Winding / rewinding RF coils

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Tom Whitworth:
I have a heathkit SB-303 receiver which works pretty good with the exception of 40 meters. I have chased it down to a tuneable slug core coil on the RF amp board. Its pretty tattered, patched and probably missing some windings. Its your basic tuned circuit with a 100pf mica cap, and 22K ohm resister in parallel with the coil to ground.

The questions I have are, if I want to rewind that section of the coils...

A: Where do I find that ultra fine insulated "magnet" wire?

B: How do I go about figuring out how many turns are required?

Roy Herman:
you go to


Tom Whitworth:
Know what? Thats a really good suggestion, and I have done buisness with them in the past. Love their color dials for the 101.

Unfortunatly they dont have RF coils for the 303 ... go figure.

But thanks  ;)

Dale Hunt:
Quote from: KE4JOY

The questions I have are, if I want to rewind that section of the coils...

A: Where do I find that ultra fine insulated "magnet" wire?

B: How do I go about figuring out how many turns are required?

A:  how fine is it?  Magnet wire is available in lots of sizes, from #8 or bigger to #40 or
smaller.  In most cases for RF coils the exact diameter isn't important, though it makes
a slight variation in the inductance and self-resonant frequency.  You do need to make
sure that the wire can carry the required current, but that is usually only an issue in
transmitters.  You can salvage magnet wire from old electromagnets, chokes, speakers,
TV yoke coils, motors, etc.  Or you can tell us what size you need and someone can send
you a few feet from their junkbox, since it doesn't take a lot.

See if you have a local shop that rewinds motors and ask to buy a few feet of the size
you need.

B:  the number of turns depends on the coil.  The first step is to count the turns on the
existing coil, which will give you a start, though your coil may have a tuning slug with
different permeability.  (You used to be able to order formers and slugs of specific types
from Amidon.)

If you know that the circuit is resonant on 40m and it has 100 pF across it that gives
you the required inductance of the coil.  Then you may need to experiment to find
the number of turns that gives you that inductance on your specific core.  You can get
a start using this inductance calculator for air-core coils:

In the end, just wire up the coil with 100pF across it and use a dip meter to get it
to tune the right range (or a little high, since there will also be a few pF of stray
capacitance in the circuit.)

If there are multiple windings that aren't tuned to resonance, that makes it more
difficult, and you'll have to make an educated guess at the relative impedances
of the different parts of the circuit.

If you can measure continuity from the input to the output connector and simply want to rewind the coil for laughs, the best way would be to simply unwind it, counting the turns as you go.  However, for me to rewind a coil of many turns, it would really have to look like crap!  An original and working coil is quite often better than a rewound coil with operator induced errors.

There are many factors to consider while doing this, from miking the wire to determine the size to keeping the family away from you while you're doing this job.

If the wire size is very small, like #40 you might be talking about a couple hundred turns....and while a few turns mistake won't mean that much it still pays to stay as close as possible to the original.

To better help you, you might provide a bit more information.... like where this coil is used; is it a coil with many or few turns, etc.

FWIW:  When winding a coil of many turns it might be worth the trouble to make a holder for a variable speed hand drill and a mandrel to hold the coil form.  Then running the drill very slowly you can unwind the coil while counting the turns.  In this case it's good to have someone else there to plug the drill in and unplug it as necessary. A foot switch would be great but an additional PITA to put together. (the speed set switch will be adjusted so that the drill starts and runs at the predetermined slow speed)

Then you use the same process to rewind the coil.  Saves a lot of finger cramping and time.


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