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Oldmics
04-20-2004, 08:37 PM
Hi Gang
Another forum member and friend of mine was in my shop today and he came up with an interesting question that perplexeds me.

When a speaker fails from D.C. voltages being present,the coil is burnt in a definate tell tale fashion.This pock marking on the coil usually either fuses the adjacent windings together(essentially lowering the ohmage) or burns the coil winding open.

Is this failure due to the current load placed on the winding material ,causing the failure of the winding material itself.Or is it due to the lamination material on the windings being burnt away from the coil,therefore exposing the bare uninsulated winding to shorting?

I realise each case will be different,but am seeking opinions from the forum.

Gentlemen?

Oldmics

John Nebel
04-20-2004, 08:51 PM
Oldmics,

The DC resistance is much lower than the audio frequency resistance, consequently the current is higher and the winding fuses.

John

scott fitlin
04-20-2004, 08:56 PM
Interesting question! I tend to think that DC at high amperage and voltage cause the very fine strands of ribbon wire to overheat extremely quickly, and the coil reaches its burning point almost instantaneously. So the wire burns up and the speaker dies an instant death.

subwoof
04-20-2004, 09:39 PM
Well remember that the AC resistance is higher than the DC resistance BUT BUT that goes out the door when 50+ volts of DC PUSHES the coil right out of the gap. A coil in the open is a length of wire...period.

So the "telltale" burn marks that the reconer sees ( and bases his warrantee decisions on ) is flawed.

A significant oscillation that is partially rectified will ALSO produce this marking along with a cabinet loading offset that restricts the cone motion in one direction...

Usually the LEAD WIRE will act as a fuse before the coil fuses ( assume it is worn from travel ) and will be scorched black. It would take a really long moderate overheat to degrade the coil wire varnish to allow adjacent winding shorts.

90% of the shorts I have seen is when the ALUMINUM coilwire ( IE: 2226 ) is rubbed against the magnet due to travel offset ( gross over-driving ). I once had a 2226G come in that worked ok BUT had a DCR of .6 ohm and almost every winding was shorted to the next !

But I still see the occasional 2226H that sees a crown MA5002 driven into hard clipping ( hello - how many harmonics are present in a pure squarewave...? ) and it really loks like the "DC burn " that is pictured in the JBL reconers guide.

sub

John Nebel
04-21-2004, 06:29 AM
All sorts of weird stuff could happen, depending on the current.

To step into the twlight zone see the below

http://205.243.100.155/frames/shrinkergallery.html

4313B
04-21-2004, 07:02 AM
Take a paper clip and put it across a 1.5 volt dry cell battery. For additional fun, make sure an open can of gasoline is close by.