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Old 15th Mar 2019, 11:27 pm   #1
Vintage_Man2012
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Default Long Wave Coil Rewind

Hello everyone,

I have finally managed to obtain a machine capable of producing "wave wound" coils with the intention of repairing the broken LW coils in several Roberts R200 and R300 radios in the shack.

I have plenty of litz wire to do the job, but I need to know how many turns there should be on these little coils - to be clear I'm referring to coils L3 and L5 on the 1602 Trader Sheet.

Does anyone know how many turns should be wound or can someone explain how to calculate the turns required for these radios?

Many thanks
Paul
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Old 15th Mar 2019, 11:54 pm   #2
winston_1
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Default Re: Long Wave Coil Rewind

You could unwind the broken coils and count the number of turns.
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Old 16th Mar 2019, 8:45 am   #3
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Default Re: Long Wave Coil Rewind

That may indeed be the solution however, my other machine with a turn counter is too beefy for the wire and it would break so I would have to do it manually - I was hoping someone may already have made a note of it somewhere!

If all else fails then I'll do it by hand!

Paul
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Old 16th Mar 2019, 10:45 am   #4
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Default Re: Long Wave Coil Rewind

I have an AVO Douglas winder which should be able to unwind and count the turns if used gently. I have plenty to do but if you have a spare coil, I could have a go.
You could weigh the coil to get a wire length and an estimate of the number of turns.
The inductance is the critical parameter and this will depend on the wave as well as the number of turns. Is the wave quicker or slower than the turns?
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Old 16th Mar 2019, 10:46 am   #5
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Default Re: Long Wave Coil Rewind

Knowing the number of turns of a wave-wound coil will be very unlikely to help you to replicate that coil. It isn't the number of turns (or resistance) of the coil that's the key factor - it's the inductance.

For a conventional 'solenoid' coil, with turns laid side by side, life is much simpler. A coil of a given diameter with a given number of turns of wire of the same gauge, will produce a coil identical or very close to the original. In a single layer coil, if the coil needs to be tapped so that the tap is 10% of the overall inductance, the tap would be at 10% of the overall number of turns. That does not apply in multilayer coils due to 'mutual inductance', so the tapping point can only be found by experimentation.

With wave wound coils, knowing the number of turns won't in itself allow you to wind a coil which has the same inductance of the original. If you look closely at a wave wound coil, in addition to the internal and external diameter and the width of the coil, you'll also see the angle at which the wire traverses from left to right, which has a bearing on how many waves there are for each turn of the coil. You'll also see how tighlty (or loosely) the turns are laid side by side, which has a bearing on how closely the turns are coupled.

No knowing how the coil winding machine used to wind the original was set up, will make it next to impossible to recreate the coil by simply knowing how many turns it had. With Litz wire you'd also need to know how many strands and of what gauge, the original wire consisted.

Despite all of this, provided you know the inductance of the original coil, you can wind a test coil on a former of the same diameter, making the test coil the same outer diameter and width, then having counted how many tuns you've put on the coil, check its inductance. If it's too high, you can progressivley remove turns checking as you go, until the inductance is correct. Likewise, if the inductance is too low, you can add turns until it's correct.

Once you've done that and have noted the settings of your wave winder and the number of turns, you'll be able to wind as many coils as you wish which will be close to the original.

Some years ago I made a simple hand wound 'wave-winder' (known as the 'Morris Gingery' winder), using a pocket calculator as the turns counter, which advances by '1' when a microswitch was operated by a cam with each turn. See pic 1 below.

I wound a number of test coils of varying dimensions, numbers of turns, and closeness of coupling of the turns. Pic 2 below shows a test coil of 500 turns, the inductance of which - when tested - was 3mH. You'll see that the turns are quite loosely coupled. If I wound another coil with the same settings and number of turns, that too would be 3mH. However, if I altered the wave setting so that the turns were closer together, the inductance would be quite different (higher). Incidentally, afterwards I dipped the coil in molten wax to fix the turns, which didn't alter the inductance.

The bottom line is that you need to wind a few test coils till you achieve the same inductance which the coil is place on the same former as the original. (EG, if the coil is an aerial on a ferrite rod for example).

Life is a lot simpler for close-wound 'solenoid' coils or even 'spider web' coils, for which online calculators are available. EG:

http://crystalradio.net/professorcoy...coylecyl.shtml

http://crystalradio.net/professorcoy...piderweb.shtml

I hope that might help a bit.

Good luck in your endeavours Paul.
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Old 16th Mar 2019, 11:24 am   #6
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Default Re: Long Wave Coil Rewind

Look at the circuit, look at the capacitance values and the range of the tuning capacitor.
From these calculate what the inductance must be.

