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Old 17th Sep 2017, 6:39 pm   #1
Voxophone
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Default Field coil speaker dismantling

Does anyone know how to dismantle field coil loudspeakers in order to remove the coil? Specifically I have one from a Ferranti 145 which is measuring high resistance.

I've removed four large bolts which I hoped would release the clamp containing the coil, but although it now feels loose I can't get it to separate from the body. I don't want to force anything I shouldn't.

Any ideas? I've attached a couple of pictures below.

Thanks

Liam
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Old 17th Sep 2017, 6:44 pm   #2
TonyDuell
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Default Re: Field coil speaker dismantling

Is there a paxoiln 'spider' in the middle of the cone (on the front side of the speaker), with a central screw? If so, that screw probably goes into the end of the central pole piece and has to come out before you can remove the bracket/pole piece to free the coil.
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Old 17th Sep 2017, 7:00 pm   #3
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Default Re: Field coil speaker dismantling

I've attached more pictures showing the front. At the centre is a hard plastic disc which rotates as the field coil clamp is wiggled. It looks like it corresponds with the end of the pole piece as you're suggesting. I can't see any way to remove it though.

Edit - the disc might be metal actually rather than plastic. Could this be the actual end of the pole piece? It feels similar to the material used to make permanent magnets (obviously not magnetised though).
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Old 17th Sep 2017, 7:19 pm   #4
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Default Re: Field coil speaker dismantling

Hi9 Liam, any further screws visible on the speaker frame at the reverse side of the cone?

Ed
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Old 17th Sep 2017, 7:28 pm   #5
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Default Re: Field coil speaker dismantling

That looks to be a hardened fabric dust cap, but it looks to be stationary on the centre pole rather than going over the whole area and guarding the gap.

There may be a screw under it.

Earlier speakers had as tony says a spider, a diecut piece of thin paxolin or tufnol with recurved arms to allow it to flex with voicecoil movement, but while keeping the coil centred. Later speakers put the spider outside the voice coil centring the voice coil against the outer pole piece and dispensing with the centre screw. There's more space outside and so a spider can allow more cone movement.

This one seems to be some sort of half-way house.

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Old 17th Sep 2017, 7:32 pm   #6
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Default Re: Field coil speaker dismantling

It looks to me as though you'll have to drill out the four large rivets that secure the pole piece and bracket to enable you to release the pole piece and gain access to remove the field coil. The speakers weren't put together with disassembly in mind. On re-assembly, you'd need to use nuts and bolts to refit the pole piece.

If the coil is loose on the pole piece, that's lucky because on the Ekco AC77 speaker that I restored the pole piece had rusted onto the coil former and the coil was reluctant to work loose. The card end cheeks of the coil former weren't up to a rewind, so I made new Paxolin end cheeks and used 2-part epoxy to secure them to the old card tube former.

It might help if you look at pics 3,4,5 in post 4, and pics 3,4,5 at post 6 at the thread below, which show the dismantling and reassembly of the speaker. You need to be careful not to damage the speech coil when you remove the pole piece, which protrudes into the coil. The speaker must be layed on a flat surface so that you can lift the pole piece off vertically, keeping it central to the speech coil as you do so.

Don't know if yours has a 'hum-bucking' coil. Mine did and can be seen in the pics:

http://www.vintage-radio.net/forum/s...d.php?t=134720

Quite a challenge rewinding the 2000 Ohm field coil - 20,000 or so turns - 2.3km of fine wire.

Hope that helps a bit.

Good luck in your endeavours Liam.
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Old 17th Sep 2017, 7:38 pm   #7
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Default Re: Field coil speaker dismantling

Looks to me like the suspension is on the outside of the voicecoil. Is that the pole piece visible in the centre of the speaker?
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Old 18th Sep 2017, 9:29 am   #8
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Default Re: Field coil speaker dismantling

Thanks everyone for your replies.

Quote:
Originally Posted by PJL View Post
Looks to me like the suspension is on the outside of the voicecoil. Is that the pole piece visible in the centre of the speaker?
I think the part visible in the centre is the end of the pole piece as you say. It feels like metal and it rotates as the slackened field coil assembly is rotated. I can’t seem to withdraw it however.

David:

I think the ‘rivets’ you’re referring to may be the holes for the fixing bolts, which I’ve already removed. Having removed them I can lift and rotate the clamp and field coil assembly somewhat, but I can’t withdraw the pole piece from the rest of the speaker and hence can’t separate the two parts. Strangely, although the field coil is not rivetted to the speaker body, the OPT is.

Very encouraging to see in your link that repairing these things is possible!

Ed:

I haven’t found any screws yet on the underside. The only fixings I can see are two soldered tags which secure the flexible copper springs holding the cone. These seem to do the job of the cloth ‘spider’ I’ve seen on other speakers (i.e. keeping the cone centred and allowing it to flex in and out). I could possibly de-solder these tags and release the spring assembly, but not sure this would help with removing the pole piece.

I’ll have another crack at it tonight.

Cheers

Liam

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Old 18th Sep 2017, 9:33 am   #9
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Default Re: Field coil speaker dismantling

Just been reading the thread that details your superb restoration, David. What a total labour of love - real concourse level.
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Old 18th Sep 2017, 12:27 pm   #10
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Default Re: Field coil speaker dismantling

Most likely it has been magnetised and will need to be prised off (very carefully).
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Old 18th Sep 2017, 1:06 pm   #11
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Default Re: Field coil speaker dismantling

You mean the pole piece itself has been magnetised and is preventing me withdrawing it? I did try to test for magnetisation with a croc clip but didn't feel any pull. I'll check this out again later today. A compass needle might do the trick.
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Old 18th Sep 2017, 6:06 pm   #12
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Default Re: Field coil speaker dismantling

It is unlikely the pole piece will be magnetised unless it has been energised recently.
It looks as if there are locating collars in the bolt holes, these may have been press fitted, but you should be able to prise the field coil assembly from the flat plate.

