[HBR] HBR-14 coil info

[Walt Hutchens]

An alternative to Breck's excellent info on using PVC pipe sink tail pieces
to make HBR coil forms is to use 1" Schedule 20 PVC water pipe.

The 1" is the nominal inside diameter:  It's actually larger than that and
the outside is very close to 1-1/4".    Because the pipe fittings go on the
outside, that's the controlled dimension and will be the same for all
strengths, etc.   So this material lets you wind coils using the W6TC specs.

I recommend the schedule 20 pipe because it's plenty sturdy enough (thicker
wall than the sink tail pipe pieces) and will fit snugly over the plastic
base of a junked metal octal tube.   You take off the metal part, save the
Bakelite plug, clean it up and it's a force fit in this PVC pipe.

A 10' length of pipe will cost under $10 and will make more coil forms than
you can ever use: Use a radial arm saw or a miter box to slice off 3"
pieces.  

Also slice 1/4" sections off each end of some 1" couplings: These can be
slid or shoved onto the top end of each coil form for reinforcement and to
act as a grip for pulling the coil.

The inside diameter of the couplings is tapered so the first slice will be a
light push fit and the third will be a tough fit.  But if you chamfer the
respective corners and have a block and hammer at hand to give it a whack,
you can always get the rings to go on.

I glue the top rings on with PVC cement before starting on the coil.  For
the HR-10 to HBR project I have experimented with leaving out the APC
trimmers on the antenna and mixer coils.   This works fine: You don't need
anything like the precise tracking required for the oscillator coils.   You
just have to select the right fixed capacitor and adjust the tap location
and turns spacing slightly to get the coil to track.

For the forms that need an APC I use a Dremel tool with a rotary file bit to
cut away inside the top until the APC I'm going to use is a snug fit.  Then
I drill the necessary holes in the form, wash, dry, and wind the coils,
bringing the leads out the bottom.

I solder on the APC leads and any required fixed shunt cap, then push it in
place.  All the leads coming out the bottom of the form are now stripped,
passed through the respective pins in the plug, and clipped with about 1/8"
sticking out the end of the pin.   The coil can now be carefully pushed into
the appropriate socket and tested.

You'll have very little trouble with poor contact between the loose leads
and the inside of the pins ... just once in a while.  If it's suddenly dead,
poke or twist the coil slightly.  However don't set the coil aside for a
couple of months and expect it to still work well in loose lead
configuration -- there'll be just enough corrosion to make things
squirrelly.   

You can do everything except oscillator stability testing this way and you
can get a rough guess about that.

Note that everything is just pushed together: Even the most dramatic
adjustments can be made without starting over, by carefully prying the coil
out -- you can't pull it because the plug isn't glued in the form -- and
disassembling. 

Of course if you are following W6TC you shouldn't need any redo, but when
using a different tuning cap, dial, oscillator circuit and tube (and pretty
much everything else, as I am) there is necessarily some cut-and-try!

When everything is okay you take the coil off the base and push out the APC.
Glue it in with PVC cement.  Put JB Weld on the base and inside the bottom
of the form, put the leads through the right holes, push it together, clean
the outside carefully and test again before the JB Weld sets.   If all is
satisfactory I leave the coil in the set for 24 hours or so, then pull it
out, solder the pins, and trim the excess wire length from each.

Clear nail polish laid on in four strips is excellent for locking turns in
place.   The oscillator will however drift after you do this for up to
several days so don't try serious stability testing right away.

In the worst case -- a coil requiring a major redo AFTER being glued -- you
can use a fine hacksaw to cut all the way around about 3/8" from each end
(use great care next to that APC!), take off the top and bottom pieces with
the plug and APC respectively, make an axial cut in each of the PVC rings,
and pry it off.   The APC can be cleaned up and reused; the wire can be
salvaged from the main part for use on a higher band.    You've wasted a few
cents worth of PVC pipe and a few hours of your time ...

The result looks as good as a commercial form or coil and it's a lot more
durable and easier to work with than the polystyrene forms.   Among other
things if you touch it with a soldering iron you get a mark, not a hole.
And of course you can heat the pins in the Bakelite tube base as much as you
want without them flopping over.

I'm still doing 40M coils ... few other things got in the way!

Walt
KJ4KV