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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 14th Aug 2018, 10:10 pm   #1
Colourstar
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Default Any cure for a rubbing speaker cone?

Hello all, I'm working on a late 60s Ferguson consolette stereo record player which has been out of use for several years and quite possibly stored in less than ideal conditions too. One of the 15 ohm 8"x5" EMI branded speakers was found to be seized. Light pressure on the cone easily got it moving again but it is very obviously rubbing (and sounding 'orrible). There is no obvious damage to the cone or suspension. Are there any possible remedies or is a replacement driver the only answer?

Luckily the other 8x5 loudspeaker in the player is fine.

Many thanks,
Steve
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Old 14th Aug 2018, 10:25 pm   #2
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Default Re: Any cure for a rubbing speaker cone?

The 'seized' was probably rust between the coil and the concentric magnet pole-pieces - you do suggest that it may have been stored in sub-optimal conditions, and any dampness is likely to cause rust.

Some people have had success by way of 'exercising' the coil for a few hours through feeding it with enough 50Hz AC to cause it to oscillate in and out over 1/2 inch or so in order to work-away the rust. Only problem here is that the rust particles are themselves magnetic and so are going to stay entrained in the areas where the magnetic field's the strongest! There's not really much you can do to get the magnetically-coagulated rust-dust out of the gap.
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Old 14th Aug 2018, 11:50 pm   #3
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Default Re: Any cure for a rubbing speaker cone?

One possible trick for cleaning debris out of the gap is to try adhesive tape. This is quite often suggested as part of a 're-coning' exercise. I imagine it's very much more tricky with the cone still in place. I'm afraid another thing to worry about is that if the outside of the voicecoil is rubbing then there is a risk that the enamel on the wire will be abraded away and partial shorting of the coil will occur. The best solution might be to remove the cone-and-coil assembly and see if you can then restore the shape of the coil. But if the coil is firmly glued to the basket then removing it might be very tricky to achieve.

Cheers,

GJ
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Old 15th Aug 2018, 6:45 am   #4
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Default Re: Any cure for a rubbing speaker cone?

Sometimes the rubbing is caused by a white compound which forms on the zinc treatment to the speaker magnet poles. If this is the case then it can be sucked out.
I have fixed this on permanent magnet speakers by first carefully removing the centre dust cap and placing the end of a round hose from a good vacuum cleaner over the round hole in the centre of the speaker, and then exercising the diaphragm by pushing the hose in and out whilst stuck to the centre hole over the speech coil.
Stop and check after a while and flex the diaphragm slightly sideways whilst moving it in and out in different directions to check where the worst, if any, of the rubbing is. continue to rub away any roughness causing the scratching, then try the vacuum cleaner again.
During this process air is sucked in through the inner bellows and down around the speech coil gap and then out through the centre gap. Because the gap is so small the air is travailing very fast and I believe it can also remove small iron oxide particles.
It doesn't normally matter too much if some of the enamel is rubbed away from the outer edge of the speech coil, so long as the speaker is not earthed.
The dust cap can be glued back on with a bead of PVA glue around the edge.

This process works especially well on energised speakers, but do it before energising them so that there is very little residual magnetism.

Mike
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Old 15th Aug 2018, 12:32 pm   #5
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Default Re: Any cure for a rubbing speaker cone?

Its usually not too difficult to separate the coil sleeve from the cone, having unsoldered the pigtails first.
Then you can clean out the magnet and pole gap thoroughly, scraping and blowing the rust out, stick paper to finish off.
Centre the coil with 3 paper slips and reglue to the cone. UHU works well.
Done a few, tricky but possible with care.
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Old 15th Aug 2018, 8:56 pm   #6
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Default Re: Any cure for a rubbing speaker cone?

Thank you all for the replies. Some interesting ideas there. I'll have a go at removing the dust cap first, as Mike suggests, although it seems to be very securely bonded so I'll proceed with caution....

Steve
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Old 15th Aug 2018, 9:18 pm   #7
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Default Re: Any cure for a rubbing speaker cone?

Try a little heat and unpick it with a scalpel.
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Old 19th Aug 2018, 9:47 pm   #8
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Default Re: Any cure for a rubbing speaker cone?

Happy to say that the speaker is no longer rubbing. I managed to partially remove the dust cap and gave it the business end of an appropriately vintage Hoover Constellation. There was no obvious corrosion or particles to be seen, even with a good light and a magnifier, but I suppose with the tiny clearance between coil and magnet, it doesn't take much.

Anyway, thanks again for the suggestions. All is sounding good and I can now reassemble the record player, ready for a 'Success Story'.

Steve
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Old 20th Aug 2018, 10:13 am   #9
Edward Huggins
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Default Re: Any cure for a rubbing speaker cone?

That's great news. A good job that old and powerful Vac did not suck the cone off!
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Old 20th Aug 2018, 5:16 pm   #10
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Default Re: Any cure for a rubbing speaker cone?

Yes you have to ensure it is placed over the voice coil area where there is a lot of strength, and hold the hose very firmly in-line with the coil former.
If you put it on the diaphragm direct it would probably suck a hole in it.
Well done Steve.

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