Stuck Voice coil magnet shift..

crassmage

This may have been discussed ad nauseum but I may need some help.
Here goes:

I am an avid Polk lover. My first Decent speakers are/were some old monitor 7s. Nothing has knocked them off my main system. So I finally stopped trying to upgrade as it wasn't happening(Polk monitor 7s are amazing). Made wife happy anyways...
Fast forward to yesterday I was at a thrift shop and son finds a pair of monitor 10a. I HAD TO! Didn't bother to even check but when I got them home 3 of the 4 drivers were seized up. Tried the PVC pipe method for a long time this evening and I'm about to give up. Is there anyone who fixes these around here? First post. Thanks for ANY help!
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lawdogg

I am more than happy to help. It sounds like you already have a re-centering tool going, so maybe I can offer some advice about how to use it? Are they just failing to re-center, or do they keep shifting while they are in the tool, or some other issue?

crassmage

Wow thanks for the warm welcome! I am in Minnesota. Not much around here for vintage audio repair.
My issue is yes I made the rig albeit my pipes are 2 inch cuts via the nice kid at the hardware store. I'm handy enough. But I just can't get it to free up the speaker. I have 2 I am trying with but neither ever gets to the point to where it moves freely..
Thanks again

delkal

Are you sure the adhesive broke and the magnets shifted (can be fixed)? Or did someone cook the voice coils (can not be fixed).

When you take off the dustcaps is the coil centered in the magnet?

crassmage

I have the dustcap off one. Centered? It's still in there. I'm not sure if cooked or what to be honest. Would a picture help? Just won't move is all. I will give it another go today. Sound/music still comes out of driver but obviously not the sound I need..thanks!

FestYboy

Yes on the pictures

crassmage

Any pic in particular that could aid in finding out? Voice coil, magnets,etc. You guys are great!

pitdogg2

all
including your jig may be of help

crassmage

Will do!

crassmage

lawdogg

Alright I see what's going on. That PVC jig won't work - let me explain. You may be able to make a cut in it and get it to work, or you might need to get another size pipe (I forget what size works ... there are actually two different sized magnets, if everyone didn't already know that). Why do you need to cut it?

The way the jig works is that is clamps the magnet, and the screws move the frame relative to the magnet / pole piece assembly ! So you need a way to clamp the magnet and hold it in place relative to the frame. To do this, cut a section out of the PVC pipe, and use a hose clamp.

Here's what your jig should look like:



Before you put the driver into the jig, make sure the pole piece and magnet are securely fastened to each other! I always epoxy them together and let it cure BEFORE the driver goes into the jig.



Once the driver is in the jig, and the hose clamp is tightened to hold the magnet in place, the screws will move the frame relative to the pole piece.



Now you need to gently move the frame so the voice coil is centered perfectly around the pole piece. I took some pics to help explain:

Bad ... Bad ...





Good!



Here's how to visualize the voice coil - if the voice coil is the electrical tape, here's how you need to center it.

Here's what happens when the "magnet" shifts, the voice coil gets clamped by the pole piece and can't move.



If you screw the frame over too far in the other direction, it'll snap the other way.



So use patience and finesse, gently turn the screws to get it exactly centered, concentric in the middle so that the voice coil can move freely.



A hard lesson I've learned - sometimes once it's re-centered, the moment you loosen screws to remove it, it'll snap back into a shifted position. Dang! So what I do is apply the magnet-to-frame epoxy BEFORE putting the whole assembly in the jig. Once it's centered, I leave it in the jig until the epoxy has cured, and then can remove the jig without fear of re-shifting.

Cheers and happy re-centering!

lawdogg

I'll add that your voice coil & former do look a little ragged. You need to make sure your voice coil is good (does it measure OK?) and make sure that the voice coil former is smooth and round. Otherwise, since the tolerances are so tight, it has a chance of not being able to center properly without scraping as it moves up and down. Been there done that!

crassmage

Thank you so much. I have some time today. Hope to find success. How do I measure the VC? Ohms? Thanks again for taking the time

lawdogg

Yes ohm it out to make sure it's not open.

crassmage

So basically we want a reading i.e. 1 and not Infinity. One on _ post one on + post.
Correct?

FestYboy

No, 1 and infinity are interchangeable on multi meters. So don't trust that.

If the coil is good, and you have one lead on each speaker post, you should read all least 3.4 ohms ( depending on the driver tested as most don't carry a nominal impedance less than 4 ohms. A DC resistance check will always read lower than nominal impedance spec.)

crassmage

Ok. Was using 1 as a number is all. They checked out ok. The 2 I have out anyways. Gonna go for it. Thanks for the input!

crassmage

So I made a similar rig. I epoxied the cap and magnet. Dried. Put in PVC pipe. Then measured where the pole plate would sit within the pipe. Put screw in there to keep it from budging. The PVC held the magnet in place. Put glue on magnet(top) to be stuck to basket plate Then put basket in and had screws to adjust that. Voila! Once it moved free I hooked it up to a tone generator and tested in the 50-60 range and adjusted screw until silent. Now will wait for glue to dry. Promising at least. Thanks for all the help everyone.

crassmage

So checked one last time and did a subtle bass test at low volume to see. Tested good. No buzz. But a little flicker and my amp blew a couple fuses. Replaced them. Still blow. Coincidence or something I could have done with this speaker? Doubtful but I'm new to all this.
Thanks...

FestYboy

In my experience, test tones in the 7-11 Hz range will show any real issues. 50-60 can still hide a rub, especially at low volume/excursion.

crassmage

I will test that thanks!

lawdogg

You mean the fuse in the speakers or in the amp?

crassmage

The amp. I replaced and they blew again.. all correct amps/volts. I was only testing the driver in the jig. Was fine no wires moving nothing...poof...never ends

crassmage

Also 7-11 hz range it was silent. If it stays in place what is good to use to "seal" the magnets? Power grab stuff?

crassmage

Well something gave overnight. Stuck again. I was able to move it back in the job but that pretty much told me my adhesive didn't hold. Sure enough was the case. Try again today.

pitdogg2

crassmage wrote: »

Well something gave overnight. Stuck again. I was able to move it back in the job but that pretty much told me my adhesive didn't hold. Sure enough was the case. Try again today.

what kind of adhesive are you using?

crassmage

I had the loctite red. Also power grab around the bottom. Of magnet with pole plate. Sat for 24 hrs. Took apart very gently.. stuck.. was able to pull it apart easily ...suggestions?

FestYboy

Cleanliness is next to godliness... And the loctite should not push out the seam (one light head in the middle of the radius).

lawdogg

I use 2-part epoxy and it's never failed me.

crassmage

I did use minimal amount. I will get some epoxy and give it another whirl

crassmage