When you carry out the reassembly the final adjustment can be done using 3 thin pieces of stiff plastic as shims. I use the hard clear plastic which the impossible to get into bubble packs are made of.

After rewinding and refitting the coil assembly, slacken the 2 bolts locating the spider behind the frame, and carefully insert the 3 shims between the speech coil former and the centre pole evenly spaced around the pole, if very tight they may need sanding with fine paper to reduce their thickness.

Gently tighten the 2 bolts on the spider and check for even tension on each of the 3 shims.
Remove the shims and check the diaphragm moves in and out with out any rubbing.

Mike
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Old 19th Sep 2017, 9:58 am   #13
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Default Re: Field coil speaker dismantling

Thanks all.

I managed to get the filed coil off eventually. All that was needed was a bit more force and persistence. The black disc visible from the front of the speaker is indeed the tip of the pole piece, which doesn’t seem to be magnetised. I’ll post pictures soon showing how it fits together in case anyone might need this information.

The field coil itself will definitely need a rewind. I had hoped I might be lucky and find a break near one of the terminals, but even on bare copper I’m still reading high resistance. There’s also a sizeable green spot of corrosion which seems to go through several layers.

The method of bringing the inner end of the coil out to the terminal seems a bit strange (to me), as it seems to pass through the middle of the winding. Both terminals are located on the outer surface of the winding. I expected one of them to be inside the coil against the bobbin.

I’ll try to replicate the original as much as possible when doing the rewind, but does the direction of the winding really matter for this application? The only possible issue I can think of is that the hum-bucking coil could end up adding, rather than subtracting, hum – but I’d think this could be as simple as flipping the coil over if it happens.

Cheers

Liam
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Old 19th Sep 2017, 1:08 pm   #14
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Default Re: Field coil speaker dismantling

Another quick quetsion regarding this. What is the most reliable way to determine the size of the original/replacement wire?

Measuring over the enamel with digital calipers suggests an overall diameter of about 0.18mm. Is this the size of wire I need to order, or do I need to strip the enamel before taking the measurement?

I did hope there would be a limited number of standard sizes and I could pick the nearest one, but .16, .17 and .18 are all available, which isn't much help.
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Old 19th Sep 2017, 1:47 pm   #15
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Default Re: Field coil speaker dismantling

On that gauge of wire the insulation will be so thin as to make little difference (typically 0.01mm).

You need to be real careful winding stuff that thin - it is really easy to snap, particularly since there seem to be a very large number of turns.

Last time I did this sort of thing it was only 1400 turns of 0.1mm wire, and it snapped three times. One second of lack of concentration is all it takes.
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Old 19th Sep 2017, 3:20 pm   #16
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Default Re: Field coil speaker dismantling

Quote:
Originally Posted by Craig Sawyers View Post
On that gauge of wire the insulation will be so thin as to make little difference (typically 0.01mm).
But there's only 0.01mm difference between the sizes avaialble (0.16, 0.17 and 0.18mm etc.), so even this small ammount could affect the choice of wire. It sounds from what you're saying though that 0.16mm is most likely for a measurement of 0.18mm (subtracting the enamel thickness on both sides). I might try measuring the Ohms per metre also as there is quite a bit of difference in resistance between these sizes.

I suppose for a field coil having the wire too thick would be better than having it too thin? If it's too thin the current carrying capacity may not be enough, but if it's too thick more turns could be added until it reaches the design resistance?
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Old 19th Sep 2017, 4:47 pm   #17
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Default Re: Field coil speaker dismantling

If the speaker is British, the wire will almost certainly be SWG. 37SWG is 0.1727mm diameter and 38SWG is 0.1524. Given your measurement over the insulation of about 0.18mm, I'd say the wire was 37SWG.

Wires.co.uk describe 0.17mm as 37SWG.
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Old 19th Sep 2017, 4:48 pm   #18
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Default Re: Field coil speaker dismantling

But you may have trouble with a thicker wire getting it all on the bobbin?
Sounds like 0.16mm is about right, try measuring it in swa wire gauge sizes, the original is not metric wire, and convert that size back to metric. A micrometer will be closer than a caliper gauge if you have one.
It is very thin and you will get breakages. I just joint it and insulate and carry on winding otherwise you may never get to the end.
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Old 19th Sep 2017, 9:24 pm   #19
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Default Re: Field coil speaker dismantling

Hi Liam, not sure how good your digital callipers will be at that small diameter, A micrometer with vernier scale may be better.

On my tables 0.1778 is shown as to overall diameter for grade 2 (double coated) wire. I suspect yours will be single coated so possibly closer to 38SWG or 0.1524 as a metric size. This is probably easy to obtain.
Is the wire interleaved on the bobbin with layers of paper and are there cheeks on the bobbin. This will all impact on the difficulty of a rewind.

Ed
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Old 19th Sep 2017, 10:45 pm   #20
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Default Re: Field coil speaker dismantling

The correct way is to measure the resistance of a known length of the wire. There are plenty of wire resistance tables on the web. You need to get the total resistance and turns about right or the hum-bucking won't work and the HT may be wrong. Modern thin copper wire is a lot tougher than you might imagine.
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