Bass speaker failure mechanism

Look for a frozen cone, ie the cone will not move when pushed in...most often caused by magnet shifting due to hard vertical drop.

jak

amp

stuck

pulse

perfectly free movement, all seems perfectly well, except the voice coil is open circuit

Check the continuity of the pigtails twixt connectors and cone

Ron(UK)

the

or

is

yes full continuity. I'm loathe to replace the speaker if there is some intermittant problem in the amp dumping full DC on the speaker, although no reported loud clicks or bangs. Is there an interim fudge like putting a 100V, 100W light bulb in series with a new speaker and running in earnest for a few hours. Its a high power 250 W or so 12 inch Mackie M1263W I think I'll ask the owner and then heat up the cone periphery and dust skirt periphery to remove the cone and look inside.

the

or

is

yes full continuity. I'm loathe to replace the speaker if there is some intermittant problem in the amp dumping full DC on the speaker, although no reported loud clicks or bangs. Is there an interim temporary fudge like putting a 100V, 100W light bulb in series with a new speaker and running in earnest for a few hours. Its a high power 250 W or so 12 inch Mackie M1263W I think I'll ask the owner and then heat up the cone periphery and dust skirt periphery to remove the cone and look inside.

Slice the dust dome off first with a razor blade, occasionally the joints where the pigtails are soldered to the voice coil fail,and you can make a temporary repair. Chances are the v/c has just failed out of badness, if there had been dc on the output it would almost certainly be either locked up solid or be rubbing.

Send it to Wembley Loudspeakers for a recone.

Ron(UK)

On Thu, 04 Jan 2007 10:55:59 +0000, N Cook Has Frothed:

You would hear that. And a healthy speaker would handle it. You have the option of sending the speaker for a recone.

theres no bodge option, its o/c. Time for a coil repair, which normally means a rewind.

NT

Didn't he concur that the fault was between the terminals and the voice coil? That's mechanical failure caused either by age (fatigue) or over-excursion...not DC. It might be fixable, but it will likely never be reliable. New speaker (or recone) is definitely indicated.

If DC 'is' suspected, it's easy to test for.

jak

The owner now says that it had been on odd occassions dropping out but immediately returning. Now I've removed the cone I can locate where the break is. About 10 turns of slightly darkened varnish covering the break somewhere underneath in the lower coil run. I assume a few turns of slightly reduced diameter so, chain only as strong as weakest link, eventually mechanically broke due to heating, rather than vaporised , so could make and break until permanently broke. No other rubbing noticeable.

I had a go at locating the break properly by scraping back small bits of varnish and the break is just at the point where the flat copper that goes axially out to the braid joins the round copper wire of the coils. This suggests a design flaw, would these thin flat copper tails be the round wire pressed to flat ? so causing a stress point at the change of form , i could see no trace of solder or anything at the juncture. The slight browning would be quite normal for those 10 or so turns and under coil may be perfectly ok, this voice coil remade is 5.7 ohm dc for 8 ohms speaker , seems reasonable. Shame the cone and skirt are now slashed. Seems no point in recommending a genuine Mackie replacement speaker.

Just unwind one turn and you've got a 7.2 ohm speaker, moving coil impedances are a bit all over the shop so it wouldnt be a problem. But if youve trashed it...

NT

goes

round

under

ohms

Another reason for not recommending a Mackie replacement. The centre of the magnet is vented to the outside via a wire mesh grill allowing any iron swarf and crud less than 1mm or so in size to pump in and out of the voice coil/ magnet space.

Not so. The magnetic circuit isn't directly in the same location. Besides, there shouldn't be anything getting in via the grille.

Graham

and

there

Do Mackie use a foam formulation guaranteed not to disintegrate to dust over time ?

Nobody does, they all disintegrate eventually.

High power speakers instrument or PA don't generally use foam or rubber for the surround, they are either paper or paper reinforced with a material such as linen.

I don't think Mackie make their own speakers, they did own RCF for a while, but I expect if you cross reference the speaker you have there, it`s most likely an Eminence.

Ron(UK)

On Fri, 05 Jan 2007 08:05:54 +0000, N Cook Has Frothed:

I have a pair of Peavey Black Widow 12's in a combo amp that have the mesh vent through the magnet structure. These are approx 25 years old and are still in use today. The amp is a 160 watt RMS tube amp. (6x6L6)

and

there

If the coarse grill was replaced with 2 larger diameter hemispheres one inside the other something like the size of a flour seive with proper filter material between the two, that would be ok

It`s really not a problem, Most pro drivers are vented. Any ferrous stuff which got in there would stick to the metalwork long before it got to the magnetic gap -not that there should be any iron filings floating around inside the cabinet. Some speakers don't even have the little grill, some have a disk of foam in the vent hole, some just have a hole. Care has to be taken to make sure that there`s no chuffing or other unwanted noises from the vent. There`s a lot of air moving in and out of there with a big low frequency driver.

Ron(UK)