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Old 20th Sep 2017, 1:38 am   #21
Argus25
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Default Re: Field coil speaker dismantling

Be aware that when you are filling a fixed volume bobbin with wire, it turns out that the resistance of the winding you end up with is inversely proportional to the 4th power of the radius (or diameter) of the wire you select. This is a very steep relationship, so a small change in the diameter of the wire from the original will result in a significantly big change in the resistance. So try to find the closest matching wire diameter you can get.

Without writing out the equations, the relationship above simply occurs because the number of turns and therefore the length of wire that will fit on the bobbin is inversely proportional to the square of the radius (or diameter) of the wire and also the total resistance is inversely proportional to the cross sectional area of the wire (which is proportional to the square of the radius or diameter) and proportional to the length as well. So the total resistance ends up being proportional to the inverse of the 4th power of the wire diameter or radius. There are only a few common fourth power relationships, and this is one of them.
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Old 20th Sep 2017, 10:02 am   #22
David G4EBT
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Default Re: Field coil speaker dismantling

Ooops - I'd misinterpreted the holes on the clamp where the four bolts had been removed, to be rivets. Glad you got the pole piece and coil removed OK!

In terms of what gauge of wire to use, looking at 'wires&co website, it shows that 0.15mm (38 SWG) is 0.975 Ohms/Metre, 0.16mm is 0.85 Ohms, and 0.17 (37SWG) is 0.73 Ohms. I think the easiest way to choose the appropriate gauge would be to measure the resistance of 10 Metres of the old wire to see whether it's closer to 9.75 Ohms, 8.5 Ohms or 7.3 Ohms.

The thicker the wire, the more robust it will be, but the coil I wound was 38SWG (0.15mm) and I wound on more than 2km of wire without a break. That coil was 2,000 Ohms - yours is stated on the Trader Sheet (763) as 1,000 Ohms. Looking at your former, whatever gauge wire you opt for, there is ample room is the coil turns out to be a larger diameter than the original.

I made a basic coil winder powered by a sewing machine motor with foot controller, naively assuming that if I wound at a fairly slow speed, I would more or less get the turns to lay neatly side by side. That fanciful notion soon went down the gurgler - the wire has a mind of it's own, so I just did the neatest job that I could, and in fact the original coil wasn't all that neat.

The original cheeks of the coil former on my Ekco AC77 were only made of cardboard, so I made some new cheeks from 2mm Paxolin sheet and fitted them to the tube ends with epoxy resin. I also supported the cheeks with plywood discs and end clamps during the winding process so that the wire didn’t force the cheeks off the coil tube as it's inclined to do. (The tube of the coil former protrudes at one end to accept the hum bucking coil).

In this instance it wasn’t an RF coil or a transformer where the inductance or turns ratio is important - it was just a 2,000 Ohm field coil (in your case, 1,000 Ohms) to mains energise the speaker and to act as a smoothing choke, so knowing the number of turns wasn’t that important, albeit I did fit a turns counter to the winder.

I simply filled the coil former with solderable enamel wire, then just touched the soldering iron to the wire to tin it for 5mm or so and tested the resistance, intending to either add or subtract turns to get it about right. (Had additional turns been needed, I'd have insulated the soldered portion before winding on the added turns). As luck would have it, it was 1,997 Ohms! As the original coil was wrapped in black plastic, having fitted the lead-out wires, I put a couple of turns of black ‘gaffa tape’ around the coil so that it looked as near as possible to the original coil.

Wires & Co sell 50g, 500g and 1kg reels. 50G isn't enough - 500g will be more than enough and with VAT & P&P will be under £20.00:

The gauges concerned will be found at this link under '0.140mm to 0.180mm Solderable Enamelled Copper Wire':

https://www.wires.co.uk/acatalog/SX_0140_0180.html

My coil wasn't interleaved and I've not seen any examples where field coils have been. Seemed to me that even if a turn came into contact with one several layers below (doubtful), the potential difference between those turns would be no more than few volts and highly unlikely to lead to an eventual short.

Don't know what your coil winding facilities are - mine were non-existent so as I mentioned earlier, I made a basic winder using a sewing machine motor. I've attached a few pics which might be of interest. Initially I fitted a 'tally counter' ('clicker') as a turns counter, but later I fitted a shaft counter from China which only cost about a tenner and is excellent. Completely irrelevant to winding a field coil, but might be useful should I ever want to wind/rewind a transformer.