Wave wind a coil which is definitely too big.

Measure its inductance

remove some turns, measure again.

Keep removing turns and measuring until you get the target inductance.

With wave coils, removing turns is easy, adding more and jointing wire is a lot more trouble.

David
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Old 16th Mar 2019, 11:51 am   #7
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Default Re: Long Wave Coil Rewind

I rewound a coil for LW LO on a Sky Casket by hand, sort of random wave winding. I since made a gadget out of Lego Technic with a finger operated LCD counter (€2.50) to count the turns, operated by a lego arm.
Anyway, I calculated what the inductance maybe ought to be. Then when near what looked like approximate size I bared the wire and measured. The second attempt at making it worked.
Tricky bit on Osc coils is getting the feedback winding right.
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Old 16th Mar 2019, 2:14 pm   #8
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Default Re: Long Wave Coil Rewind

Many thanks for all the advice regarding the winding of the replacement coils.
There is much more to think about here than regular linear winding.
I’m away for the weekend but when I’m home next week I will have to look at the unbroken coils in another identical set and try and do some calculations.
Apologies for being a bit of a novice here but can someone explain how to measure the inductance of one of the “good” coils I have?

Thanks again for all your advice everyone
Paul
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Old 16th Mar 2019, 2:30 pm   #9
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Default Re: Long Wave Coil Rewind

Inductance bridge/meter or resonate with a known value of capacitance and enter F & C into this calculator to solve for L, or if you know the total value of the capacitance across when it's in the receiver when tuned to the LF end of the band use those to figures to input into the calculator:

http://www.1728.org/resfreq.htm

This video shows deriving L using a fast edge pulse and a 'scope:

https://www.youtube.com/watch?v=74fz9iwZ_sM

Lawrence.

Last edited by ms660; 16th Mar 2019 at 2:50 pm. Reason: addition clarification
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Old 16th Mar 2019, 3:49 pm   #10
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Default Re: Long Wave Coil Rewind

A good traditional method is to connect a known capacitor and use a grid dip meter. If you've never heard of one of those, you have a real treat coming. Rarely mentioned nowadays, but a thing of great usefulness.

David
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Old 16th Mar 2019, 3:58 pm   #11
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Default Re: Long Wave Coil Rewind

You can get unboxed devices for under £10 which will measure inductance, capacitance and transistor parameters or you can go posh and get a Peak meter.

I have not had a good experience when trying to measure inductance of iron cored transformers, possibly because they were faulty.
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Old 16th Mar 2019, 4:00 pm   #12
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Default Re: Long Wave Coil Rewind

Ebay has LCD screen PCB mounted testers with a ZIF DIL socket. Tests diodes, transistors, fets, thyristors etc identifying pins and showing voltage so you know if Ge, Si, Schottky. Measures inductors, capacitors, resistors, pots. Measures parallel & series resistance of capacitors (ie leaky or poor ESR). No good for leakage of higher voltage caps, I use a modified electronic flash charger from a recyclable camera with 2M Ohm from + and neon with 100nF from - to test cap leakage. A 1Hz flash is about 1uA, flicker means very bad, > 5uA.
A fraction of the price of a S/H bridge or "Grid Dip"/"FET Dip" and usually more accurate.
I have a nice FET Dip meter, however it's not much use below 900KHz, too much gain. Works to VHF. It's most use 2MHz to 150MHz.
The Dip meter is fine for SW coils or to inject a signal into a loop aerial, most are poor on LW for a "Dip".
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Old 16th Mar 2019, 5:06 pm   #13
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Default Re: Long Wave Coil Rewind

I admit to having had only a bit of experience in wave-wound coils. But I've partly convinced myself that wave winding, rather than layer-winding, has only a small effect on inductance - provided the overall dimensions are the same.

With linear winding, each turn will be closely coupled to the turn next to it. So inductance is significant.

With wave-winding, it won't be closely coupled. But it will be partly coupled to several other turns, so by the time you add these in series, the voltage at the two terminals, caused by the same dI/dt that you push through the coil, will be the same for both winding methods.

Wave winding, of course, does result in a much lower self-capacitance for an inductor, which is one reason that it's used. Another is that the coil is much more self-supporting.

Faced with the problem, I'd do as Dave RW suggests - wind one with more turns than is likely needed, then strip off until it tunes correctly.

Let us know how you do - this is going to be interesting!
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Old 16th Mar 2019, 6:15 pm   #14
Vintage_Man2012
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Default Re: Long Wave Coil Rewind

I must say this has provoked far greater discussion than I envisaged!

Looks like I’m going to need some new instrumentation to play with (any excuse for a new toy)..

