Weird speaker impedance behavior

[Randall]

I have a Frontman 212R in for no sound. Customer had attempted to remove chassis from cabinet and got it stuck. I got it out and determined the chassis was working, but that both speakers were open. I was surprised, but the customer OK'd Eminence replacements. I ordered a pair and then pulled the bad ones out. While they were out, one definitely is open, but somehow the other one measured good. I checked the wires and connectors, everything looks good. This is when I discovered something strange.

While measuring the one I thought was good, the readings would go all over the place from 7.2 to open for a time. I thought it must be my probe contacts, but it did it whether connected to the terminals, the speaker leads from the push on connectors, or the braid itself. I can make the static impedence reading change by exercising the cone a few times. It sat at 7.2 ohms for a while, causing me to make a dash to the phone to cancel one of the new speakers I had just ordered. But, then I find I can make it go from anywhere from 7.2 ohms to a few hundred 0ccasionally for a few seconds, and watch it drop down to 25 or so. Mostly it just stays between 9 and 16 ohms.

Now, I realize I will see change while the ohmeter is connected and the cone is flexed. It is in the following several seconds after it stops that I get these random readings. It has been sitting at 13 ohms for a minute now. I just flexed it and now it is at 9.3 ohms. Did it again and now it is 11.2 ohms. Also, I can make it change by rapping on the frame with a screwdriver. Another flex, and back down to 7.2 ohms.

What is causing this? I am hearing no coil rub.

[Enzo]

You are aware that moving the cone generates a voltage, which will confuse the ohm meter. Best to place a speaker face down on a table to prevent motion. (You can use a speaker as a microphone in fact)

But if doing that and clipping wires to speakers it still varies, first thing I'd do would be to grasp the little tinsel wires and flex them. Make a change? I'd suspect the tinsel wires were just failing.

[g1]

Do you have a good speaker to compare to (while flexing cone) . It's basically a motor/generator, so DC resistance reading will only be stable when cone is stable.

edit: simulpost. what he said ^^^^^

[Randall]

As I said, I understand the speaker makes a signal when it is moving. I have known this since I was 12, and terrorized my little brother by putting a car speaker under his bed and used it to both talk and listen. I am talking about it sitting statically and not moving after moving the coil. I tried flexing the copper braids to the coil while clipped to the probes, this does not change anything. I have tried it facing up and facing down, applying pressure from both sides. How can it read 7.2 ohms one moment, and with everything secured so no wires or leads are moving, after flexing the cone a few times, it sits a 12 or 16 ohms for a minute, or until the cone is flexed again and I get yet another different reading?

And after doing this numerous times in varying ways, suddenly it stops doing it. It is now staying within 7.2 -7.8 ohms after multiple cone movements over a 10 minute stretch. Nothing else was changed.

Then, just now as it was sitting at 7.3 ohms for an hour, I rapped the frame with a screwdriver, and it rises to 10.3 ohms and stays there. Flexing the braid leads, and unclipping and reclipping to them at various places make no change. So how is it possible that it rose 3 ohms just from rapping on it? If it were going down in impedence I could understand it as being a shorted winding, but rising? What would do that?

edit: I just unclipped my meter from the speaker in question that is sitting at 10.2 ohms. I connected to another speaker in a DRRI that is here. It measures 7.2 ohms, and goes back to 7.2 ohms after flexing every time. I reconnected to the speaker in question, it is still sitting at 10.2 ohms.

This is real guys.

[Enzo]

MY first suspects are always tinsel wires, but inside the thing, they terminate at the voice coil, and those connections can fail just like a solder connection inside an amp. Your speaker is intermittent. The solder between voice coil and tinsel is failing, sometimes the tinsel to terminal joint fails, but it sounds like you resoldered that.

[J M Fahey]

Youhave a dead speaker and an iffy one, replace both.

[The Dude]

Often times when I see this, the tinsel lead is burnt right behind the speaker terminal. You need to cut the burnt part off before you resolder it. If it's long enough after cutting, you can just resolder it. If not, you'll need more tinsel lead to either extend or replace it. Look for a darkened area or burn mark on the leads. Sometimes you can bend the terminal in towards the center of the cone to get more slack in the tinsel lead. I hope that makes sense.

Edit: A good way to see if this is the case is to check resistance with your probes on the tinsel leads themselves instead of at the terminals.

[vintagekiki]

I am an old school representative and for all break measurements I use an analog ohmmeter. The advantage is twofold because in addition visual, I also have an acoustic overview.
In the case of unprovoked speaker interruptions, cause was almost always is cold solder at point where moving coil connect with braid (between suspension and dust cap).

[Randall]

The issue is not with a burnt or bad soldered copper braid to terminal, or tinsel lead as Enzo refers to it. As I have said, I am measuring by clipping to the copper braid behind the terminal, close to where it goes into the cone, and then verifying my readings by clipping to the terminal and/or the length of wire coming off the push on connectors. They are very close to identical. I am 100% convinced the issue is not an outside connection.

