any stripe: I rushed the job. Big mistake.

I started with the cord replacement, which was straightforward enough. It involved replacing the old coax output cable, which looked like RG-8x or similar, with skinny
RG-174, which made room for the new 3-wire power cord without needing a new hole in the chassis. Adding the in line fuse was not a problem either, there was enough room in the under chassis to squeeze it all in. The problem arose when I looked for a convenient stud to attach the third grounding wire. There wasn’t any.

Now it’s close to 11:00 PM on a week night, getting near bedtime, but if I could button up this cord replacement, I could take the VFO upstairs, plug it in and just give a listen. I was so close… Okay, no problem, I’ll just drill a 1/8th inch hole in the chassis, put a bolt and nut in, and Bob’s your uncle – chassis grounding. Without giving it much thought, (obviously) I chucked in a bit and started drilling.

These old boat anchors are built tough, no flimsy soft aluminium chassis here – nope this was good heavy gauge steel and it was taking some effort to get through. I was putting some pressure on the bit, and just as some part of my hindbrain was screaming “No, no lighten up your going to pop through…” suddenly the bit popped through. Right into the main tuning tank coil.

In that instant, the thought that I should have put a little wood block in there behind the hole (there was ample room) flashed through my head, way too late to be useful. I think I said “oh, darn” loudly a few times. Maybe that wasn’t exactly what I said.

Carefully modified main tank coil

Carefully modified main tank coil

I surveyed the damage. It was pretty bad, but not cataclysmic. If I had shattered the coil form, it would have been game over, but all I had accomplished was to destroy the windings. This, I thought, could possibly be fixed. I went to bed.

The next day, I pondered my options. Obviously, all I had to do was wind a new coil on the ceramic form, with the same inductance as the one I had destroyed. No problem, I’ve wound lots of coils. I just had to look up the inductance of the main tank coil in the manual. Every coil in the circuit was called out by value in the parts list except the main tank coil. It was just referred to as Knight Kit part number 152014, “coil, oscillator, tank”.

Okay, this wasn’t going to be that easy, but I wasn’t about to admit that I had murdered my beautiful VFO with a power drill. I’ll just count the turns, and compute the inductance.
There then ensued over the next day or so a great deal of careful, methodical turn counting. Of course, with roughly 30 gauge wire, and a chunk of the wire actually missing (bits were cut by the drill and fell off) it was actually impossible to get an accurate count. I even tried taking a macro photo of the damaged coil, and using a photo editor putting little tick marks on the image every ten turns so I wouldn’t lose count. In the end I had an estimate plus or minus 5 turns, not quite close enough for engineering work, as they say.

Okay, plan B occurred to me. The dial calibration instructions called for adjusting two trimmer caps separately, one for 80 meters, and one for 40 meters and up. This meant that the tank coil, which was obviously a fixed inductance, had to resonate the main tuning capacitor plus the each trimmer at two different frequencies, which are known. Since the trimmer caps are identical, I just had to find an inductance that could resonate both frequencies within the range of the trimmers, with the main cap set at the calibration point.
(Footnote here: I mistakenly thought both ranges were calibrated to the same point on the main tuning cap. This is not true. This error may explain some of the new weird behavior I saw later on. But at this point I was feeling awfully clever.)

I then embarked on a course of feverish calculation: inductance by wire diameter by gauge and number of turns, resonant frequency at various values of capacitance – finally I zeroed in on the target value, and I started to wind my new coil.

I use the penny and quarter technique. I laid out 10 pennies in a row, and 8 quarters (I needed 81 turns total). After winding each turn, I moved a penny to a new location on the table, gradually building a new row. When ten pennies were moved, I moved a quarter to represent 10 turns. Lather, rinse, repeat. It’s a foolproof method of guaranteeing that you don’t mis-count, and it only costs $2.10.

penny and quarter technique, afer turn 23

penny and quarter technique, afer turn 23

But alas, I did not nail it on the first try. Two thirds of the way through, I was running out of room on the coil, and realized I had used the wrong, too large, wire gauge. Okay, start again with the right gauge, only to run out of room again. Re-compute number of turns for smaller gauge wire and start again. Lather, rinse, repeat.

Finally my coil was done, but there was another nagging issue. The original coil had been treated with an application of “coil dope”, a thin varnish. Reading on the web seemed to indicate that using coil dope was a good idea, especially in oscillator circuits, as it minimizes drift by stabilizing the turns of wire against movement or vibration. But where to get some? You can buy it mail order, but it is expensive and I didn’t want to wait. So I looked for recipes of home-brew coil dope and discovered that it is dead easy to make. Smelly, inconvenient, and extraordinarily flammable, but easy.