Engineers may wince at the uses of mild steel endplates for the winding shaft rather than bearings, but it worked fine, and was hardly worthwhile going to too much trouble given the limited us to which the winder is being put. I did make a crude spring loaded 'tensioning device' to tension the wire and prevent the coil from 'freewheeling' and getting tangled up when the motor stopped.

Hope that of interest.

Good luck in your endeavours.
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Old 20th Sep 2017, 11:20 am   #23
Voxophone
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Default Re: Field coil speaker dismantling

Thanks David, that’s very useful. Nice job on that coil winder!

I think I’ll try the resistance measurement before ordering the wire. The bobbin does have plenty of space so I don’t think overfilling would be an issue.
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Old 20th Sep 2017, 1:58 pm   #24
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Default Re: Field coil speaker dismantling

Handy hint for winding thin wire, don't pull off the reel unwinding it, pull off the top leaving the reel still. This assumes you have a proper reel with smooth ends.
 
Old 20th Sep 2017, 2:27 pm   #25
Boater Sam
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Default Re: Field coil speaker dismantling

I bought the cheapest manual winder from China, came from a UK stockist, £17 if I remember, and it is smashing, does everything I need, 2 speed.
I've done 6000 turns a few times without expiring.
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Old 20th Sep 2017, 8:51 pm   #26
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Default Re: Field coil speaker dismantling

Hi Sam, can you post a pic of it please.

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Old 20th Sep 2017, 9:35 pm   #27
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Default Re: Field coil speaker dismantling

I can only send a picture of it on the auction site, I'll send you the link in a PM to avoid contravening the rules.
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Old 20th Sep 2017, 10:16 pm   #28
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Default Re: Field coil speaker dismantling

There are a number of ways to make coil winders. I made a micro-lathe multi-task for the job adding a turns counter by gluing a magnet to the chuck. This is much easier than a mechanical counter. The black box on the photo contains both an RPM meter and a turns counter, both ebay items (The rpm meter is handy when using the chuck to drive mechanical rpm meters for calibration). With this I have been able to wind coils and transformers with many 1000's of turns of wire such as 37 swg. The photo attached shows the winding of a bifilar coil.

( with regards to my post above about the 4th power relation, it is worth an example: if say you wound a coil of the same height (volume) as the one that is there, where the wire you use was only 15% bigger in diameter than the original wire, then the resistance would end up being about 1/(1.15 to the power of 4) smaller, or 57% of what it should be)
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Old 21st Sep 2017, 9:29 am   #29
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Default Re: Field coil speaker dismantling

I measured the resistance of 1, 5 and 10 metres of the field coil wire and got about 0.75R/metre each time. Based on the figures at wires.com, it looks like 0.17mm is what I need.

I unwound the old copper from the bobbin last night. It took about 40 minutes! Total weight of wire is about 270g so a 500g bobbin will be ok.

It looks like there was definitely a break in the coil. Interestingly, I also came across a soldered joint covered with tape half way through. It must have broken once during winding in the factory.

Now I need to repair the bobbin. It seems quite robust with paxolin end plates, but one of these worked loose when prising it off the pole piece. Hopefully I can reuse it.
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Old 21st Sep 2017, 9:59 am   #30
David G4EBT
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Default Re: Field coil speaker dismantling

Quote:
Originally Posted by Voxophone View Post
Now I need to repair the bobbin. It seems quite robust with paxolin end plates, but one of these worked loose when prising it off the pole piece. Hopefully I can reuse it.
I'd suggest that rather than use Araldite, you use Plastic Padding 'Super Steel' two-part epoxy, which is much stronger and sets rock solid.

Despite its name, it isn't conductive.

It's really important that the end cheeks are firmly fixed and supported or there is a tendency during winding, for the wire to force the cheeks off.

Good luck with the re-wind.
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Old 21st Sep 2017, 10:49 am   #31
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Default Re: Field coil speaker dismantling

There is also the trusty baking soda and superglue trick. Lots on Youtube and also

http://www.vintage-radio.net/forum/s...d.php?t=123112
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