It has been suggested the issue may be where the braid connects to the voice coil wires, but I don't know how you'd verify that. It is covered in goo.

Juan suggests it is an "iffy speaker". Well, we know this already, but I am struggling to understand why. The braid to v.coil connection seems the most viable at this point. I guess since one speaker was blown, it is not hard to believe the other one is damaged, especially since it likely was taking all the wattage for a time.

BTW, this is rated at 100 watts, and it comes with two 50 watt speakers and a hard distortion mode, so it's not a big stretch as to why it blows it's speakers. This is the second one I have replaced both in.

[g1]

If you cut off the dustcap, you may be able to see the solder joints on the voicecoil itself. (the points VK mentioned)

[Enzo]

A really sharp Xacto blade or a razor blade, you can cut off the dust dome right at its edge. That exposes the voice coil and connections. If you are able to repair it, a new dust dome can be glued in place.

[J M Fahey]

The issue is not with a burnt or bad soldered copper braid to terminal, or tinsel lead as Enzo refers to it. As I have said, I am measuring by clipping to the copper braid behind the terminal, close to where it goes into the cone, and then verifying my readings by clipping to the terminal and/or the length of wire coming off the push on connectors. They are very close to identical. I am 100% convinced the issue is not an outside connection.

The frayed/open point usually is *exactly* where tinsel wire touches the solder blob if a hollow rivet as used (what I do, makes future repairs easier) or goes through cone.
Test it yourself: solder a wire (any wire) to an eyelet on a board, then shake wire back and forth (which happens all the time when a speaker works), you tell me where it will break 99% of the time.
Clamping crocodiles to wire does nothing if break is between wire and joint.

The second weak point is where voice coil wires, glued to cone, *just* reach said hollow rivet if used or are twisted around a short braid stump.

Not a cold solder but wire broken there.
Of course, reflowing the supposedly cold solder usually catches on the exposed voice coil wire and connection is restored.
Manufacturing and guaranteeing my own speakers made me be VERY keen on studying and solving those problem, since for me itīs a loss ($$$) and loss (prestige) situation.
So I both use a generous length of tinsel wire, and bend it in an S shape, to give it ample room to move without stretching but also not flapping against the cone, and also releigiously surround and glue said hollow rivet to cone with very rigid Epoxy.
And IF after many years tinsdel wire breaks, itīs very easy to solder a new piece from speaker terminal eyelets to cone eyelets, without even pulling bthe dustcap.

It has been suggested the issue may be where the braid connects to the voice coil wires, but I don't know how you'd verify that. It is covered in goo.

Lazy manufacturers typically use somewhat flexible goo, just black oloured vinyl (Elmerīs) glue.
I did too, now only use strong Epoxy, even if I "waste" 5 extra minutes applying it and curing it with a heat gun.
Well worth the "waste" of time.

Your hair thin voice coil wire may very well have broken under the black goo.
If you want to recover those speakers, heat goo with a heat gun, it will soften a lot, pull it away with tweezers and slightly scratch voice coil wire glued to cone to measure continuity.
If so, tin it and solder new tinsel wire over it, then covr joint with epoxy.
Heat it with a heat gun so it partially melts (but avoid bubbling) and it both "wets" cone better and dries harder.

Juan suggests it is an "iffy speaker". Well, we know this already, but I am struggling to understand why. The braid to v.coil connection seems the most viable at this point.

See above and below



See also I use THICK tinsel wire, as thick as possible still going through 1/8" eyelets.

My speakers are ABUSED for decades and keep working.

[pdf64]

I've got an old Fane that developed an intermittent open circuit. On removing the dust cap (very carefully!), the wire to the voice coil had broken just as Juan describes. I scraped the varnish on it back a little and soldered a small bridging wire between the tinsel's terminal on the cone and new exposed copper on the topside of the wire (it was still stuck on to the cone). With a bit of protective epoxy around the repair, and the old dust cap re-glued back in place, it's still working fine 20+ years later
I was also relieved that the amp survived unscathed, as that Fane had been only load on the amp, which I'd been pushing pretty hard at a gig when the issue manifested.

[Randall]

I cut open the confirmed dead one, and this is the amount of goo we are faced with. It's a cheap speaker to begin with, not worth the time and effort in this case in my opinion. I can see in Juan's environment where that would be what one would be motivated to do, but my customer is OK with a couple of $70 75 watt Eminence replacements. So onward and upward.

[J M Fahey]

Copper flexing *will* break eventually.

Maximum flexion is when tinsel wire*just* reaches cone, and often vibrating wire shakes rivet or soldered end around and voice coil wire breaks there.
Thatīs why strongly fixing joint to cone, so it moves back and forth but does not flex is important.