You make coil dope by dissolving styrofoam packing peanuts in toluene. Toluene is a highly volatile, flammable, toxic and generally nasty solvent use as paint thinner, which can be had at any hardware store in inconveniently enormous quantities. I needed about an ounce, and was only able to find it in a gallon container. At this point, I’m all set with regard to future toluene needs.

The packing peanuts are easy, right? I mean one always has scads of the things from incoming packages in boxes in the basement. Right? Well, we had recently recycled every last one of the little buggers at the local retail shipping store, there was not one to be had on the premises. I had to wait until I got to work the next day to get some at the office. Sensibly, our IT guy has never thrown out a styrofoam peanut since we moved into that office space. You never know when you will need some.

Making the coil dope was done outside, despite the still chilly late winter temperatures, and even at that I nearly passed out from the toluene fumes. But finally, after dissolving an amazing number of peanuts in two ounces of toluene, I was rewarded with a maple-syrup-like consistency, evil-smelling fluid. Voila, coil dope!

Artisanal Coil Dope

Artisanal Coil Dope

Using a cotton swab, I covered the coil in a light coating which dried in seconds, and looked pretty official when it was all done. And I still have 2 ounces – 1.5 milliliters of coil dope left for future endeavors.

Rebuilt main tuning coil

Rebuilt main tuning coil

The finished coil popped right back in place and I quickly soldered back the two connections. I went through the calibration procedure, and everything seemed okay, but the story wasn’t over yet.

I took the VFO on my trip to the upstate QTH on schedule the following weekend, and installed it in the Novice station. It looked good, and I fired it up on 80 meters. I was using the technique where you turn the VFO on (not keyed) and key the transmitter. This gives rise to a phenomenon called “backwave”; if your receiver is not muted you hear the VFO tone behind the sound of your keying. It sounds odd, faintly disturbing, like listening to the receiver with it and your head in a garbage can – but I was determined.

V-44 in Novice Station

V-44 in Novice Station

A little tune up, and a couple of quick CQs. Not hearing any response I headed off to the computer to check Reverse Beacon Network, to see where I was heard. Okay, lots of stations were picking me up on 80 meters, definitely a good start. Next, on to 40.

I tuned everything up on 40, and gave it a go. Lots of good reports on 40. And, lots of good reports on 80. What? Yes, it looks like I was a candidate for the Worked All Bands Simultaneously award. Definitely not a good thing. Subsequent tests with a dummy load showed that I was getting strong sub-harmonic signals that were getting through the output tank in the transmitter. Not a good thing at all.

I knew that the oscillator in the V-44 generates strong harmonics from two fundamental frequencies: a 160 meter band fundamental generates the first harmonic which provides 80 meters, and an 80 meter band fundamental provides harmonics used for 40 meters and up. Something was definitely out of kilter, perhaps output filtering, which looks kind sketchy from the schematic, or maybe my home-brew tank coil resonating someplace it shouldn’t. More research is needed, but for now sadly the V-44 is on the bench.

In light of this revelation, I went back to playing with the HG-10, and in doing so made a remarkable discovery. If you don’t use fancy two-diode keying arrangements, the HG-10 works fine with the Knight T-60. Take away the diodes from the keying circuit, and the funky instability goes away.

The old 2-diode trick for simultaneous keying of the VFO and transmitter

The old 2-diode trick for simultaneous keying of the VFO and transmitter

I know from prior experience that the T-60 is notoriously hard to key with anything other than a plain key that shorts to ground. It has a fairly large dropping resistor in the keying circuit, which has the advantage of presenting a low DC voltage to the key when it is open, removing any shock hazard. The downside of this arrangement is that if there is any significant resistance in the key, the rig won’t key. (I had this happen with a soviet military key, which had 20 ohms resistance internally in some kind of built in filter.) And apparently, the T-60 doesn’t like diodes in its path to ground. With a regular key, the HG-10 drove the T-60 perfectly well, I was mistaken in the assumption that the low drive was at fault.

The HG-10 is now part of the Novice station, and the V-44 is back in the shipping box, awaiting further diagnosis to figure out the weird sub-harmonic behavior. I may re-visit the winding of the tank coil. I may find a “for parts” V-44 and replace my homebrew coil with a real one. But for now the V-44 is sidelined and the humble HG-10 is doing the job.

THe HG-10 takes its place in the Novice Station

THe HG-10 takes its place in the Novice Station

So my Novice station is now VFO enabled, with backwave, using the HG-10, but I really wanted a setup where the VFO was keyed (no backwave), no chirp (a common side effect of a keying the VFO), and a sidetone with receiver muting would be nice. This gave me another idea, involving 21st century microcontroller implementation of 1950’s circuit, but that’s a story for next time. Until then,

de N2HTT

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2 Responses to A tale of two VFOs

  1. Pingback: Together Again (for the first time) | 73, de N2HTT

  2. Pingback: Novices. | 73, de N2HTT

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