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Woofer, yes touchy...

Copa1934

Copa1934

My ears are bleeding
I've had some odd things happen and I've read some odd offerings, but this one stumps me but good.

I have a woofer that has poor output, hooked straight up, no xover. Push lightly on the dust cap and it comes to life in a big way. Checked the leads, while still hooked up and playing tunes and no change. But press lightly on the cone and the volume goes way up. Finally, press from the backside and the volume decreases further!

What is up with this?
 
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coil not at the proper depth in the magnet? Magnet too weak? Is this a Alnico mag or ferrite? Magnet loose on the basket or sepeated any?
 
similost said:
coil not at the proper depth in the magnet? Magnet too weak? Is this a Alnico mag or ferrite? Magnet loose on the basket or sepeated any?
Click to expand...
Proper depth? Going by the spider, it's level with the mounting surface. Looking through the vent holes in the base of the cone I can see through, so if the VC was too far out I would think those holes would be blocked. Holes are very close to the mounting point of the spider, by the way. I wouldn't eliminate this possiblity as I'm not the original owner so no way of knowing if this was how they were new or if they got this way after time. Only know that the tweeter was shot in this one. Possibly damaged the VC in some way?

Magnet weak. Can't say, and not sure that would be the case, assuming it were weak I wouldn't think pushing the VC in further would result is such a huge change in performance. Modern woofer, definitely ferrite. Magnet is not loose, nor is it off center (no rubbing at all). No signs of separation.

Baffling eh?
 
Not a short, open (intermittent) circuit.

Check the flex leads to see if they are frayed enough to have failed at either end. Often though, this will happen where the lead wires pass through and attach to the ends of the voice coil wires. Flexing or over-current application over time can kill any of those weak points.

John
 
I used to have a set of baby advents that worked perfectly. After dropping one of them the woofer behaved exactly as you described. My problem was the magnet shifting ever so slightly. The voice coil didn't rub, but it was no longer centered.

Out of curiosity, try pressing on the spider at different spots to shift, or cock the voice coil in one direction or the other.
 
Ceallach said:
I used to have a set of baby advents that worked perfectly. After dropping one of them the woofer behaved exactly as you described. My problem was the magnet shifting ever so slightly. The voice coil didn't rub, but it was no longer centered.

Out of curiosity, try pressing on the spider at different spots to shift, or cock the voice coil in one direction or the other.
Click to expand...

I would agree with either a shifted magnet, a shifted pole piece, or a voice coil that is hung up or dragging - possibly from a bad refoam, or possibly a loose or sagging spider, or a damaged voice coil (usually swollen or misshaped from overheating.

First is try to use while in different vertical orientations like when mounted. Rotate the woofer and try in each vertical orientation every 30-90 degrees if possible. If one works and others don't, its a spider sagging issue.

If you can feel the voice coil dragging, you have a couple of options. One is to take off the dustcap and check the gap visually. If its off center to the voice coil or vice versa, you'll need to loosen the spider and outer edge of surround and take out the whole cone/coil assembly and check for a uniform gap spacing. If its out of round, then you'll have to search for a few of the threads which discuss this. If there is reasonable space to the gap, then inspect the voice coil for damage. If still ok, then you could try shimming the coil in the gap, reglue the spider, then reglue the surround, let dry and try again.

You may also want to have a competent reconer or speaker repair shop do the above.
 
Copa1934 said:
I've had some odd things happen and I've read some odd offerings, but this one stumps me but good.

I have a woofer that has poor output, hooked straight up, no xover. Push lightly on the dust cap and it comes to life in a big way. Checked the leads, while still hooked up and playing tunes and no change. But press lightly on the cone and the volume goes way up. Finally, press from the backside and the volume decreases further!

What is up with this?
Click to expand...

Most of the voice coil winding width that is within the magnetic gap is fried, shorting out just that section of the coil. Pressing down on the cone places the remaining un-burned coil windings into the gap and output increases. The DC resistance is likely much lower than it should be. It isn't real common for a voice coil to burn to that extent without losing continuity, but it does happen.
 
Bob Orme said:
Most of the voice coil winding width that is within the magnetic gap is fried, shorting out just that section of the coil. Pressing down on the cone places the remaining un-burned coil windings into the gap and output increases. The DC resistance is likely much lower than it should be. It isn't real common for a voice coil to burn to that extent without losing continuity, but it does happen.
Click to expand...
I've had just that happen before. I had refused to admit that the woofer was blown, because I would get it to work by fiddling with it. Ultimately, I ended up replacing the woofer just the same, and performed a postmortem on the original. The windings were just about fused together on the lower portion of the coil, but looked fine in the area above where the pole piece would end. Had a DCR of about 2Ω, whereas the "good" driver from the other speaker measured ~7Ω
 
M Jarve said:
I've had just that happen before. I had refused to admit that the woofer was blown, because I would get it to work by fiddling with it. Ultimately, I ended up replacing the woofer just the same, and performed a postmortem on the original. The windings were just about fused together on the lower portion of the coil, but looked fine in the area above where the pole piece would end. Had a DCR of about 2Ω, whereas the "good" driver from the other speaker measured ~7Ω
Click to expand...
I'll check with my meter next.

