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Author Topic: Winding / rewinding RF coils  (Read 8286 times)
KE4JOY
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Posts: 1358




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« on: June 15, 2013, 03:39:29 PM »

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?
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WB2EOD
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Posts: 217




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« Reply #1 on: June 15, 2013, 06:32:59 PM »

you go to www.oldheathkitparts.com.

73
WB2EOD
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KE4JOY
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« Reply #2 on: June 15, 2013, 07:21:11 PM »

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  Wink
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WB6BYU
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Posts: 13143




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« Reply #3 on: June 15, 2013, 07:23:11 PM »

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:

http://hamwaves.com/antennas/inductance.html

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.
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K8AXW
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« Reply #4 on: June 15, 2013, 08:34:35 PM »

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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WB6BYU
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« Reply #5 on: June 15, 2013, 09:52:33 PM »

I find that mounting the former (without the core) on a shaft chucked into the vice
makes it easier to wind, especially if I can arrange some sort of a handle to turn it.
For example, put a nail in the vice, slip the former over it, and attach a brace to the
former to turn it.  That makes it easy to count the turns as you wind it.

One recommendation when winding transformers was to turn your phone off so you
didn't get interrupted half way through and lose count.

An easy way to count the turns is to run the top if a toothpick or other pointed object
along the coil and count the "clicks" as it drops down between each turn.  Takes some
practice.
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KA4POL
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Posts: 1965




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« Reply #6 on: June 15, 2013, 09:52:55 PM »

A good source for magnet wire is: http://www.planetengineers.com/default.asp?cat=Wire%2C+Magnet
There is a Yahoo group of Heathkit enthusiasts. They provide all kinds of information.
I'd calculate the windings from the 80 m coil L108.
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G3RZP
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Posts: 4466




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« Reply #7 on: June 16, 2013, 02:39:07 AM »

If it's tuning to 40m with 100pF, then it's about 5 microhenries. Probably a bit less to allow for stray capacity around the bandswitch.
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KE4JOY
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Posts: 1358




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« Reply #8 on: June 16, 2013, 11:39:27 AM »

Lots of great suggestions, thank you very much.

I found a good source for the wire, an old am/fm portable radio, the am slug antenna was wrapped with the fine wire.

The 'calculator' that was linked looks quite effective but I am afraid its a little over my head.

Anyhow thanks very much for the input!  Smiley
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K0IZ
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Posts: 737




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« Reply #9 on: June 16, 2013, 06:41:38 PM »

Another source of magnet wire is Hobby Lobby and similar places, where they carry hanks of several sizes for use in projects.
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G3RZP
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Posts: 4466




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« Reply #10 on: June 17, 2013, 10:02:23 AM »

Check the wire from the slug antenna - make sure it isn't either Litz or bunched conductors with several fine wires served  with cotton, silk or a fine plastic serving.  You want a single conductor.
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KD0REQ
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Posts: 896




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« Reply #11 on: June 18, 2013, 10:18:15 AM »

crack open the handbook for the handy formula of turns on a core, or try this online tool

http://easycalculation.com/physics/electromagnetism/inductance.php
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KE4JOY
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Posts: 1358




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« Reply #12 on: July 16, 2013, 12:25:12 PM »

Just an add to this thread.

I discovered another cheap source of this ultra fine wire.

A cheap set of headphones/earbuds, I went to repair a set and was quite surprised to find three bunched sets of fine enameled conductors no other insulation in sight. The common appears to be bare Cu and the other two 'legs' were enameled.
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KE3WD
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Posts: 5694




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« Reply #13 on: July 16, 2013, 02:53:48 PM »

A dial caliper is a great tool to have.  You can pick up a digital one from Harbor Freight that will read out in both inch and metric at the  push of a button. 

Handy to find out the actual diameter of scrounged wire, plug into the calculator to get the right coil the first shot. 


73
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