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Old April 17th 16, 04:46 PM posted to rec.radio.amateur.moderated,rec.radio.amateur.homebrew
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Default [N2HTT] A tale of two VFOs


73, de N2HTT

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A tale of two VFOs

Posted: 16 Apr 2016 05:38 PM PDT
https://n2htt.net/2016/04/16/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 havent 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.
Thats 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 supplyÂ*at the top of case, with the
tuned circuits below, so the heat from the supplyÂ*wont add to thermal
drift. This gives the cabinet a kind of cool portrait rather than landscape
aspect ratio. Its rather charming.
Knight V-44 VFO

Since it contains a built-in high voltage supply,Â*I knew I would have to
re-cap it. I also found it interesting that the manual warns the V-44Â*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 wasnt any.

Now its 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, Ill just
drill a 1/8th inch hole in the chassis, put a bolt and nut in, and Bobs
your uncleÂ*Â*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 wasnt exactly what I said.
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, Ive 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 wasnt going to be that easy, but I wasnt about to admit that I
had murdered my beautiful VFO with a powerÂ*drill. Ill 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 wouldnt 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 BÂ*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 he 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. Its a foolproof method of guaranteeing that you dont
mis-count, and it only costs $2.10.
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 withÂ*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 didnt 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, Im 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

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 IÂ*still have
2 ounces 1.5 milliliters of coil dope left for future endeavors.
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 wasnt 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 receiverÂ*with it and your head in a garbage can but I was
determined.
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 providesÂ*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
shouldnt. 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 dont 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

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, removingÂ*any shock
hazard. The downside of this arrangement is that if there is any
significant resistance in the key, the rig wont key. (I had this happen
with a soviet military key, whichÂ*had 20 ohms resistance internally in some
kind of built in filter.) And apparently, the T-60 doesnt 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

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 1950s circuit, but thats a story for next
time. Until then,

73

de N2HTT


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