At least I now have something to occupy my time with during a very wet weekend in Wales!

Will keep you posted on developments!

Thanks again
Paul
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Old 17th Mar 2019, 10:27 pm   #15
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Default Re: Long Wave Coil Rewind

I thought it may help if I uploaded a photo of the coil I am trying to replicate so here is a photo of one of the broken coils I've removed.

Paul
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Old 17th Mar 2019, 10:54 pm   #16
kalee20
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Default Re: Long Wave Coil Rewind

It's quite loosely wound, is my first thought.

If you want to measure inductance, a pretty good little item is this. No connection - I'm just a satisfied user, and it agrees pretty well with the more advanced instruments at work.

However, if it is a tuning coil, then initial advice holds - wind more than necessary, and then strip off turns till it tunes correctly. If the coil is connected to a multi-gang capacitor, you may want initially to connect it to its very own, independently-adjustable capacitor, to tune it - and after stripping turns, readjusting the capacitor till it's approximately equal to the capacitor it will eventually be connected to.
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Old 17th Mar 2019, 11:23 pm   #17
Vintage_Man2012
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Default Re: Long Wave Coil Rewind

Yes, compared to some of the other coils in my other sets it isn't as closely wound.

I do like the instruments made by Peak, I have one of their semiconductor analysers and I've been very pleased with it so far so I may order one of their LCR meters.

The winding machine I have is driven by a single cam, it's not like a Douglas Wave Winder - but hopefully it will be enough to produce something that will at least allow me to get the set working again.

As for the litz wire, its 10 strand 0.04 mm which is visually similar to that used originally.
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Old 18th Mar 2019, 1:04 pm   #18
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Default Re: Long Wave Coil Rewind

A cheaper and more versatile option to consider to check the inductance would be one of the many LCR/Transistor 'multi-testers' that abound, such as the one at the link below. It has the advantage of using three test clip leads, with a separate plug-in ZIF socket, whereas many similar device use only a ZIF socket, which I find an annoyance.

Although this thread concerns only inductance, which the tester will measure from .01mH to 20H, in addition, it will also measure capacitance from 30pF to 100mF, including ESR above 2uF, resistance up to 50 MegOhm, and bipolar NPN/PNP transistors, Mosfets, Jfets, thyristors and diodes:

https://www.ebay.co.uk/i/191873654353

Back on the topic of winding coils, in 2015, when I was experimenting with winding replica Repanco 'DRR2' coils on my little home-brew wave winder, I made a 'Coil Coverage Test Unit' unit based on a 'suggested circuit' in Radio Constructor many years ago. Using a built-in calibrated capacitor, you can set the capacitor to the lower and upper capacitances of the capacitor used in the radio, and with the homebrew coil under test, can sweep the signal generator across the band to note the upper and lower frequencies at the dip on the meter. You can also connect a frequency counter and/or scope if you have one.

Hardly worth building for the very occasional use to which it would be put and I've used it little, but I enjoy building simple bits of test gear in the 'MFJ' ethos ('Made From Junk'). I wrote it up on the forum, and later in the BVWS Bulletin:

https://www.vintage-radio.net/forum/...d.php?t=114238

Hope that might be of interest.

Good luck with your coil winding endeavours Paul.
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Old 18th Mar 2019, 3:36 pm   #19
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Default Re: Long Wave Coil Rewind

MFJ, I did wonder what it stood for, rather tickled me.
 
Old 18th Mar 2019, 6:04 pm   #20
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Default Re: Long Wave Coil Rewind

Quote:
Originally Posted by David G4EBT View Post
For a conventional 'solenoid' coil, with turns laid side by side, life is much simpler. A coil of a given diameter with a given number of turns of wire of the same gauge, will produce a coil identical or very close to the original. In a single layer coil, if the coil needs to be tapped so that the tap is 10% of the overall inductance, the tap would be at 10% of the overall number of turns.
I'm just picking up on this, it's not quite right. Even in a single-layer coil, there is significant coupling between turns.

With zero coupling between turns, you would indeed get 10% of inductance at 10% of turns. But that would mean the turns would have to be massively far apart, several times the diameter of the former they're wound on. With 100% coupling, 10% inductance would be at 31.6% of turns. In practice, 10% inductance will be somewhere between 10% and 31%.

Wheeler's formula for a single-layer coil is useful: L (μH) = a²N² / (9a + 10l) where N is turns, a is radius, l is length, in inches. If the coil length is short, then the 9a (constant as you add turns) term in the denominator dominates, so L is approximately proportional to N². If the coil is long, then length (which is proportional to turns) dominates, it cancels the N² in the numerator, so L is approximately proportional to N. These are extremes: real-world coils are midway between.
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