Thanks!
 
You'd probably hear it when the driver springs to life loudly, if the coil had that kind of damage. I have $5 that says Sir.Byrd and I are on the right track here... I have spent years looking at failed drivers and that failing tensile lead issue is so common you wouldn't believe it, as is the failing connection between the lead wire and the voice coil, usually where it passes through the cone or coil.

John
 
jdurbin1 said:
Not a short, open (intermittent) circuit.

Check the flex leads to see if they are frayed enough to have failed at either end. Often though, this will happen where the lead wires pass through and attach to the ends of the voice coil wires. Flexing or over-current application over time can kill any of those weak points.

John
Click to expand...
8-ohm speaker, which should read around 6-7 typically, reads 0.5. And if I push the cap it reads up to almost 2.0, so not good. Sounds to me like a short, lower reading means less resistance, correct? If frayed wouldn't the resistance value be larger?
 
Well, couple of things come to mind. One, you are generating voltage in the coil when you move it in the magnetic field, so unless you mean that the DC impedance goes to 2 ohm and stops there with the cone held motionless at that position, the change in reading is meaningless - it'll jump all over the place when the cone is moved.

Also, are you sure the 0.5 ohm reading is accurate? If so, yes - I would agree with you about a short (though you would normally be hearing something rattly or distorted if the coil is burnt - and I wouldn't be connecting it to anything at 1/2 an ohm, either). I have also seen shorts where the layers fused to each other at the top of the coil which could cause the low resistance w/o having the rest of it cook enough to be loose on the former. Wouldn't expect that reading to change much with cone held at different positions though, has to be some other damage elsewhere if so.

But, it's still increasing when you press it which could still consistent with the lead wire issue Sir.Byrd and I mentioned. Moot point if the load is 1/2 an ohm at rest though.

I don't think a shifted magnet would cause any of that unless it also pinched and rubbed the coil enough to cause the shorting.

We can all speculate about it, but the summary is this driver is likely toast. If you can't see and fix any external damage in the lead wires, or can't carefully disassemble it to get to the coil, it's a goner. If you want to know why, post-mortem after cutting the moving parts out of the driver is about the only way to tell.

John

Copa1934 said:
8-ohm speaker, which should read around 6-7 typically, reads 0.5. And if I push the cap it reads up to almost 2.0, so not good. Sounds to me like a short, lower reading means less resistance, correct? If frayed wouldn't the resistance value be larger?
Click to expand...
 
jdurbin1 said:
Well, couple of things come to mind. One, you are generating voltage in the coil when you move it in the magnetic field, so unless you mean that the DC impedance goes to 2 ohm and stops there with the cone held motionless at that position, the change in reading is meaningless - it'll jump all over the place when the cone is moved.

Also, are you sure the 0.5 ohm reading is accurate? If so, yes - I would agree with you about a short (though you would normally be hearing something rattly or distorted if the coil is burnt - and I wouldn't be connecting it to anything at 1/2 an ohm, either). I have also seen shorts where the layers fused to each other at the top of the coil which could cause the low resistance w/o having the rest of it cook enough to be loose on the former. Wouldn't expect that reading to change much with cone held at different positions though, has to be some other damage elsewhere if so.

But, it's still increasing when you press it which could still consistent with the lead wire issue Sir.Byrd and I mentioned. Moot point if the load is 1/2 an ohm at rest though.

I don't think a shifted magnet would cause any of that unless it also pinched and rubbed the coil enough to cause the shorting.

We can all speculate about it, but the summary is this driver is likely toast. If you can't see and fix any external damage in the lead wires, or can't carefully disassemble it to get to the coil, it's a goner. If you want to know why, post-mortem after cutting the moving parts out of the driver is about the only way to tell.

John
Click to expand...
I'm not attached to this speaker (or driver for that matter) and the driver is a cheap driver anyway, so repairing is not an economical idea.

Yes when at rest 0.5-ohms and I'm using a digital meter that has been accurate, up to this point anyway. Yes when held motionless, as best I'm able, it peeks around 2-ohms.

I'm almost done with replacement woofers. Huge magnets, larger voice coil, yet same frequency response, well actually "rated" down to 43hz, but we'll see.
 
jdurbin1 said:
You'd probably hear it when the driver springs to life loudly, if the coil had that kind of damage. I have $5 that says Sir.Byrd and I are on the right track here... I have spent years looking at failed drivers and that failing tensile lead issue is so common you wouldn't believe it, as is the failing connection between the lead wire and the voice coil, usually where it passes through the cone or coil.

John
Click to expand...

I'll take that bet! :) The giveaway clue is low output during regular play. If the problem was frayed leads, there would be no output, or intermittent output. Frayed leads are an on or off problem, not an output reduction problem.
 
Took the dust cap off today, actually peeled off relatively easily. Actually held on with double sided tape, along the perimeter. Anyway, didn't see any burns or scoring on the inside of the VC. Too much adhesive on the connections, leads to VC for anything to come loose or for any movement. I'm leaving it at that. Maybe, if I'm inclined, I'll do post mortum, but for now it's, well, a dead issue, pun intended.

Thanks for all the insight, might come in useful with other projects and troubleshooting.
 
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